August, September, and October Weather Discussion Archive – Retrospect



October 17 2005 – 18:30 UTC

Tropical Storm Wilma is the 21st named storm of this incredible 2005 hurricane season. 2005 is now tied with the 1933 season for the most named storms in a single year. Wilma has strengthened steadily over the last 24 hours, and that should continue for an additional 24 hours. Sea surface temperatures and oceanic heat content are conducive for rapid strengthening, but the favorable anticyclone aloft has shifted to the west and is now anchored just east of the Yucatan coastline. As a result, the northerly clockwise flow around the Watsonwille and  Dallas is knocking down convection over the surface circulation with 10-20 knots of shear. It is only a matter of time until Wilma positions itself underneath this ridge, and rapid intensification will likely begin once that happens. Category three of four status may be Wilma’s peak intensity in the northwest Caribbean Sea before being drawn north by an approaching trough. Once the upper level trough begins to interact with the hurricane, upper level southwest winds may begin to weaken the cyclone to some extent.

The synoptic pattern has not changed over the last several days. Wilma is caught between two larges areas of high pressure; the subtropical ridge over the central Atlantic and another ridge near Texas and Mexico. A shortwave trough over the east coast is lifting out and will not be much of a steering influence. Wilma will not move much in any particular direction until a mid to upper level trough exiting Canada and the midwest begins to erode the eastern flank of the Mexico high. Any northerly progression may not begin for another 48-72 hours. This idea is supported by the 12Z UKMET, CMC, GFDL, GFS, NOGAPS, and ECMWF. The GFS and GFDL have been flip-flopping between the Yucatan and Cuba, but the upper air pattern progged by the GFS clearly shows strong westerly winds developing in the Gulf of Mexico ahead of the approaching trough. Furthermore, the European has been very consistent with recurvature through the Yucatan Channel the last several days. It is also interesting to note that the NOGAPS has been pushing Wilma deep into the Yucatan, and that it has been the more westerly outlier during most of this season. The 12Z has suddenly shifted in favor of the other recurvature solutions. The probability of a direct hit near the northeast tip of the Yucatan and western tip of Cuba are about the same for now. The threat to both areas will have to be evaluated on a day to day basis; it is too difficult to pinpoint such a small location given the current lack of movement. All of this would also suggest a threat to central and southern Florida. All interests south of Spring Hill, Florida and Daytona Beach, Florida, should remain abreast of the situation. This includes Key West, Miami, Orlando, and Tampa. Landfall would occur between St. Petersburg and Naples if this scenario evolves as forecast. Again, the trough responsible for recurvature would also result in increasing southwesterly shear prior to a Florida landfall, so if Wilma were to peak as a major hurricane in the northwest Caribbean, then a hurricane landfall along the lines of a Category two would be more likely. Timing of landfall would be 5-7 days. Southern Florida should not be worried just yet, but there should be some concern. As we’ve seen with Katrina and Rita, a medium range hurricane forecast is subject to a large degree of error. In this case, if the southern Florida scenario does not evolve, then the track could be drastically different. Scenario #2 is Wilma getting caught under the ridge to the west and the trough to the north is not as strong as forecast. This would result in a Bay of Campeche track, but that is considered unlikely due to model trends (now an entire lack of model support) and the upper air pattern that has been outlined by some of the models for several days. A moderate landfalling hurricane over southern Florida in the medium range is the best bet for now.

September 22 2005 – 09:00 UTC

This is a special morning update on Category 5 Hurricane Rita. There have been a few changes in the forward heading of the storm and the available model guidance. First, Rita has gradually shifted to more of a west-northwest heading, with possibly even a slight wobble to the northwest within the past couple hours. Second, nearly all model guidance is shifting from central Texas to upper Texas between Galveston and the Texas/Louisiana state line. The 00Z UKMET and GFS an are between Galveston and Port Arthur. Even the southern outlier over the last few days, the NOGAPS, is a hair east of Galveston as of 06Z. The 00Z GFDL is into Galveston Bay. Apparently, the ridge is breaking down faster than earlier anticipated. Galveston and Houston are still in the thick of it. So if you’re in that area, then get out. For southwest Louisiana, a track closer to the LA line means 1.) be ready for a major storm surge if you live along coastal LA south of Lake Charles and 2.) A slight deviation to the right means you’re staring at a majorhurricane coming at you. If you have the ability to leave southwest Louisiana then i’d recommend you do so, especially with these easterly shifting models. A direct hit in southeast Louisiana is not a concern, but heavy rainfall and coastal flooding is.

Another VERY important aspect of this potential shift in track: The level of oceanic heat content ahead of the center would NOT decrease. As you go westward towards the Texas coast along 25N subsurface temperatures gradually become cooler. However, a more WNW or NW track will take Rita over another section of the loop current, which is very warm. In conclusion, if this more northerly track does take place, then we could be dealing with at least a strong Category 4 at landfall. GET AWAY from the upper Texas and lower southwest Louisiana coastline.

September 21 2005 – 01:00 UTC

An upper low responsible for light to moderate southwest shear over Rita the last few days has moved away, and the hurricane has intensified under a developing anticyclone aloft. Upper winds and sea surface temperatures will remain condusive for steady strengthening through the Gulf of Mexico. The Statistical Hurricane Intensity Prediction Scheme (SHIPS) has become much more aggressive on intensification this afternoon, and is now calling for Rita to hit the Gulf Coast as a 145mph Category Four on the saffir simpson scale. Rita’s central pressure continues to fall, satellite reveals an organizing hurricane, and the storm has several days to intensify under favorable conditions. Therefore, the SHIPS forecast is very reasonable, and that is why the National Hurricane Center suddenly increased their intensity forecast at 5PM EDT.

The last discussion posted two days ago called for a landfall in extreme northern Mexico, but it did not downplay the potential threat to Texas. Since then, both dynamical and tropical models have started to converge in one area, and that focal point is the central Texas coast. The margin of error still includes the entire coastal Texas area (and even extreme W LA and N MX), but the most likely target appears to be between Corpus Christi and Galveston this evening. The CMC, UKMET, GFDL, and ECMWF are in general agreemwnt with a track in that area. The GFS is the northernmost model, which is closer to metro Houston. Meanwhile, NOGAPS is the southern outlier and is just north of Brownsville. If you live along the Texas coast, then PLEASE follow all directions given by your emergency management officials.

The rest of the Atlantic Basin is quiet. Philippe is no threat to land.The same goes for all active tropical cyclones in the eastern Pacific.

Tropical Depression 17W will need to be monitored by Hong Kong and areas south of the city. The official JTWC forecast is calling for 17W to become a typhoon within the next few days. Tropical Depression Saola could also threaten Japan as a typhoon in a few days as well.

September 19 2005 – 03:00 UTC

There are two big developments for southern Florida this evening. First, Rita is temporarily moving more northerly. Second, recon is finding that Rita is of at least moderate tropical storm intensity. Both of these developments place the southern Florida peninsula at a higher risk for a moderate hurricane landfall. Ridging aloft and favorable sea surface temperatures should allow for Rita to become at least a Category 1 hurricane within the next 12-24 hours, and probably even a Category 2 by the time it reaches southern Florida. Hurricane force winds may be felt in southern Florida and northern Cuba within 24-36 hours. The main event still isn’t expected to occur until Rita enters the Gulf of Mexico, where it will have favorable upper level conditions, sea surface temps, and time to strengthen before threatening northern Mexico or southern Texas as a major hurricane. The tropical model suite is still focused on the central and eastern Gulf Coast, but they do not have the dynamical edge the global models have. The GFDL was just east of New Orleans yesterday, but has been trending west near the border over the last 24 hours The CMC, NOGAPS, and ECMWF are south of the border. The UKMET is over the TX/MX line, and the GFS is over S TX. CIMSS steering analysis shows a strong mid to upper ridge over the southeast that was accurately progged by the global models, and it should hold. The slight relocation to the north tonight should have no impact on the track in the medium range due to the ridge. Landfall is expected in the same general area as Emily struck earlier this season. Texas must pay close attention to this situation, however.

Interests in the Bahamas need to keep an eye on Philippe’s progress.

The ative tropical cyclones in the eastern Pacific are no threat to land. Jova is curving north as the models indicated it would.

September 18 2005 – 01:30

95L INVEST has become better organized today and has been classified Tropical Depression 17 by the Tropical Prediction Center. Convection has consolidated near a surface circulation, banding features have become more apparent, and outflow has expanded in all quadrants, especially in the western semicircle. The improving structure of TD17 can be attributed to an upper high building over the surface low. Conditions are expected to remain favorable over the next five days, and intensification should continue. SHIPS develops TD17 into a Category 2 hurricane by 120 hours. It is reasonable to assume that TD17 could be the 5th major hurricane of the season in 4-6 days. Luckily, it is unlikely that the storm will directly threaten the Lesser Antilles. All model guidance is in full agreement that the northerly motion will continue along a weakness in the subtropical ridge.

96L INVEST, located north of Hispaniola, remains the most dangerous threat to land. An upper low to the west will continue to hinder organization over the next few days and the chances of it becoming a significant tropical cylone prior to reaching southern Florida or Cuba are low. The risk of moderate intensification begins once it enters the central Gulf of Mexico. Conditions will likely be favorable, and all global models are now indicating tropical cyclogenesis. In fact, the European model is depicting a significant hurricane strengthening just east of Mexico next weekend. Most models are still showing a continued westerly track towards northern Mexico through the period; the exceptions being the tropical model suite and the GFDL. The GFDL indicated a threat to the central Gulf Coast earlier today, but is already beginning to shift west. The northerly solutions once in the Gulf do not make sense. All global models depict a strong mid to upper level ridge along the central gulf coast through day six, and unless 95L crawled at a snail’s pace, then the ridge will force the storm into Mexico. The only interesting development in today’s track guidance is a northwest turn shown at Day 6 in the 12Z and 18Z NOGAPS runs. The bottom line: in 5-8 days a hurricane may threaten the northern Mexican coast, and southern Texas is not in the clear.

Tropical Storm Ophelia is becoming extratropical as it begins to pull away from the Canadian Maritimes.

A tropical wave in the eastern Atlantic near 35W will be monitored for the possibility of slow development.

Interests in Hawaii should continue to monitor the progress of Hurricane Jova. All guidance remains east of the islands, however. Lidia and Kenneth are no threat to land.

Tropical Storm Vicente is dissipating, and is no longer expected to strike Vietnam as a classified cyclone.

September 17 2005 – 01:00 UTC

Discussions return as Katrina recovery continues…

95L INVEST, a tropical wave east of the Lesser Antilles, has not improved much in organization over the last 12 hours. Convection is being divergently enhanced by southwest winds from a TUTT cell, but those winds are not conducive for consolidation. There is a mid to upper level ridge over the eastern side of the disturbance, but overall upper conditions are nothing more than marginal for tropical cyclogenesis at this time. Slow development into a tropical depression is possible over the next few days. The weak surface low along the wave axis has been moving west-northwest today rather then due west yesterday. This change in heading is likely a sign of things to come as a moderate trough over the central Atlantic will likely pull 95 more northerly with time. All global model guidance has been consisently taking the wave north of the Lesser Antilles and Puerto Rico, and the tropical model suite has shifted a bit north this afternoon. 95L will probably pass just north of the islands, but interests north of Martinique are advised to monitor this system closely.

Newly formed 96L INVEST, located north of Puerto Rico, also has some potential for slow development in the medium range. An upper low just west of the tropical wave will prevent 96L from developing rapidly. Furthermore, land interaction with Hispaniola and Cuba will limit development over the next 24-96 hours. 96L will not have much of a chance to organize until it reaches the Gulf of Mexico as it pulls away from land and upper winds become more favorable. 96L has little to no chance of recurving due to a massive ridge of high pressure building over the southeast United States out of Texas. The only hint of a trough over the southest or east coast is Tropical Storm Ophelia, which will be northeast of the Canadian Maritimes by Monday afternoon. The upper pattern is not going to change through the period, and 96L will have a free shot at the western Gulf Coast, specifically Mexico. South Texas is not in the clear either, but all guidance is currently south of Brownsville and the pattern seems to support such a track. The end game may be a tropical storm threatening Mexico in 5-6 days.

The Ophelia forecast is straightforward. Ophelia will continue northeastward along the western periphery of the subtropical ridge. Massachusetts and Rhode Island are still within the margin of error, and the track can vary quite a lot even as the center approaches that latitude. Interests in the official warning area should be ready for tropical storm force winds and heavy rainfall for about a 6 hour period.

The remainder of the Atlantic is generally quiet. A tropical disturbance near 33W will be monitored once it draws closer to the Lesser Antilles. None of the models are latching onto this feature yet.

In the eastern Pacific, Hurricane Jova is no threat to land. However, Hurricane Kenneth should be monitored in Hawaii as there is a slight chance that the islands could be hit with tropical storm force winds in the medium range. But chances are that Kenneth will either dissipate or pass just north of HI. A tropical wave near 108W may also develop, but it is not expected to threaten land.

Tropical Depression 16W (Vicente) is forecast to strike northern Vietnam and extreme southern China as a tropical storm in a couple days.

August 22 2005 – 23:00 UTC

It turns out that another call for development in the medium range has verified, but of course we had to once again back off just prior to the in initial stages of formation. The mid level circulation over the Yucatan Peninsula yesterday moved into the Bay of Campeche last night and quickly developed under favorable conditions. There isn’t much left to forecast other than calling for landfall as a moderate to strong tropical storm in southern Mexico within the next 18 hours.

The remnant of ex-TD10 just won’t quit. The disturbed area of weather from yhe Bahamas to Hispaniola is the area under investigation. The synoptic setup is a tropical wave and the partial remnant of TD10 being divergently enhanced by an upper low to the north. This upper low will either lift north or weaken as the wave continues westward towards Florida and eastern Gulf of Mexico. Meanwhile, the large anticyclone along the Gulf Coast is expected to extend into much of the Gulf of Mexico by the medium range. Therefore, the wave will be moving into an area with favorable winds aloft, and very warm sea surface temperatures. In fact, portions of the eastern Gulf of Mexico contain some of the highest levels of Hurricane Heat Potential in the Atlantic Basin. It’s no wonder that every global model is either hinting at or showing tropical cyclogenesis in the eastern Gulf within 4-6 days. On a scale of one to ten, the probability of development within the next 6-7 days is probably an eight. Development may not even take that long. There is a chance that development may occur even prior to the developing low pressure area making landfall in southern Florida. Fortunately, the strongest this system could probably get east of Florida is tropical storm status, and heavy rainfall would still be the biggest issue.

Once in the eastern Gulf, the track is highly uncertain. First, any medium range forecast (4-7 days) will always be difficult. There are also some model disagreements, not surprising considering the timeframe in question, but overall they have converged some today. The 18Z run of the GFS, a model that earlier took it westward towards Mexico, now slows it down and has it landfalling along the Florida Panhandle in 8 days or so. Afterwards, it actually moves the storm back to the Gulf, only to move north again and make landfall along the Panhandle several days later! However, this scenario is long range, subject to robust change, and has little chance of verifying. The 18Z NOGAPS and 12Z ECMWF are also far east and slowly moves a tropical cyclone towards the Panhandle/Big Bend area by Day 7. The 12Z CMC and UKMET are further west, with the former moving a formidable storm in the general direction towards Texas. The UKMET is slower and places a storm in the central Gulf in 6 days. Given the sea-level pressure pattern displayed at the end of the run, UKMET may be hinting in a turn towards the northern Gulf coast beyond the forecast period. Overall, the precise track details are not set in stone at this time once in the Gulf, and it is largely dependant on how far the western flank of the subtropical ridge extends. Right now we are favoring the northern Gulf coast, but everywhere along the coast should stay in tuned on this feature. There are many reasons to believe there will be some type of tropical cyclone in the Gulf of Mexico or along Florida into next week.

Elsewhere, we are also keeping an eye on a tropical wave and associated broad area of low pressure east of the Cape Verde Islands. This system is moving west to west-northwest in the tropical Atlantic and only generating sporadic convection due to the cool water temperatures at its present location. As it gains longitude, water temperatures will warm. Furthermore, upper-level conditions are fairly favorable for organization. Dry air will be the main issue, and should prohibit rapid intensification. Nonetheless, slow development is possible and this may become a tropical cyclone over the next few days. Model guidance suggests it will get caught in a ridge weakness and curve out to sea. This is the most likely solution, though it should be noted a slower developing system may stand a better chance to miss the trough connection and make it further west.

In the East Pacific, Hurricane Hilary is beginning its weakening phase in cooler waters. Please see our front page for information on Supertyphoon Mawar and TS Guchol in the West Pacific.

August 22 2005 – 01:00 UTC

The tropical wave that exited the coast of Africa yesterday has not changed much in organization over the last 24 hours. Upper level winds aloft are conducive, but dry air has wrapped around the mid level circulation, which is north of the thunderstorm activity. Development is not expected in the short term due to the dry air, but slow development beyond a couple days cannot be ruled out. All global models continue to take the wave well north of the Lesser Antilles in response to weakening of the subtropical ridge.

A tropical wave and the partial remant of tropical depression ten near Hispaniola is interacting with an upper low. No development will occur within the next few days. However, the global models are lowering surface pressures near the Florida Straights in 4-6 days. The upper low shearing the wave is forecast to liftout, and an upper level ridge near Georgia is forecast to expand over the entire Gulf of Mexico and Florida. Upper level winds and sea surface temperatures may be favorable enough for slow development by that period. Beyond Day 5, It is uncertain where the surface low would travel if one were to develop.

The enhancing phase of the Madden Julian Oscillation is spreading over the Atlantic Basin, and the operational GFS continues to spin-up several tropical cyclones over the next 2 weeks. The GFS should not be taken literally beyond 5-7 days out, but trends can be an indication.

Typhoon Mawar has rapidly deepened into a dangerous 125 knot typhoon. Mawar is forecast to strengthen even further into SUPER typhoon status (130 knots +) with winds sustained winds in excess of 135 knots over the next 48 hours. Thereafter, Mawar will be approaching a frontal boundary and upper level winds will become less favorable. Additional weakening is also expected as a result of land interaction prior to the inner core making landfall in southern Japan. However, it is increasingly likely that Mawar will be a formidable typhoon by landfall. All interests between Kure and Tokyo, Japan, need to be preparing for the possibilty of severe impact damage. Yes, a couple models do take Mawar into Tokyo, but the overwhelming consensus still points a tad west of the city. Tropical Storm 12W is no threat.

August 21 2005 – 02:00

Typhoon Mawar has been upgraded by the Joint Typhoon Warning Center. One minute sustained winds are an estimated 80 knots, and the pressure has dropped to 963HPA. Mawar is showing signs of rapid intensification. Outflow is expanding in all quadrants, and an eye has become easily visible on all satellite imagery. In fact, dvorak satellite estimates from both the Satellite Analysis Branch and UW – CIMSS are 5.0. A 5.0 estimate supports 90 knot winds. Conditions appear condusive for additional development over the next couple days. Mawar could easily peak with 100-110 knot winds, and the few intensity models that predicted RI show the typhoon peaking in this territory. Once Mawar begins to approach Japan, some weakening prior to landfall will occur. A mid to upper level trough is oriented southwest to northeast across Japan, and the southwest winds asssociated with that trough will begin to shear the typhoon. However, in light of the recent round of rapid intensification, it is very possible that Mawar will still be a classified typhoon at landfall. It is too early to forecast exactly where Mawar will come inland and how strong, but all interests in Japan should be closely monitoring this storm. Initial model guidance are all in agreement with a track similar to the one posted by the Joint Typhoon Warning Center.

Tropical Depression 12W is not expected to become any stronger than a minimal tropical storm due to its close proximity to Typhoon Mawar. The anticyclone ventilating Mawar is strengthening the upper wind flow east of the typhoon, and 12W is being sheared apart. The low level circulation appears to be partially exposed northwest of the bulk of convection. Additionally, 12W will recurve east of Japan as a result of the same trough forecast to steer Mawar north and eventually northeast.

Neither the tropical disturbance in the western Caribbean nor the remnant of Tropical Depression Ten have become any better organized over the last 24 hours. The remnant of TD10 will still be monitored as it continues westward toward southern Florida and the Bahamas. The main area to watch in the Atlantic Basin this upcoming week may be the tropical wave that has exited the coast of Africa. This is one of the more impressive waves to exit the coast this year. If the wave can hold together over the Atlantic the next 24 hours, then it will be a candidate for development. Fortunately, only the GFS model is developing the wave, and all of the global models agree on a track well north of the Lesser Antilles. Focus on this wave will ramp up tomorrow if it holds together, which is quite possible.

In the eastern Pacific, Tropical Storm Hilary will likely be upgraded to hurricane status within the next 6-12 hours. The good news is that Hilary is no threat to land.

August 20 2005 – 00:00 UTC

The remnant of tropical depression ten is no longer showing signs of regeneration. Development is not expected, but it will continue to be monitored as it moves towards the Bahamas and Florida. The disturbance that appeared to have some chance of development in the Gulf of Mexico in the medium range is located near Honduras. The global models are no longer hinting at development, and conditions do not appear all that favorable at this time.

Tropical Depression 08-E should have no problem becoming a hurricane within the next few days. Upper level winds and sea surface temperatures are very condusive. Beyond 72 hours, the cyclone will begin to weaken as it moves into cooler sea surface temperatures. It is highly unlikely that TD8 will threaten land, but interests in the Baja should monitor its progress over the next few days.

Tropical Storm Mawar (08W) is forecast to become a typhoon as it passes just west of Iwo Jima. While the NOGAPS and GFS are struggling to initialize the tropical cyclone, the models clearly show an 850HPA vorticy tracking similar to the official forecast through day five. It is likely that Mawar will pose a signficant threat to Japan in 5-6 days.

A Tropical Cyclone Formation Alert has been issued for the tropical disturbance located east-northeast of Tropical Storm Mawar. This disturbance will likely develop into a classified tropical cyclone, but the threat to land appears to be low at this time.

August 16 2005 – 00:00

No changes from the discussion posted two days ago….

Official advisories on Tropical Depression Ten are not being written at this time, but they will likely be reinstated within the next couple days if the remants continue to organize. The orignal surface circulation has remained intact, and the low level cloud field is organizaing. Thunderstorm activity near the center has been gradually increasing over the last 48 hours. A mid to upper level trough over the central Atlantic and Caribbean Sea is preventing an upper level ridge from developing directly over the tropical low, but upper level winds are forecast to become slightly more favorable for intensification over the next few days. The remnant of TD10 should redevelop into a depression and possibly strengthen further into “Tropical Storm Jose” by the end of the period.

The forecast path of redeveloping TD10 is straightforward through Day 5. A deep layer subtropical ridge extends from the central Atlantic westward into Texas. No mid to upper level troughs are expected to weaken the ridge within the next five days. TD10 should remain on its west-northwest track through the period, and it should remain north of the Lesser Antilles. The forecast becomes tricky beyond Day 5. Some models are showing Td10 being caught into a weakness just off the East Coast while others show a continued westerly track towards the Florida peninsula. It is too early to jump to any conclusions, but interests in the central and northern Bahamas and the East Coast USA from Florida northward to the Carolinas should monitor the progress of this system.

Elsewhere in the Atlantic Basin, the potential for tropical development in the Bay of Campeche or southern Gulf of Mexico by Days 4-7 is increasing. The global models, especially NOGAPS, have been consistently indicating some sort of development in the Bay of Campeche for the last several days. The disturbance under investigation will originate from Central America and the extreme southwest Caribbean Sea. The convection currently over the Yucatan Peninsula is associated with a tropical wave near 85ºW, but this wave is only partly responsible for the possibility of TC formation in the medium range. A second wave of low level energy near Panama and Costa Rica will slowly move northwest and reach the Bay of Campeche by Day 4. Upper level winds are forecast to be favorable for tropical cyclogenesis during that period. Steering Layer Analysis and initial model guidance suggests that if a tropical cyclone were to form, it would likely threaten the Mexican coast near Tampico. Interests in Texas and Louisiana should still keep an eye on the Gulf as this TC is not expected to form until another 96 hours at least.

August 14 2005 – 02:00 UTC

Tropical Storm Irene has not strengthened today. While convection has deepened, the outflow and symmetry are poor. Irene only has 2 days left to strengthen, and the envionment appears marginal at best. That said, it would not take much to become a minimal hurricane, especially once Irene passes over a warmer eddy associated with the Gulf Stream. Irene is slowly moving northward and a bend towards the northeast is likely over the next few days as it gets intertwined with the mid-latitude westerly flow, far enough to avoid direct impact on any landmasses. Extratropical transition should commence after Day 4.

The main focus is now Tropical Depression Ten, located near 15ºN/45ºW. A ball of moderate convection is noted to the northeast of the center, but there is hardly no outflow, and the misalignment between the center and its convection is due to shear. The shearing itself is owing to the presence of a deep upper-level trough, which is also inducing dry air into the depression’s vicinity. This unfavorable setup will likely prevent significant strengthening over the next few days. After Day 3 or so, this feature will become less of an issue and if a low-level circulation still exists by then, the environment will shift and allow more intensification. In fact, global models are already hinting in a broad upper-level ridge to the northeast of the Leeward Islands and central Atlantic, which would help ventilatation of the system. Therefore, it is entirely possible that this will become a hurricane in 6-7 days.

TD10 is currently moving northwestward as it feels some influence from the aforementioned upper-level trough. However, the 700-850MB steering layer (used to dictate shallow storm movement) indicates a consistent and general easterly flow across the central Atlantic. As a result, the pull of the trough should not completely carry it north; rather only impart a modest northerly component to the track as we’re seeing right now. Global model guidance agrees that a general northwestward motion will continue over the next 3 days or so, taking it higher in latitude than the northernmost Leeward Islands. Thereafter, the GFS and ECMWF both indicate the subtropical ridge will build, gradually forcing the system on a more west-northwestward or even westward course. The CMC, NOGAPS, and UKMET likewise show a reinforced ridge, though they do not pick up on TD10 at all. The timeframe where the system should bend further west corresponds to the approximate period when it will have a greater opportunity to intensify. Again, assuming the system will still be alive when this happens, there is ample reason tonight to believe a hurricane may be heading for the southeast US coast early next week. This is not set in stone, just an option we feel is the most likely to actually occur. If such is the case, it is too soon to speculate on where it would hit, if it actually hit at all. Regardless, this could be an interesting storm to monitor in the coming days.

Another lesser area of interest is a tropical wave in the central Caribbean Sea. Although there is some deep convection, it is being divergently-enhanced by an upper-level low to the west. Due to this shearing environment, no development is expected in the near-term. However, as this wave moves west, conditions may become more favorable in the medium-term. Global model guidance suggests very low shear and possibly even strong upper-level ridging across the Gulf of Mexico and western Caribbean Sea, the likely destination for this system. NOGAPS has been hinting in weak tropical cyclone development the past few runs, whereas other global models are only picking up on low-level energy, if anything at all. Given the way the conditions are progged and the wave’s current status, there is a slight possibility of formation several days from now when it is further west. It should be noted that several global models also missed the genesis of Bret, Cindy, or Gert.

In the East Pacific, Hurricane Fernanda is slowly weakening at sea, and Tropical Storm Greg is slowly strengthening…also at sea. In the West Pacific, Typhoon Sanvu made landfall along the China coast and is now inland weakening.

August 13 2005 – 01:00 UTC

Irene is almost a hurricane. Aircraft recon finally investigated the system today and found maximum flight level winds of 64KT and a minimum pressure of 997MB. Another mission will take place overnight. Satellite imagery depicts a ragged area of moderate convection just to the east of the center and prominent outflow to the north and west. Upper conditions are favorable for further intensification; dry air is a non-issue now that the upper-level low near the Bahamas is sliding southward and weakening. There are no troughs or upper-level lows near by inducing significant shear either. SHIPS continues to peak the storm at a minimal hurricane followed by slow weakening as it moves through cooler water. Global model guidance curiously does not show much in the way of intensification either in spite of the seemingly favorable environment. The most logical explanation for this is most models take it northward in the next 48 hours, over cooler waters partially upwelled from Franklin and Harvey. If this happens, Irene will have a more difficult time sustaining cold cloud tops due to no appreciable source of latent heat. Weighing all the guidance and synoptic conditions, Irene should become a hurricane sometime tomorrow, and no more than a weak category 2 hurricane within 72 hours due to the cooler water temperatures.

It now appears Irene will indeed stay offshore the US east coast. The forward motion has been northwest over the course of the day, but the past few hours indicate that it has significantly slowed down. This is likely owing to the fact that the storm is caught in a col between several very crucial steering features. The aforementioned upper-level low to the southwest is trying to tug it westward, as is a ridge rebuilding from the west. This ridge is very evident on water vapor imagery to the north of Irene, with all low pressure-involved weather remaining along its northern periphery over Canada and the northeast. On the other hand, a longwave trough, elognated west-to-east, represents the break in the ridge and the consequential poleward flow very near Irene. Given this opposing steering flow, part to the west and part to the north, a continued slow northwest motion is anticipated over the next day or so. A stall is even possible. In 36 hours or so, a key feature will be a second upper-level low, currently over the Mississippi Valley. This is progged to scoot eastward over the southeast US and thus induce a stronger southerly flow upon Irene. This will also erode the ridge to the north, allowing the strong trough to dig further south to the point where it should be able to lift the storm poleward.

There is still some uncertainty in how rapid these changes will take place. All the dynamical and global have converged on a track offshore the US east coast, even the ECMWF, which was earlier showing a North Carolina brush. However, the dynamical models have been extremely inconsistent from run to run, not surprising since most are not designed to handle tropical cyclones in baroclinic environments such as Irene. The majority of the global models have also been slightly underestimating the strength and westward extent of the subtropical ridge. Today’s geopotential height observations versus the heights initialized by the models indicates mean amplitude differences of 10DM at least. It should also be noted that the 00Z global models do not come out in a few hours, and these will be the first incorperating today’s aircraft GIV data. Given the evolving synoptic setup towards a more southerly stream, a recurve offshore the east coast is forecast. However, the slight model errors regarding the high’s initialization suggest it may track a little further west than the TPC track and model consensus, perhaps to 72ºW, before recurving to the northeast. High swells and dangerous rip currents will plague the east coast, but otherwise Irene is not expected to be much issue.

INVEST 96L has developed today from a low pressure in the deep tropical Atlantic. Satellite imagery suggests this feature is gradually becoming better organized, with enhanced outflow particularly to the west. QuikSCAT indicated a closed circulation and strong barbs. Convection is still scattered and not very intense, and dvorak estimates only at 1.0, both likely the reasons for no upgrade as of yet. Environmental conditions are favorable for further strengthening. Saharan Air Layer is in a weakened state, upper-level shear is light, and SHIPS takes very near hurricane strength in 120 hours. One thing that stands out is none of the global models show much strengthening in this system, and given the weak convection it is very feasible to assume it will be a slower process than indicated above. Nonetheless, a tropical depression may form tomorrow, and a tropical storm thereafter. The mid to low-level steering flow is easterly over the area, and thus a general west-northwest should continue. Early tropical models take it northwestward, though this will likely be adjusted as they tend to have early right biases. The BAMM, perhaps the most reliable for this kind of shallow storm in the tropics, shows a bend to the west in the 120 hours. It is too early to speculate on where this will track in the longer run, though early indications are that it may get picked up by the same deep trough set to pick up Irene in a few days.

Elsewhere, Hurricane Fernanda and Tropical Storm Greg remain out at sea in the East Pacific. In the West Pacific, Sanvu (10W) is continuing to move northwest towards China.

August 12 2005 – 08:00 UTC

Special Update

The mid level circulation to the north is strengthening while the surface circulation that has been tracked all along is dissipating. What does all of this mean? Irene has relocated to the north and is now susceptible to moving through the initial weakness in the subtropical ridge. All of the model guidance and forecasts that targeted the Carolinas were based on the surface circulation to the south remaining intact, and bypassing the first trough. The probablity of a landfall along the East Coast USA has diminished significantly. Based on the recent development, the National Hurricane Center may even shift more towards the east at 5AM EDT or later this morning. With that being said, there is still an outside chance that the ridge could extend west earlier than anticipated, and interests along the coast should still closely monitor Irene’s progress.

August 12 2005 – 2005 – 03:00 UTC

Tropical Storm Irene has behaved as forecast over the last 24 hours. Our forecast has been in relative agreement with the TPC track through 72-96 hours over the last day or so, and Irene is following the expected short term west-northwest track. While sustained winds have officially increased to 50mph, Irene does not look much more impressive than it did 12-18 hours ago. The southern semicircle of the surface circulation is completely void of convection, and there is only moderate convection along the northern edge. Without SSMI and channel 2 shortwave infrared imagery, it is very difficult to even locate the surface center on standard infrared imagery. If one were to simply look at enhanced infrared animations, then the northern half of Irene would look suspect. However, the circulation spotted to the northwest is nothing more than a mid level circulation, and SSMI imagery confirms that the surface circulation is still following the 5PM EDT TPC forecast track. The primary inhibitor of development is an upper low located just to the east-northeast of the central Bahamas. As mentioned in the afternoon TPC discussion, this upper low is responsible for funneling dry mid to upper level air into the cyclone. Irene is also being affected by a weak southerly shear axis generated by the upper low. Intensification will be limited in the short term. The mid to upper level feature will dissipate over the next couple days while Irene slides north and west of the southerly shear axis. Upper level winds will become slightly more favorable and dry air should not be as much of a nuisance as it is now. The 00Z SHIPS intensity model forecast is indicating that upper level winds will remain fairly low after 24 hours, and Irene will likely peak as a Category 1 or 2 Hurricane in a few days. Beyond Day 4, the model is indicating that upper winds will shift and emanate from the East Coast. However, quasi-geostrophic flow will likely limit immediate and significant weakening as Irene will be moving more in a northerly and eventual easterly fashion.

Several tropical and global model forecasts are also suspect. Last night, we discussed how the tropical models become less reliable once north of 20ºN latitude. Earlier today, the tropical models shifted further east, and away from North Carolina and the forecast. However, this evening’s 18Z and 00Z tropical runs would appear even less reliable. BAMM, GFDL, and LBAR are indicating a bend to the west by 4-5 days and a drastically slower forward motion. Meanwhile, BAMD and NHCA98E initially indicate recurvature, but then they bend every so slightly to the right with hardly any movement by 96-120 hours. The tropical models cannot handle a complex pattern at this latitude. This may sound redundant but it must be said. The tropical models are based off of initialized GFS data, and the GFS is not initializing anything more than a weak tropical wave. A weak tropical wave would be steered by the low level steering layer, and in that steering layer the subtropical ridge has not re-intensified behind first trough or weakness that exited the East Coast. Therefore, BAMD and NHC98E are taking GFS-initialized “tropical wave” Irene through this weakness. However, the ridge that was over Missouri yesterday evening is now over the Tennessee Valley and is continuing to move east. By the end of the period, this ridge of high pressure will reinforce the central Atlantic subtropical ridge, and the two models become lost once this reinforcement is in position. That explains the BAMD and NHCA98E tracks, and they can easily be thrown out since Irene is not likely to pass through the first weakness to begin with since it is stronger than the models initialized it as. Also note that BAMM, GFDL and LBAR shifted west because the models show Irene missing the first trough followed by being caught under the reinforcing ridge. Furthermore, if any tropical model was worth looking at, it would be the GFDL and it is one of the westerly members. The GFDI, which was one of the best dynamical perfoming model last season, is also indicating a bend back towards the west.

There has been no significant change in the ECMWF and NOGAPS tracks over the last 24 hours. Both models have shifted ever so slightly offshore near the Outer Banks, but that is still very close considering those positions are 4-5 days out. It is interesting to note that when the NOGAPS model is indicating less development, the more it shows an easterly track and vice versa. This goes back to the weakness in the low level steering layer discussion. It is conceivable that if Irene intensifies more than what NOGAPS is indicating as of 18Z, then it may take a slightly more westerly path. The 12Z ECMWF forecast is difficult to explain given that it barely shows such a weak 850MB low near the Outer Banks, but it may be a result of the ECMWF’s superior handling of the synoptic pattern. The 12Z UKMET and CMC appear to be suffering from the same problem as the BAMD and NHCA98E models. In fact, the UKMET is suffering from yet another problem. The initial weakness in the ridge is not as significant as the model is indicating. The 12Z RAOB sounding from Bermuda indicated a 5950DM ridge while the 12Z UKMET initialized the ridge at 5940DM. Both models are being discounted for this evening’s forecast.

With all this in mind, Irene may bend slightly to the left, but still remain on a general west-northwest heading underneath the reinforcing ridge that is now exiting the Tennesse Valley. A secondary weakness will likely erode the western periphery of the western Atlantic ridge in 4-5 days, and a northerly bend into the Outer Banks may occur. Irene would not recurve out to sea east of NC like some models are indicating if this storm were to gain as much longitude underneath the ridge as we are expecting. A direct hit or brush on coastal North Carolina as a minimal or moderate hurricane is still forecast; we are barely west of the TPC track at 120 hours. It should also be noted that the remainder of North Carolina and central/northern South Carolina is not out of the woods. The Mid-Atlantic States seem to be in better shape this evening as Irene will likely recurve quickly once being influenced by the secondary trough, but interests in this area should still monitor Irene’s progress.

There are no other areas of interest in the Atlantic Basin. It is interesting to note that the enhancing phase of the Madden Julian Oscillation should arrive in the Atlantic sometime within the next 5-7 days, and activity along the Intertropical Convergence Zone may begin to increase.

August 11 2005 – 03:00

Forecast highly uncertain…

Tropical Storm Irene is still struggling with dry air and the Saharan Air Layer. Nevertheless, Irene has shown slow signs of organization under increasingly favorable winds aloft, convection is consolidating near the surface circulation and outflow is improving in all quadrants. As Irene continues to gain longitude, Irene should pull away from the SAL and begin to deepen at a faster rate. Upper level winds are now highly conducive for development, and sea surface temperatures are more favorable for intensification in the western Atlantic. The reliable SHIPS intensity model ramps up the cyclone into a minimal hurricane by Day 5, and that forecast appears reasonable.

The track forecast depends highly on the rate of intensification over the next few days. Currently, Irene is very weak and is being steered by a shallow steering layer. Irene is approaching a weakness in the subtropical ridge at the low levels, and a slightly more poleward motion has been apparent over the last 12 hours. If Irene remained a tropical depression or minimal tropical storm over the next several days, then it would be pulled northward by an approaching trough near the East Coast, and it would be shunted east of the Carolinas. However, if Irene were to strengthen into a strong tropical storm or minimal hurricane, then it would remain on a west or west-northwest track for a longer duration because the subtropical ridge is stronger in the mid levels. This reasoning can be incorporated when analyzing the 12Z and 18Z models. The GFS model is still failing to initialize Irene as anything more than a weak tropical wave, and it even dissipates the wave within a couple days. The tropical model suite, and the 18Z GFDL model is being initialized with data that is initializing the GFS. Remember, a weaker Irene would result in a more northerly track due to the weakness present in the low levels of the atmosphere. The tropical models have shifted north for two reasons; poor GFS intensity initialization and they have little skill forecasting tropical cyclones north of 20º latitude as it is. The tropical models cannot handle baroclinic (nontropical such as mid to upper troughs) steering mechanisms, and they can only perform well when a tropical cyclone is clearly south of an intense subtropical ridge. Therefore, the GFS, GFDL, and remaining tropical models are being thrown out. The CMC and UKMET are also being thrown out due to strong rightward biases in the short term and they do not indicate further development. This leaves us with the 12Z ECMWF, the 18Z NOGAPS, and 00Z NOGAPS run which is now available through 72 hours. First, the ECMWF has the best model accuracy for the North American sector, and the NOGAPS has been the best performing tropical cyclone model in the Atlantic Basin this season. It appears as if the NOGAPS model has had the best handle on the situation thus far. The model has been more westerly than any other model the last few days, and it has also been the most accurate. The NOGAPS is showing intensification, which is what we expect. The 00Z NGP is also indicating the recent west-northwest motion being seen now. One more note about the steering scenarios before we go into further detail regarding NOGAPS. The weakness is also present in the mid levels, and the upper trough approaching the western Atlantic is currently strong enough to prevent a direct hit on the East Coast. However, a mid level ridge near Missouri is progged to replace the trough by the medium range, and the NOGAPS is indicating that Irene will be caught under this soon to be westward extention of the subtropical ridge before curving north. The NGP clearly shows a westward bend to Irene’s track by 72 hours. The 12Z ECMWF run keeps a low center just east of the Outer Banks of North Carolina, but it is not depicting intensification either.

If Irene was to directly hit or brush the East Coast, it would likely do so as a minimal or moderate hurricane. A major hurricane cannot entirely be ruled out until Irene is observed once clear from the suppressing Saharan Air Layer. After detailed analysis of the intensity forecast being corporated with with the steering layer and model guidance, a track towards the Carolinas, primarily the Outer Banks, seems very reasonable at this time. A direct hit or brush on coastal North Carolina as a Category 1 or 2 hurricane is forecast. However, this could easily change over the next 24-48 hours due to the lack of a clear model consensus and the somewhat uncertain, yet highly important intensity forecast.

August 10 2005 – 08:00 UTC

A few notes regarding the new 06Z tropical model suite tracks…

06Z tropical model guidance graphic

1.) Irene was once again initialized moving due west. The depression is obviously caught underneath the subtropical ridge and has missed the first weakness.

2.) The track map proves how flawed the CMC, UKMET, GFS, and GFDL tracks are. All of the aforementioned global models are performing horribly in the short term, and it is resulting in disastrous northerly model biases in the medium range.

3.) The remaining tropical models are now aimed at the Carolinas, and their short term tracks are MUCH more believable.

4.) This is looking more and more like an East Coast hit….mainly looking at the Carolinas right now.

August 10 2005 – 00:00 UTC

Irene has become better organized today. Outflow and banding has finally appeared, particularly to the north and east of the center. The center itself is still hard to locate, though it is estimated to be under or just west of a mid-level cyclonic rotation very evident on infrared satellite imagery. The environment ahead of Irene continues to become more conducive. Shearing has markedly decreased, and upper-level ridging is noted to the west of the system, which will soon help ventilate it further. A possible short-term hinderance is a remnant trough just ahead, which is inducing some mid-level shear nearby. This will lift out with time and should not be a huge problem, though it will at least prevent any significant intensification in the next 36 hours at least. The air is still somewhat stable, though this is becoming less of an issue. Irene is expected to regain tropical storm strength tomorrow, with slow strengthening shortly thereafter. By Thursday, a faster intensification rate may commence and Irene should approach hurricane status by the end of the week. How strong exactly it gets is still uncertain, and while it would be extreme for us to call for a major hurricane…that possibility has not yet been entirely dismissed.

We are continuing to become more confident on Irene’s future track. The storm is currently being steered westward by low-level steering flow associated with a ridge of high pressure. There are no weaknesses to curve Irene poleward anytime in the immediate future, though a trough will be passing on the north side of the ridge. Models differ on how much this feature will erode the ridge. The GFS and CMC both erode it the most and curve Irene northward very quickly. In fact, both keep it well east of Bermuda and lose it completely by 72 hours. However, this solution seems very dubious. For one, the GFS shows a northwest motion in 24 hours, which is obviously not happening given recent motion and the steering flow. Also, both keep it very weak, which in this unusual case would make it more susceptible to connect to weaknesses. Finally, both models have tended to have right biases for storms this year. Therefore, they are discarded for now.

This leaves us with the UKMET, ECMWF, NOGAPS, and to a lesser extent, the tropical models. The 12Z UKMET recurves Irene near 65ºW and takes it northward near Bermuda. The initialization was actually fairly accurate, so this model does warrant some consideration. That said, the UKMET has been too far right with this storm for a while, so it is possible the right bias may still be at play. This solution does not match with the evolution of the steering layers either. The 12Z ECMWF on the other hand takes a weak system towards the South Carolina/North Carolina border on Day 7. The ECMWF has been indicating a Carolinas landfall or threat for the past 3 runs now, and considering its accuracy for medium-range, this is worth notice. The NOGAPS, another model that has perfomed well with tropical system tracks this season, continues to shift further west with every 6-hour run. The latest 18Z run takes Irene very close to the Carolina coast on Day 5, followed by a turn to the north as it feels an upcoming trough. The NOGAPS has too been indicating a Carolinas threat. Finally, the latest GFDL for a change does not dissipate the system, and has it making a left hook further north late the forecast period. The medium-layer BAM is very far south, and shows a hook to the west-southwest and what would be an extrapolated Florida threat.

Based on the models, it appears a track out to sea is becoming less likely for Irene. All interest along the southeast coast should closely monitor its progress over the next few days.

August 9 2005 – 02:00 UTC

Irene is truely a fighter. Over the past 3-4 days, the storm has endured cool water temperatures, dry Saharan air, and formidable westerly shear from an upper-level low to the north. In spite of all of these obstacles, the storm has managed to generally maintain a constant intensity, and it is somewhat surprising that it has not succombed and dissipated. However, the storm has been and still is poorly organized. The pattern over the past several days has been for convection to fire to the east of the center, get blown away by shear, and then redevelop again due to the marginal conditions. That said, there is evidence that Irene will slowly be moving into a more favorable environment. The upper-level low that has been shearing it is slowly sliding to the north, not to mention Irene itself is moving west; thus the distance between the two features is increasing. The influence of Saharan air lessens the further west it gets, and sea surface temperatures are warmer as well. An upper-level ridge centered in the central Caribbean Sea is progged to expand and slide north, possibly allowing more ventilation in Irene. A second upper-level low northeast of the Bahamas should scoot northward, though there stands a chance it could have a slight negative impact if the global models are too fast in pushing it out as they often are with upper features. Nonetheless, all of the global models except the GFS/GFDL indicate some strengthening in the next few days. In fact, 18Z SHIPS takes the storm to near hurricane strength by Day 6. Given the guidance and the apparent improving conditions ahead, slow reintensification is expected in 24-36 hours, followed by a quicker rate. Irene should be at least a moderate tropical storm by the end of the week, and there is a chance it will be a hurricane.

The future track of Irene is also a bit complicated. The system is currently being steered just north of due west by low-level steering flow associated with a surface ridge. A weakness in the ridge associated with extratropical storm Harvey is now behind Irene’s longitude and thus will not have an influence. Now the question is, will it lift poleward in response to the next trough set to exit the continental US around midweek, or miss it and stay along the periphery of the subtropical ridge? Global and dymaical models are rather diverged on the track of Irene. The CMC, UKMET, NOGAPS, and the past several runs of the GFS all curve Irene near Bermuda’s vicinity in response to the digging trough. However, there are a few issues with these solutions. The 18Z GFS actually develops a second vortmax to the southeast of Irene’s current center in the next 24 hours and merges the two shortly thereafter. This slows the storm down and as a consequence, the model has it being picked up by the trough sooner. Furthermore, it loses the system completely by Day 4, which seems dubious as well based on what was outlined above. The UKMET is a problem because it initializes it incorrectly and shows too much northerly motion in the short-term. The NOGAPS does not appear to have any issues, and although it recurves it similarly, it has been trending further left with each 6-hour run today. The BAM models, on the other hand, take Irene westward and apparently have it missing total connection with the upcoming trough. The 12Z ECMWF is the most interesting, as it shows a landfall along the South Carolina/North Carolina border on Day 7. No other global model yet shows this solution, though it is not one to be dismissed. The ECMWF generally performs well in the medium-range when it initializes storms correctly, and given vorticity maps, it has initialized Irene quite well. Second, the aforementioned models are slowly as a whole trending stronger with the ridge of high pressure and westward with the storm. Finally, Irene has already been tracking further south of earlier model runs today, largely due to being a shallow system. Tomorrow’s model guidance will be very crucial in deciding which scenario pans out: either Irene will catch the trough and recurve prior to 70ºW, or miss it and be guided very close to the southeast US coast in the next 7 days.

The remainder of the Atlantic is quiet. A disturbance in the East Pacific, INVEST 97E, is showing some signs of organization. The West Pacific has calmed down as well.

August 7 2005 – 22:00 UTC

Relocation makes it a done deal…

The surface circulation made a second relocation to the north during the overnight hours. Every gain in latitude makes the cyclone increasingly susceptible to getting picked up by the trough no matter how weak it may be. All of the global models, with the exception of the Canadian, and all of the tropical models are indicating that the mid to upper level trough near 55ºW will turn the cyclone more northerly over the next 72 hours. The system is now too far north to avoid recurvature. Any threat posed to Bermuda and the East Coast has diminished significantly over the last 12 hours, but interests in Bermuda should still monitor its progress.

The circulation also developed closer to the most intense convection, and QuikSCAT data indicated 40 knot tropical storm force winds this morning. The NHC decided to upgrade TD9 to Tropical Storm Irene at 11AM EDT. Since then, upper level westerly winds associated with the persistent upper low to the north has once again sheared Irene apart. The cyclone has likely weakened to tropical depression status but the NHC probably decided to hold onto tropical storm classification at 5PM out of continuity. If convection does not begin to flare near the surface circulation within the next 6-12 hours, Irene will likely be downgraded. Nonetheless, conditions may become slightly more favorable for slow development in a few days if Irene can survive the current round of shear.

The rest of the Atlantic Basin is quiet, and none of the global models suggest immediate development over the next few days. The same goes for the remainder of the northern hemisphere.

August 7 2005 – 02:00

Tropical Depression Nine remains disorganized this evening. The surface circulation is still displaced well to the west of the deepest convection. With that being said, there is actually convection to speak of compared to the lack thereof 24hours ago. CIMSS AMSU data is indicating a 1004.9MB surface pressure, which is down from 1006.5MB early this morning. QuikSCAT data is indicating 30 knot surface winds, and that is not quite enough to warrant an upgrade to tropical storm status. Additionally, Dvorak intensity estimates are only 25 knots. The cyclone is forecast to strengthen very slowly over the next 24 hours as an upper low nearby to the north is still generating strong westerly shear over the system. Beyond 24 hours, TD9 will be entering a more favorable environment with warmer sea surface temperatures and lessening westerly winds aloft. Tropical Storm Irene should easily arrive within the next 24-36 hours.

The depression continues to be one of the more difficult systems to forecast track-wise. At 5PM AST, TD9 was located near 19.0ºN/43.2ºW. The latest position estimate from the Satellite Analysis Branch as of 2345 UTC is 18.8ºN/43.8ºW. The difference in those two position estimates indicates that TD9 has at least temporarily taken a west-southwest jog. Shortwave infrared floater imagery confirms the wsw motion, and it is continuing. The recent change in direction has placed the center just south of the NHC forecast track, but any wobble towards the north could change that. As noted in previous discussions, TD9 is caught in a strong low level easterly steering flow since it has not intensified. In other words, Tropical Depression Nine is a very shallow cyclone, and it is not being guided by mid to upper level winds, which is where the most significant weakness in the central Atlantic can be found. A general westerly heading should continue for another 24-36 hours. If TD9 does make the northerly turn, it would likely do so near 20ºN and 55ºW-60ºW. Nearly all model guidance is suggesting a northerly bend in the track at that time. However, all model guidance has been too far north in the short term, and their medium range tracks should not be taken literally. Likewise, Tropical Storm Harvey, which is partially enhancing the weakness in the subtropical ridge, is lifting out, the low level flow is not weakening, and TD9 continues to head generally westward as a minimal tropical cyclone. These are significant factors that are indicating the mid to upper trough will not be deep enough to pull TD9 northward. We still expect TD9 to pass just north of the Lesser Antilles in 4-5 days as a moderate tropical storm. Thereafter, TD9 could continue to move west-northwest for a while, but it is too soon to determine if the subtropical ridge would support a track towards the East Coast. However, just to put things into perspective, it is quite conceivable that soon to be Irene could split the distance between Bermuda and the East Coast. Interests in Bermuda and along the East Coast should still be monitoring the progress of this cyclone.

The weak surface low near Mobile, Alabama, and the disturbed area of weather east of South Carolina have dissipated. There are no other areas of interest in the Atlantic Basin this evening.

There are no organized areas of disturbed weather in the eastern Pacific.

The western Pacific has become quiet since Typhoon Matsa made landfall in eastern China. The GFS, NOGAPS, and ECMWF models do not indicate immediate tropical storm formation within the next few days.

August 6 2005 – 02:00 UTC

Tropical Depression Nine has not behaved as forecast. The low level circulation became detached from the deepest convection during the overnight hours, hence significant weakening. Since then, convection has not refired near the center. The more northerly motion has resulted in a track over cooler sea surface temperatures and Saharan Air Layer entrainment. Significant intensification is not expected within the next day or so. The only chance TD9 has of becoming a named storm is if it takes a westerly route. A weakness in the subtropical ridge is located near 25ºN/55ºW. This weakness would have been a more significant steering mechanism had TD9 been a more intense cyclone moving at a faster rate of speed. The depression is weak and is caught in the low level easterly steering flow. Furthermore, the trough/weakness is being enhanced by Tropical Storm Harvey, which is quickly moving out of the picture. A continued west to west-northwest track is expected through the forecast period. A Caribbean threat is not anticipated, however. If the westerly track verifies, then the risk of SAL entrainment will have diminished, and water temperatures will be more favorable for intensification. Intensification or regeneration is forecast by the time it reaches 55ºW-60ºW as long as the low level circulation remains intact. A long range threat to the East Coast is still a possibility.

The low level circulation near Mobile, Alabama, is not forecast to develop into a tropical cyclone. While upper level winds and sea surface temperatures are favorable for TC formation, proximity to land will hinder organization. None of the global models depict a surface low moving southward into the northern Gulf of Mexico and the current low level steering layer is very weak. Little movement is expected over the next few days followed by a northerly track into the southeast US in the medium range. the remainder of the Gulf of Mexico is quiet.

The two areas under investigation in the eastern Pacific have dissipated. A new disturbance near 10ºN/95ºW has some potential to develop in 3-5 days.

Matsa (09W) has made landfall in China, well south of Shanghai, as a minimal typhoon. Dissipation will occur within the next 48 hours, but significant inland flooding is likely.

August 4 2005 – 18:00 UTC

Tropical Storm Harvey is moving to the east-northeast away from Bermuda. The satellite structure is well-organized, after looking very subtropical yesterday. Dvorak T numbers are now 3.5, or 55KT. Considering those estimates have been consistently underestimating intensity, Harvey’s actual strength may now be 60KT or even 65KT. Regardless, some slight intensification is possible over the next 24 hours before it reaches cooler water out at sea. If it’s not already a hurricane, Harvey has a shot of becoming one.

The main focus today is recently-upgraded Tropical Depression Nine, near 13ºN/32ºW. The convective pattern is not quite as concentrated as it was 6 hours ago. However, the low-level circulation was partially exposed to the north earlier, whereas it now surrounds it. The latest GOES Floater-2 frames even shows a burst of deep convection now firing directly over the center, signifying another round of organization may be about to commence. Furthermore, the system still has an extremely strong equatorial outflow channel on the right side. Outflow to the north and west is not as prominent. TD9 is located under a very large anticyclone aloft, which is being naturally enhanced by a strong upper-level low well to the west. This feature is expected to follow the system throughout the forecast period, and thus I see no problems with shear ahead. Given the large and well-organized structure of the storm at such a low intensity, favorable upper environment, and very warm water temperatures, strengthening is a near certainty. Hurricane status is likely in the next few days, and there is a good chance it will be a major hurricane sometime next week. The name given when it achieves tropical storm strength will be Irene.

The future track of TD9 is rather interesting. Global models were earlier sold on a recurve and pickup by the same central Atlantic trough that is pulling Harvey away. However, this consensus has shifted. The 12Z GFS keeps the storm on a general west-northwest track over the next 6 days or so but then turns it westward under a rebuilding ridge of high pressure, a pattern that would result in a southeast US threat. The GFS has been showing this more consistently in its runs, somewhat surprising for a storm in this location since it usually shows early recurves. The 12Z CMC also has the storm missing the central Atlantic trough and has it being pushed west or west-northwestward on Day 6. Again, earlier runs of the CMC were far-out recurvatures. The NOGAPS has barely even picked up on the system at all, and the 12Z run keeps it as a marginal TD or even weaker through the period. Thus, it is not a surprise it shows a more leftward motion towards the Leeward Islands, since it assumes the system will be weak enough to be steered by low-level easterly flow. The UKMET and ECMWF both show track more to the right, with the ECMWF being a sure recurve. However, the ECMWF has TD9 located at 20ºN/45ºW in 3 days or so; for the system to move there, it will have to turn northwestward right now. The model is either initializing it incorrectly or showing too much of a northerly component in the short-term. The ECMWF also baroclinically bombs Harvey, which seems suspect and may be a product of how it is incorrectly handling the trough. There is no doubt the UKMET is initializing it well to the north and east of its actual location, and so its storm solution is certainly off already.

Given these outlined issues, no model has a perfect handle on the storm itself over the next week. The GFS and CMC, in spite of their trending, are probably more accurate than the UKMET and ECMWF due to their short-term errors, and the NOGAPS for its doubtful intensity forecast. It is the overall pattern where the models are starting to agree. The central Atlantic trough and Harvey or whatever is left of Harvey will bypass to the north, though will probably induce enough of a weakness where a slight bend more to the northwest will occur. However, it now appears the trough will not be intense or far south enough to fully pick up the storm. The only model that for certain does this is the ECMWF, and it has many issues as aforementioned. The other models indicate the ridge will rebuild behind the bypassing trough off Canada; the ECMWF actually shows this as well, but it recurves the storm too quick to be influenced. There is still too great of uncertainty to really speculate on where the storm will go or how strong it will be after a week’s time. If current indications are correct though, there’s going to be a hurricane threatening the southeast US coast late next week.

August 2 2005 – 03:00 UTC

INVEST 92L has organized somewhat today. Moderate convection on the northern end of a high-amplitude wave has continued to blossom between the Bahamas and Bermuda. Satellite imagery depicts at least one, possibly two exposed surface circulations to the southwest of the main convection. A pronounced upper-level west of the system is the cause of this structure by inducing shear and cutting off outflow aloft. That said, the shear has lessened tremendously since yesterday. Overall, the environment remains a bit unfavorable, though it is slowly improving. An upper-level anticyclone over the eastern Caribbean Sea is progged to slide further west over the next 24 hours, which in turn will help nudge the upper low a little to the west as well. The retrogression of both features will not be sufficient enough to make the environment absolutely favorable, especially since the anticyclone will be positioned too far south relative to the INVEST. However, the already decreased shear should have less of an influence, and the system should thus slowly organize further.

The subtropical ridge located to the north of 92L has significantly eroded today due to the progression of a shortwave trough off the northeast states. The steering flow has therefore dramatically shifted more poleward, which highly discards a westward track towards the US. Furthermore, global model guidance has become more converged on the future track. The 12Z CMC and UKMET both shifted slightly to the west from prior runs, though still keep the system offshore. In CMC’s case, Bermuda would still be threatened. Today’s ECMWF and GFS showed no drastic change and likewise have a weak storm closer to Bermuda than the Carolinas. The 12Z NOGAPS is the furthest west and still shows a threat to the Carolinas, though the 00Z run coming in so far is a whole lot further east.

Based on the strong and converging model consensus and the evidence displayed in the recent CIMSS steering layer maps, a slow motion towards the north is expected to commence in the next 24 hours. This should be followed by a bend towards the north-northeast or northeast, taking it to the left of Bermuda and well away from the US. Given the marginal environmental conditions, tropical depression or tropical storm formation remains possible in the next few days. However, a significant tropical cyclone is becoming less likely given the slight adjustements in track and the fact that it will probably become intertwined in mid-latitude westerlies by Day 4. Interest in the Carolinas, even though the threat has diminished, and Bermuda should watch the storm until it has become completely void of concern.

Elsewhere, the tropical Atlantic is fairly quiet. The tropical wave that was once 93L in the eastern Caribbean Sea is showing no signs of organization and lacks much convection at all. Conditions are not conducive for any development over the next several days, coinciding with the unambiguous lack of model support. 94L, a non-tropical low in the northern part of the basin is also of little interest. This should continue to slowly move north into cooler waters without acquiring tropical characteristics. A wave moving off the coast of Africa displays sporadic convection and faint signs of an anticyclonic upper-level regime. Conditions are only marginally favorable for devlopment, though a few models do indicate a tropical cyclone. The UKMET is by far the most bullish, and has been for several consistent runs, with the CMC, ECMWF, and GFS showing a much weaker system. This wave will be monitored for formation over the next few days as it begins its trek westward.

In the East Pacific, a tropical wave and associated 1008MB low well south-southwest of Acapulco continues to become better organized. This system, 96-E INVEST, was earlier being sheared to the east, though this seems to have subsided. Further strengthening is likely and a tropical depression or tropical storm may form within the next 24 hours as it moves harmlessly at sea.

The West Pacific is currently governed by Tropical Storm Matsa (09W). This system is gradually intensifying and should become a typhoon tomorrow, possibly a major one afterwards. A northwestward motion will continue, taking it midway between Tawain and the island of Naha in a few days.

August 1 2005 – 01:00 UTC

The main area of interest today remains INVEST 92L.

92L, a tropical wave and associated mid-level circulation well north of Puerto Rico and Hispaniola is showing a few signs of organization today. Yesterday and Friday, the convective regime was extremely linear with little to no signs of outflow. This restriction was mainly attributed to a strong upper-level low that was located northwest of the wave, and as such an organized system could not take shape. The environmental setup aloft has begun to shift over the last 24 hours, and as such the system has a far improved appearance on satellite imagery than it did just yesterday. Over the past 24 hours, the upper-level low has weakened considerably and elongated in a more north-to-south orientation. This has allowed the wave to fan to the north and take on a more symmetrical, healthier structure. Furthermore, high-resolution GHCC visible satellite imagery suggests increased low-level inflow and possibly even a weak low center developing on the southern portion of the main convection.

For the next few days, global model guidance progs the upper-level low to continue to weaken and slide further west. As this happens, the environment around 92L should improve even further, and allow for additional organization. One problem is that no model seems to fully dissipate the feature anytime soon, but rather keep it near the Florida Straits and Bahamas. Thus, while the negative influence upon 92L will become less noticeable, it should persist to some extent through at least Day 3. Moreover, a smaller high aloft is progressing into the Caribbean Sea to the southeast. This feature may inhibit outflow to the south for a limited time period, however it is not expected to be significant.

92L is located under a surface ridge of high pressure, inducing low-level easterly steering flow in over the region. As such, the system should be steered westward over the next 24 hours. A mid-latitude trough will be moving off the northeast US coast soon afterwards and should thus erode the ridge somewhat. This will shift the flow and at some point in time the system will turn towards the northwest or north. How sharp and when it turns is the big issue. 18Z GFS is the easternmost model, closing off a weak low and taking it towards Bermuda in the forecast period. The 12Z CMC is a little further west but still has it well offshore the southeast US, not to mention it has a sharp easterly turn after 72 hours. The CMC is also the most agressive in terms of intensification. Today’s ECMWF and 12Z UKMET show a weak low slowly moving generally northwest then east-northeast, taking it fairly close to the Carolina coast. Finally, the 12Z NOGAPS is the furthest west of the reliable global models, showing a weak system brush the southeast US coastline from Florida to the Carolinas in the next 3-6 days.

Given how persistent 92L was under extremely harsh conditions and land interaction a few days ago (probably owing to its high amplitude) and how well its organization has improved due to upper-level low weakening, focus should be given on the system. The environment is only going to improve, and all models do develop at least a surface low. The SHIPS even takes it close to hurricane intensity in 96 hours, though that scheme does often over-strengthen systems not yet classified as tropical cyclones. Some strengthening is likely, and a tropical storm could develop this area over the next couple of days. All global models show recurvature to one variance or another. The GFS and CMC are notorious for recurving systems too quick and too far east, and this has shown so far this year. The NOGAPS so far this season, on the other hand, has demonstrated some of the best track performances. The ECMWF is rather reliable in the medium-range, and the UKMET has been the most consistent. Taking this into account, plus the current stength of the ridge to the north, we favor the more westward track taking it near or just offshore the southeast coast later in the week. Interests in the Carolinas are advised to closely monitor the progress of this system.

93L INVEST has not shown signs of organization over the last 24 hours. Some upper level easterly shear is present and dry air persists over most of the Caribbean Sea. Furthermore, none of the global models have been able to initialize the wave correctly, and none of them are indiciating tropical cyclogenesis. Tropical development, if any, will be very slow to occur as it moves in the general direction of the northwest Caribbean Sea.

The nontropical under investigation in the northeast Atlantic is not expected to affect any landmasses. Furthermore, it is quite possible that the low will never become subtropical or tropical.

In the eastern Pacific, a broad low near 125ºW has a slight chance to develop within the next 24 hours before being sheared apart. This low will not affect any landmasses. Meanwhile, a much larger low pressure system near 10ºN/110ºW has some potential to develop within 2-4 days. This system is not expected to affect land either.

In the western Pacific, Tropical Storm Matsa (09W) has been classified by the Joint Typhoon Warning Center. Matsay is forecast to be a significant 110 knot typhoon by Day 5. Early indications are that Matsa will be steered towards or just south of Shanghai, China.

Leave a Reply