This is the fourth Atlantic hurricane season forecast issued by the Independent Weather Information Center. Although we have had some successes in the past three years, our 2005 landfall forecasts did not verify well. The expectation of reduced activity in the Gulf of Mexico could not have been further off. As tempting as it was to accept the 2005 season as a rare anomaly and return to traditional forecast methodology in 2006, we could not do that. Upon an intense post-review, we now believe several of our hypothesized correlations were flawed due to misrepresentative statistics. It would be completely imprudent to use these same correlations again this year after showing no skill whatsoever in 2005.
With this in mind, we have abandoned many of the techniques used in years past. However, those that have read our seasonal forecasts over the years should not conclude that we are starting from scratch altogether. The methods that have consistently worked in the past are still being applied. Yet, we are introducing some new, promising methods based on data and research obtained from November through May. The primary parameters that are known to affect the frequency and steering of tropical cyclones in the Atlantic Basin have been closely analyzed throughout the offseason. A thorough explanation and forecast of these factors are outlined. In addition, up to 56 years of climatological evidence have been included to coincide with the anticipated 2006 summer pattern over the northern hemisphere. All of the aforementioned data has been utilized to not only forecast seasonal tropical cyclone activity, but also month-by-month activity, regional activity, and landfall probabilities.
There are a few verificational guidelines. First, any tropical cyclone development beyond November 30th will not be included within the verification of this seasonal forecast. Second, only forecast totals of tropical storm, hurricane, and major hurricane activity are provided in our regional and landfall sections. Tropical depressions that don’t strengthen further are not included in any of our forecasts. Third, a tropical depression that develops in a particular region and moves into another region before becoming a classified tropical storm counts as a tropical storm development in the region it officialy becomes a storm in. Fourth, any tropical cyclones that hit the western side of the Florida peninsula and make it into the Atlantic Ocean are not considered a hit to the eastern side and vice versa. The same goes for the Yucatan Peninsula. Additionally, any storm that hits the Gulf Coast and attempts to regenerate over the East Coast during recurvature does not count as an East Coast landfall.
Although no long-term forecast has ever been absolute, we certainly strive for accuracy each and every year. Also keep in mind that this forecast is experimental and unofficial, and therefore we are not liable for one’s actions based upon the information being presented. Please read our disclaimer.
II. El Nino Southern Oscillation
The El Nino Southern Oscillation, commonly referred to as ENSO, is an important parameter concerning Atlantic Basin tropical cyclogenesis. ENSO is most notably characterized by significant variations in sea surface temperature anomalies, or SSTAs, in the equatorial Pacific Ocean, though alterations in sea level pressure, trade winds, and convection are also observed. Warm SSTAs are associated with El Nino episodes, whereas La Nina episodes correspond with cool SSTAs. If SSTAs are neither warm nor cool, ENSO is considered neutral.
Once an El Nino event takes hold, strong upper level westerly winds dominate the low latitudes of the Atlantic Basin. As a result, hurricane development from tropical waves and disturbances along the intertropical convergence zone is typically suppressed. In the mid-latitudes, a higher frequency of upper level troughs and other dynamics that support the development of cutoff lows is typical. High latitude tropical and subtropical activity is thus increased during El Nino events. The reverse is true for La Nina. There is only a small difference between the number of named storms in an El Nino versus a La Nina, but far more hurricanes and intense hurricanes are observed in a La Nina or even neutral ENSO conditions than in an El Nino event.
Current Status of ENSO
During the last two months of 2005, the neutral ENSO began showing signs of transitioning to a La Nina. SSTAs across the equatorial Pacific Ocean cooled to values below -0.5 degrees Celsius, and the global pattern reflected this change during the Northern Hemisphere winter. As such, La Nina was officially declared in January. However, this La Nina remained generally weak and did not intensify much further after classification. In March of this year, the SSTAs began slowly moderating above the La Nina threshold. This trend has continued through May, with values currently leaning slightly above average in all ENSO regions except 1.2, just off Peru. Furthermore, there has been a significant change in the depth temperature anomalies, or DTAs, across the equatorial Pacific Ocean. The strong cool pool that extended down to 200 meters below the surface in tandem with the weak La Nina event has almost completely disappeared. A static, small warm pool has been present under the western equatorial Pacific Ocean for several months. Such a warm pool is fairly typical in a La Nina or neutral ENSO, and can expand and progress if a Kelvin Wave passes over the area. When this happens, it is common for an El Nino to initiate. However, there have been no observed Kelvin Waves yet this year, and thus it is no surprise that the pool has not shown signs of moving.
Another important aspect of the ENSO is the Southern Oscillation Index, or SOI. It is defined by sea level pressure differential between Tahiti and Darwin, Australia, with negative values corresponding to El Nino, and vice-versa. The SOI has remained largely positive over the past few months, coiniciding with the La Nina. While it is not as strongly positive as it was a few months ago, it is somewhat odd that the SOI has remained positive through this month, even though the ENSO is back to a neutral state. One responsible factor for the persistence is Cyclone Monica, which helped significantly lower pressures over Darwin during the last half of April. Since then, daily values have been hovering closer to normal, thus slowly drawing the more crucial 30-day and 90-day downward. With the cyclone season essentially over, and the overwhelming signs for a neutral or warming ENSO, it is expected the SOI will catch up to the equatorial Pacific SSTAs over the next month or sooner.
With the cyclone season essentially over, and the overwhelming signs for a neutral or warming ENSO, it is expected the SOI will catch up to the equatorial Pacific SSTAs over the next month or sooner.
While climatology cannot always be trusted alone, it can be a helpful tool in long-range predictions of the ENSO. For climatological research we use the Multivariate ENSO Index instead of just SSTAs, as it also accounts the SOI, equatorial convection, and other global conditions that help determine ENSO. The MEI this past winter, from December to March, hovered around -0.450, plus or minus about 0.100. Upon examining the MEI over the past 56 years, we found 14 years that best matched the MEI this winter: 1957, 1960, 1961, 1965, 1967, 1968, 1972, 1982, 1984, 1985, 1986, 1996, 1997, and 2001. Six of these years saw the development of an El Nino in the summer, with one other year producing a very late fall El Nino. True La Nina conditions did not persist through the summer in any of the years, though a La Nina did rebound in three years, with others maintaining a weak cool bias. Clearly, climatology does not offer much help this year, except for eliminating an already-unlikely prolonged La Nina. If one takes the spring MEI values into consideration, which have so far not decreased much since the winter, the two best years out of this sample are 1985 and 1996. Interestingly, both never ventured away from cool bias ENSO conditions for the rest of the year. However, the equatorial Pacific Ocean SSTAs in 1985 were much cooler during the spring than this year, and a little cooler in 1996. Considering the faintly positive SSTAs this month, there is reasonable concern that the MEI will fail to stay negative through the summer, unlike those two years.
ENSO Prediction Models
Coupled and statistical ENSO models have been in use for several years to try to accurately predict the evolution of ENSO up to eight months in advance. Each individual model has a bias to one extent or another, and at certain times of the year. Therefore, it is better to look at the model consensus as a whole, and more importantly, the model consensus trend. This spring, the average of all the models combined is a continued slow warming over the summer, leading towards a possible weak El Nino by the end of fall. Some, such as the POAMA, were recently forecasting a substantial El Nino to develop over the summer. However, the POAMA now forecasts neutral or slightly cool bias ENSO conditions during the hurricane season. Ironically, several other models have trended warmer over the past few months. The CLIPER, a relatively good statistical model, is currently forecasting a weak El Nino during the hurricane season. As can be seen in the chart of selected models below, the majority favor neutral conditions during the last part of 2006.
|ENSO Models||September 2006||December 2006|
|NOAA LINEAR INVERSE||Neutral||Neutral|
2006 Summer ENSO Forecast
The short-lived La Nina that ruled the past Northern Hemisphere winter is just about gone. As aforementioned, SSTAs have warmed in all ENSO regions of the equatorial Pacific Ocean over the past few months. The SOI has stayed positive with help from Cyclone Monica but is now slowly declining away from La Nina values. Additionally, the DTAs have a much more neutral appearance, with the large cool pool gone. MEI climatology for both winter and spring suggest cool-biased conditions, though with this year’s SSTAs a bit warmer, a notch higher in status appears likely. Finally, it is worth noting that the combined best five global predictors for forecasting ENSO a half year to year in advance, courtesy of Seseske 2004, suggest a neutral ENSO. Considering all of this, neutral to slightly warm conditions are expected over the summer. This is in line with most of the statistical and dynamical ENSO models. Later in the year, it is very possible that weak El Nino conditions will emerge. However, it generally takes a moderate El Nino to have a noticeable negative impact on the total amount of Atlantic tropical cyclone activity. In fact, some of the most active hurricane seasons in recorded history were during periods when ENSO was in a neutral state. 2005 is one of those seasons. The seasonal ENSO this year will overall neither help nor inhibit tropical cyclone formation. Therefore, the remaining factors known to influence Atlantic hurricane activity will carry more weight.
III. Atlantic Thermohaline Circulation
The Atlantic Thermohaline Circulation, or ATC, is a density-driven circulation in the Atlantic Basin that undergoes cycles on decadal timescales. When the ATC is in its warm or strong cycle, hurricane activity is increased. During the strong cycle of the ATC, the most notable consequence is warmer SSTAs across the tropical Atlantic Basin, which provides more energy for a tropical storm or hurricane to sustain deep convection. Additionally, lower vertical wind shear and sea level pressure anomalies, or SLPAs, are typically observed across the Atlantic Basin with a strong ATC. The exact opposite is true for the weak or cool phase of the ATC, which is inhibiting to tropical cyclone formation. Since 1995, the ATC has been in a strong cycle, which explains the significantly above average hurricane seasons that have occurred over the past 11 years. However, even during the ATC’s warm phase, it does occasionally fluctuate for several months. These small fluctuations are difficult to forecast in advance, but observing SSTAs in the Mean Development Region over a monthly period is an easy way to monitor such trends.
Since mid 2003, the SSTAs in the eastern Atlantic Basin have for the most part remained much warmer than average. Spring 2005 SSTA values in the Mean Development Region were record high, and although the SSTAs are not as high this year, they are nonetheless above average. Mean Development Region SSTAs during April averaged at 0.37 degrees Celsius above average and should be even higher this month. Given the mode of the ATC and warming trend observed in SSTAs, there is no apparent reason why a SSTA cooling would occur this summer. A continued warm ATC will act as a robust enhancing factor for tropical cyclone formation.
IV. Other Factors
The Quasi-Biennial Oscillation, QBO, is a periodic variation in the direction of stratospheric winds across the deep tropics. The two phases, easterly and westerly, generally last from 12 to 16 months, with the easterly phase often having a slightly longer duration. Unlike ENSO, the QBO’s alternation between phases is timely, making it quite simple to forecast. The easterly phase of the QBO peaked last November, with a value of -29.55. Afterwards, the QBO rapidly rose and just recently this year has transitioned to the westerly phase. The QBO will continue to rise and peak later this year, with the next transition probably not occurring until sometime in early 2007. Traditionally, this is judged to be an enhancing status on Atlantic major hurricane activity, though the observed relationship has somewhat faded over the past few years.
North Atlantic Oscillation
The North Atlantic Oscillation, or NAO, is the fluctuation in 500 millibar heights in the northern Atlantic, primarily between western Europe, Iceland, Greenland, and the Canadian Maritimes. Although it plays an important role of the steering pattern across the Atlantic Basin, one problem that has prevented reliable techniques in forecasting the NAO months in advance is that it is extremely variable on a monthly and even weekly basis. It would be foolish to try to forecast its evolution through the rest of the year without the proper understanding that most have yet to acquire. However, our 56-year climatology does indicate that spring NAO values may have a lag influence on the conditions that set up during the hurricane season. The significance of the negative NAO that was in place this past March will be discussed in detail further down.
Pacific Decadal Oscillation
The Pacific Decadal Oscillation, or the PDO, is a multi-decadal pattern of high and low pressure systems in the northern Pacific Ocean, sometimes seen as a longer-term version of ENSO. However, unlike ENSO, the PDO index is calculated by spatially averaged monthly SSTAs over the northern portions of the Pacific Ocean, not the equatorial. In terms of its variability, the PDO is most similar to the ATC, given that it too fluxes on a multi-decadal basis between a cool and warm cycle. The cycle of the PDO switched to warm around 1977, and began showing signs of reverting back to the negative cool phase in the late 1990s. However, since late 2002, the PDO has been predominantly positive. Monthly values have been positive since last November, further suggesting that a full shift back to a cold-cycle has not yet occurred. Although the PDO is not known to exhibit impacts on Atlantic Basin tropical cyclone activity, it does correlate with geopotential heights over the United States and the adjacent Atlantic region. These heights in turn play a role in storm movement.
V. Activity By Region
The Atlantic Basin has been divided into separate regions so that the amount of tropical cyclone activity in each sector can be analyzed.
Mean Development Region
The Mean Development Region is the area between the Lesser Antilles and the western coast of Africa south of 20 degrees latitude. This is where tropical waves traverse and often develop into hurricanes either in the region itself, or later on in the Caribbean and western Atlantic Basin, hence its name. Although common logic dictates that the warmer SSTAs will result in increased activity in this region, the ingredients are not that simple. 2005 saw record warm SSTAs in the tropical Atlantic, but as active as the season was overall, the Mean Development Region escaped with only two weak named storms. This conundrum was a subject of study this offseason, and sure enough we discovered a potentially very important factor for Mean Development Region activity: the March NAO.
After analyzing data 56 years of data, it is our conclusion that when the March NAO is positive, activity in the Mean Development Region is enhanced. The opposite holds true when it is negative. For warm ATC seasons, the average number of named storms, hurricanes, and major hurricanes in the Mean Development Region when the March NAO is positive is 6.4, 3.7, and 1.4 respectively. Conversely, the average of the same parameters for negative March NAO seasons is 3.2, 2.0, and 0.4. The average number of named storms and hurricanes are doubled for positive NAO years, and the average number of major hurricanes is tripled. The difference in these values is rather noteworthy. Since the NAO in March 2006 was -1.3, it is expected that Mean Development Region tropical cyclone activity will be hindered this year.
To further solidify this hypothesis, it was found that every single year with a negative March NAO, including 2006, also had high geopotential heights over the tropical Atlantic or Cape-Verde area, and vice-versa for positive March NAO years. High heights in hurricane development zones do not correlate with high hurricane activity, but instead tend to increase atmospheric stability and higher SLPAs. These conditions just prior to the onset of hurricane season will suppress the intertropical convergence zone and help reduce convection in tropical waves until they reach the Caribbean Sea. The one parameter that might suggest a little more activity in this region than last year is the QBO. The westerly phase of the QBO has a faint but existent correlation with activity east of the Leeward Islands. Taking into account the QBO, ATC, ENSO, and most importantly March NAO, we expect about four named storms to originate in the Mean Development Region, with two or three becoming hurricanes, and one becoming a major hurricane.
Subtropical Atlantic Basin
The subtropical area of the Atlantic Basin is generally the place to look for frontal lows acquiring tropical cyclone status in years with El Nino or cool ATC conditions. Neither is the case this year, so frontal activity in the high latitudes will be restricted to at least some extent. Nonetheless, the combination of a warm ATC, neutral ENSO, and negative spring NAO is climatologically favorable. The resulted pattern in this type of setup involves increased trough activity across the western Atlantic Basin. The increased SSTAs in tandem with the ATC allow better dynamics for cut-off lows to develop, especially early and late in the season. Furthermore, there is always the possibility of a strong tropical wave not developing until it is in close to the Bahamas, a very common occurrence in last year’s devastating season. Finally, a couple of tropical cyclones are forecast to enter the western or central Atlantic from the Mean Development Region. Taking all of this into account, above normal tropical cyclone activity is expected in this region, with about four to five named storms developing in the area, two to three being hurricanes.
We first begin with the eastern Caribbean Sea, often referred to as the dead zone of the tropical Atlantic Basin. Although tropical cyclones have passed through this part of the basin with no trouble, it is rare for a storm to actually develop here due to the summer and autumn presence of the Tropical Upper Tropospheric Trough, or TUTT. This feature induces strong southwesterly shear and thus usually keeps the region in check as far as development potential is concerned. It has been hypothesized that the easterly phase of the QBO during the summer and below normal SSTs could increase the effects of the TUTT on tropical cyclone activity. Since neither is the case this year, an inhibiting TUTT appears highly unlikely, and thus a favorable environment should prevail for storms entering the region. Having said that, there is a limit on the number of storms crossing the area considering the aforementioned problems with the Mean Development Region. Based on past warm ATC and negative NAO analogs including 1951, 1958, 1981, 2001, 2005, one to two named storms will probably enter or form close to the eastern Caribbean Sea this season.
In the western Caribbean Sea, the environment has supported many of the strongest Atlantic hurricanes on record, including last year’s Hurricane Wilma. This year, the western Caribbean Sea should be very favorable, largely owing to the above average SSTAs and lack of El Nino. Furthermore, the negative March NAO and resulted restraint on tropical wave development in the Mean Development Region only increases the likelihood of development in this region further west. The fact that at least one storm should move into this favorable area from the east spells major hurricane trouble. There is also concern for at least one system in this area late in the season, which will be discussed in further detail in the monthly breakdown section. Overall, three to four named storms should exist in this part of the Caribbean Sea, with one to two being major hurricanes.
Gulf of Mexico
Tropical cyclone activity in the Gulf of Mexico should be less active than the record amount seen last year, but far from absent. When neutral ENSO conditions prevail during the peak of hurricane season, as will be the case this year, one of the main dictating factors in Gulf of Mexico activity becomes the winter ENSO. An El Nino in the winter typically increases Gulf of Mexico tropical cyclones, whereas a La Nina or neutral winter often causes a reduction. Since the latter was the case this year, there will be a limit on Gulf of Mexico activity, particulary that of in-situ type. Moreover, two datasets of past warm ATC years were recorded: one consisting of years since 1950 and another for years since the recent uptrend, 1995. Both cases equally suggest the average number of named storms forming in the Gulf of Mexico is 45 percent higher when an El Nino is present in the preceding winter. The average number of storms in warm ATC years following Neutral/La Nina winters is approximately two. There is nothing to suggest that Gulf in 2006 will be more active than these sampled years, thus approximately two named storms are forecast to develop in the Gulf of Mexico. Furthermore, the springtime 850mb weather pattern would suggest most or all of these named storms that do form in the Gulf of Mexico would do so in the central or western Gulf, west of 88 degrees longitude.
Having said that, one also has to take into account storms entering the Gulf of Mexico from either the Caribbean Sea or western Atlantic. It appears that most storms that develop in the western Atlantic will track poleward and away from the Gulf of Mexico, an idea we will elaborate on further down. That leaves us with potential tropical cyclones traversing the central and western Caribbean Sea threatening the southern Gulf. One or possibly two named storms are forecast to enter the southwest Gulf of Mexico from the Caribbean Sea this year. An additional one or two named storms emigrating the western Caribbean may recurve into the southeast Gulf. In summary, four to five named storms are forecast to intersect the Gulf of Mexico this season. A few of these storms may strengthen into minimal or moderate hurricanes, but the potential for one or more major hurricanes in the Gulf is low based on our methodology. The reasoning behind our Gulf forecasts are explained in much more detail in the landfall sections below.
VI. Local Landfall Data
The theory of being able to forecast which areas of coastline will be directly affected by tropical cyclones months in advance is and should be the long-term goal of seasonal forecasting. In order for such predictions to progress, its boundaries must be tested. Long range forecasting also becomes more accurate with an increase in experience. Last year did not fare so well, but in the spirit of trying until success, experimental landfall forecasts will be included in IWIC’s seasonal forecast for a fourth year. Many researchers insist that landfall probabilities cannot be given with accuracy since the synoptic pattern is responsible for the steering of tropical cyclones. However, we argue that analyzing the overall synoptic pattern during spring, in combination with analysis of factors known to affect Atlantic hurricane activity, can enhance the accuracy of seasonal landfall forecasting.
This year, we are investigating entirely new methods based primarily on the 500mb and 850mb geopotential heights across North America, Atlantic Basin, and adjacent regions. Geopotential heights refers to the potential energy per unit mass of a body as a result of the earth’s gravitational field. Generally, low geopotential heights usually correspond to low pressure and vice-versa.
Texas and northeast Mexico
The coastlines of Texas and eastern Mexico, the western portion of the Gulf coast, faces the risk of seeing several tropical cyclones in 2006. One source of storms will be from the western Gulf of Mexico itself. As mentioned above in the regional section, the springtime 850mb geopotential height pattern this year consists of a ovoid ridge that stretches across the whole Gulf of Mexico from Florida to Texas. The ridge is centered over and south of Mississippi and Louisiana, so it is thus imparting north-northwesterly flow across much of the western Gulf of Mexico. Since we are forecasting two named storms to develop in the central or western Gulf of Mexico this year, and this layer is most critical for weak storms, one can put the two together. Unless a storm forms extremely close to the Mexican coastline and has no option but to move inland as a weak system, the steering flow supports tropical cyclones in the western Gulf of Mexico to have a more poleward component in their motion. This increases the risk towards Texas as opposed to Mexico from in-situ weak storms.
Having said that, several “weak” storms that have hit Texas in the past are notorious for rapidly strengthening just prior landfall. It is difficult, if not impossible, for us to say with certainty whether such intensification will occur this year. However, we can say with ease that if Texas is hit by a significant storm, it will not strengthen into one until very close to landfall. This is because the 500mb pattern in the Gulf of Mexico is a bit different: it is simply dominated by a strong, circular ridge centered over the Texas-Louisiana border. Therefore, the flow is westward at the 500mb level in most of the Gulf of Mexico. If a major hurricane enters the western Gulf of Mexico, it would stay south of Texas and strike Mexico under this type of pattern. Such a storm is not out of the question, but the other option is that a tropical cyclone hits the Yucatan Peninsula and enters the Gulf of Mexico as a weak system. A weaker storm would get steered more towards Texas.
Around three named storms are forecasted to make landfall in eastern Mexico or Texas this year. Texas is slightly favored over Mexico simply because at least two of these storms will be steered by the 850mb heights while in the Gulf of Mexico. Given the warm SSTAs this year, it would not be surprising to see at least one of these storms to strengthen into a hurricane before landfall. In-situ major hurricanes in the western Gulf of Mexico, while not unheard of, are rare enough to not forecast, though it is always a possibility. Finally, the final landfall of another storm that we expect will come from the Caribbean Sea will be determined by how strong it is when it reaches the Gulf of Mexico.
Louisiana, Mississippi, Alabama, and Florida Panhandle
It is a significant understatement to say the coastline from Lousiana through the Big Bend of Florida was hard-hit last year. This region was forever changed from Hurricane Katrina, not to mention the barrage of three other hurricanes. I, Jason Moreland, am a witness of the destruction and heartache along the entire central Gulf Coast. As a native New Orleanian, my life has forever been changed by the effects of Hurricane Katrina. As anyone that has been devastated by a hurricane can tell you, it’s no fun losing your home and having to unexpectedly move across the southeast for eight months. We can only hope that mother nature spares the central Gulf Coast from additional landfalls in the near future. According to the Army Corpse of Engineers, it won’t be until July or August when New Orleans’ levee protection system is completely restored to pre-Katrina levels, not to mention if Category Five protection will ever be implemented. Additionally, Louisiana, Mississippi, and Alabama are still far from even completing the clearing of debris process. All in all, Katrina recovery will take several more years, not months, and even then some areas will never be the same. I can also report that many residents of the Gulf Coast have been irritated by the lacking national media coverage of the grueling road to recovery many have undertaken.
Subsequently, one of the leading questions heading into the 2006 hurricane season is whether a tropical cyclone is likely to threaten the central Gulf Coast. Our outlook is encouraging for those in this area. First, we analyzed seasons that followed neutral ENSO or weak La Nina winters and had near neutral ENSO conditions by August, September, and October. Second, we eliminated the seasons that did not have a springtime weather pattern similar to spring 2006. In spring 2006, the central Gulf Coast was dominated by a mid-level 500mb ridge and an 850mb ridge near the surface. Past spring patterns including a dominant central Gulf Coast ridge are 1953, 1955, 1967, 1974, 1985, and 2000. 1953 was an active year for the Florida panhandle. Two named storms and one hurricane struck this area. In 1955, southeast Louisiana was struck by two minimal tropical storms. The Gulf Coast got a break in 1967 with no landfalls. The 1974 hurricane season brought Category Four Hurricane Carmen into central Louisiana. In 1985, a total of four hurricanes struck the central Gulf Coast. The Gulf Coast got another break in 2000 when only one minimal tropical storm struck the Florida panhandle. So even though we narrowed it down to six analogs, there is still quite a differential.
In consequence, we broke down the analogs further. In 1953, the Gulf Coast ridge was strong enough to protect Louisiana, Mississippi, and Alabama from tropical cyclones. However, it was too far west and weak to protect the Florida panhandle. In contrast, the 2006 ridge is setting up directly over the central Gulf Coast, and it is much stronger than the ridge witnessed in 1953. In 1955, the 500mb mid level ridge was strong enough to protect the area from significant tropical cyclone landfalls. In contrast, the 850mb ridge was oriented in a manner that allowed weak, southeast Gulf storms to head toward southeast Louisiana, hence the two minor landfalls. In 2006, a strong ridge at both the 500mb and 850mb levels is forecast to shunt any threatening storms westward toward Texas or Mexico. The 1967 analog is probably the best one for the Gulf Coast 2006 season. The 1967 ridge was positioned a little north of where it is this year, but its intensity over the coastline makes up for the greater distance. No landfalls occurred that year. In 1974, the only tropical system that impacted the central Gulf Coast was Hurricane Carmen. That year, The strong 500mb ridge over the coast kept Carmen on a westerly path up until the time it rapidly weakened over the Yucatan Peninsula. At that point, Carmen became influenced by the 850mb steering flow, which was vastly different and was oriented to favor minimal tropical cyclone landfalls along the northern Gulf coast. Carmen did rapidly intensify as it inched closer to Louisiana, but it was too late to cause the storm to steer back to the west before landfall. This year, the 850mb ridge is expected to be more protective, thus any landfalling Yucatan tropical cyclone will likely remain on a west or west-northwest heading rather than turn north. In 1985, several hurricanes struck the area despite the strong ridging that was present. Even so, the steering pattern over thewestern and central Atlantic made it favorable for tropical cyclones to enter the southeast Gulf, and it turns out that those such storms later struck the Gulf Coast. This year, the pattern over the western Atlantic is not conducive for tropical cyclones entering the eastern Gulf from the direction of the Bahamas or Florida. The remaining two landfalling tropical cyclones in 1985, Danny and Juan, originated in the central Gulf and were steered straight into the Gulf Coast because the 850mb ridge was anchored closer to the East Coast rather than the northern Gulf, creating a low-level southeast flow. Again, the low-level steering pattern of 2006 is expected to keep minimal tropical cyclones south and west of the central Gulf Coast. Finally, the 2000 season delivered one tropical storm to Pensacola. The ridging over the panhandle is stronger this year.
In summary, little to no tropical cyclone activity is forecast along the central Gulf Coast in 2006. The Gulf Coast ridge is expected to previal through the peak of the season. Minimal tropical cyclone development near the coast is always possible, but none of our data suggests that such a scenario is likely this year. This forecast is similar to the one released in May 2005, but we have developed an entirely new landfall forecast methodology soley because of the unexpected central Gulf landfalls last year. Our new methodology has yet to be tested, and we have the utmost confidence in it. With that being said, remember this is an experimental and unofficial forecast product.
Western Florida Peninsula
Once again, landfall analogs were determined by listing all seasons that followed neutral ENSO or weak La Nina winters and had spring patterns similar to spring 2006. The most notable springtime features we looked for was a strong Gulf Coast ridge and troughing over the central and western Atlantic. Six analogs met this critera: 1953, 1955, 1967, 1974, 1985 and 2000. Much like the original central Gulf Coast analogs, the number of tropical cyclones affecting western Florida in the following years varied quite a lot.
In 1953, the Gulf side of the peninsula was struck by three minimal tropical storms. Two originated in the northwest Caribbean while one developed in the central Gulf. The level of ridging extending from the Gulf Coast ridge this year is quite similar to the level of ridging witnessed over the peninsula in 1953. The height fields may be a little stronger, but not enough to dismiss the threat of similar tropical cyclone landfalls along the peninsula this year. 1955 met basic analog criteria, but the core of the Gulf Coast ridge was partially anchored over the Florida panhandle, and that does not match 2006 as much as one would like. The panhandle-anchored ridge significantly lessened the potential for southest Gulf storms to recurve over the peninsula. It’s no wonder that no landfalls occured that year. 1967 is better but not by much. The 1967 Gulf Coast ridge was centered far enough away from Florida, but positive height fields over Jacksonville, Georgia, and the Carolinas were much greater in 1967 than 2006 thus far. Moreover, heights in the central/western Atlantic have been a lot lower this year in contrast to 1967. No landfalls occurred over the Gulf side of the peninsula in 1967, but less ridging over the southeast and lower heights over the western Atlantic may increase the likelihood of tropical cyclone recurvature over the peninsula this year. In 1974, a subtropical storm formed in the Gulf of Mexico and struck Florida just south of Tampa. The positive height fields extending from the Gulf Coast ridge resemble 2006 much like 1953. One interesting differential between 1953 and 1974 is the ovoid orientation of the height fields over Florida. The 1953 orientation of the fields were more southwest to northeast whereas they were more west to east in 1974. Thus, the orienation of the ridge in 1953 may have made western Florida more suscepitble to multiple recurving cyclone landfalls than in 1974, despite similar height anomaly values. In 1985, one tropical storm formed west of Sarasota and headed east into the peninsula. Much like 1953 and 1974, height fields over the Florida peninsula in 1985 are very similar to those in 2006. 2000 is one of the worst analogs out of the selection, but the overall pattern this year is still close enought to provide clues. The core ridging in 2000 was centered over southeast Canada and the Great Lakes, but they extended well into the central Gulf of Mexico. However, had the the high been centered closer to the Gulf, the two tropical cyclones that hit the panhandle probably would have struck the western side of the Florida peninsula instead.
In conclusion, one to two named storms are forecast to develop or move out of the northwest Caribbean and southeast Gulf of Mexico and eventually recurve into the western side of the Florida peninsula. The threat of one or both of these named storms strengthening into minimal or moderate hurricanes prior to landfall cannot be ruled out.
Eastern Florida Peninsula and Keys
Interestingly, eastern Florida landfalls are not very common in years following neutral or La Nina ENSO during the winter. Only six out of 26 years with such winter ENSO conditions and neutral ENSO conditions during the peak of the respective seasons had eastern Florida landfalls. These six years are as follows: 1950, 1960, 1979, 1981, 1984, 1985. First off, the only year on this list that had a negative March NAO similar to this year is 1981, and one could even argue that the Florida tropical storm landfall was more of a hit on the southwest. Moreover, none of these years equally matched the strength of spring 2006’s Gulf ridge or central Atlantic trough. Aside from 1981, which again had a disputable and weak east Florida landfall, all of these years featured early ridging in the western Atlantic during the spring, which enabled storms east of Florida to feel the influence of easterly flow and track towards the state. Such a steering flow is not likely given the incresed troughing in the subtropical Atlantic this spring. A few of the seasons that had no landfalls matched this 2006 springtime pattern much better than those with landfalls. Based on this information, we do not expect any landfalls along the Atlantic side of the Florida peninsula this season.
Georgia and the Carolinas
The common theory these days is that if the Gulf of Mexico has an active season, which was the case in 2005, the following season will have more activity along the East Coast. That is not always the case, however. If multi-annual trends were that simple to follow, then a lot of government and independent agencies probably would have solved the seasonal landfall forecast problem by now. Our newly developed methodology this year yields little threat of tropical cyclone landfalls between Georgia and North Carolina this season, much like a good portion of the U.S. Gulf Coast.
First off, the negative March NAO translates into fewer storms developing in the Mean Development Region, the birth place of many Carolina-bound hurricanes. This alone puts a significant damper on the major hurricane threat for the Carolinas this year. To be fair, a majority of years with neutral ENSO or weak La Nina winters and neutral ENSO summers had landfalls in this area. Nevertheless, only one of these landfall years, 1952, had a pattern resembling the one we are expecting to prevail through 2006. There are even problems with using 1952 as an East Coast analog. The steering pattern in 1952 was more susceptible to fluctuating or shifting, and even when the pattern best matched 2006, the central Atlantic trough did not extend far enough into the southwest Atlantic. This year, we anticipate stronger troughing in the central and western Atlantic, thus eliminating the threat of long tracking storms slamming into the Carolinas or Georgia. Several storms are forecast to recurve out to sea north of Hispaniola and Puerto Rico. Tropical cyclones that develop close to the East Coast and hit land like the ones observed in 1953 and 1961 are less likely this year. Any storms that do develop over the eastern Bahamas or southwest Atlantic will be more prone to recurvature rather than riding up the East Coast. The ridging over the Gulf Coast only extends as far east as the Florida peninsula, and troughing has been prominent across much of the southwest Atlantic. One of the basic analogs that did not have any Georgia or Carolina landfalls was 2000. 2000 carries more weight than 1952 because Gulf Coast ridging and central Atlantic troughing was present, whereas both features had difficulty coinciding with each other in 1952. Due to North Carolina’s natural, geographical vulnerability, a brush from a storm forming immediately offshore can never be ruled out, but no tropical cyclones are forecast to slam straight into the Carolinas or Georgia this season.
Mid-Atlantic States and New England
There is not much more to add from what has already been stated in the Georgia and Carolinas landfall section. Since 1950, there were no direct hits from tropical Atlantic-originating storms in years with negative NAO during March except for 1971 and 1954. The pattern in 1971 featured a longwave trough situated over the East Coast with ridging to the east, so it is no wonder a storm made it to New England this year. If one counts all direct hits on New England, regardless of origin, no storms forming in the western Atlantic hit the area in a neutral season ENSO and a cool ENSO in the preceding winter. It also must be noted that, barring 1954, the only times the Mid-Atlantic States and New England were affected by tropical cyclones were when storms had begun recurving after striking the Carolinas or Gulf Coast. The setup with ridging over the Gulf Coast and troughing over the central Atlantic should significantly lower the probabilities of central Gulf Coast or East Coast hits this year, thus we do not anticipate any recurving storms of that nature this season.
Other forecast agencies have pointed out 1954 as an analog for the 2006 season, respectfully. The 1954 season included two moderate hurricanes landfalling in the northeast, and one major hurricane landfalling in North Carolina. Obviously, any group using 1954 as an analog are indicating that the Northeast and East Coast are at high risk this year. On the other hand, we don’t see the connection. Heading into the 1954 season, there were already signs of stronger than normal ridging developing over the central Atlantic and below normal heights over the Carolinas and Northeast. This year, there are more indications of central Atlantic troughing and the East Coast is under the influence of the eastern flank of the Gulf Coast ridge. A congregation of storm tracks may develop over the central/western Atlantic this summer, but not along the East Coast.
An above average number of tropical cyclones are expected to recurve in the central and western Atlantic, and the evolving steering pattern is indicative that some of the westernmost recurving storms may pass very close to the Maritimes. After analyzing the behavior of dozens of tropical cyclones that recurved over the central Atlantic in our analogs, we do expect one to two tropical cyclones to threaten the Canadian Maritimes this season.
The majority of threats are forecast to originate in the northwest Caribbean Sea rather than central Atlantic this season. The 500mb steering pattern that has steadily evolved over the course of the year is conducive for several tropical cyclone recurvatures east of the Bahamas. On the other hand, tropical cyclones that develop in the central or northwest Caribbean may also be susceptible to recurvature over Cuba and the Bahamas. One to two tropical cyclones may threaten the Bahamas from the southwest this season. The possibility of these cyclones being minimal or moderate hurricanes cannot be ruled out. Further elaboration of the 2006 steering pattern can be read in the Cuba section.
Considering most storms that strike southwest Florida initially pass through Cuba, the same forecast methods used to forecast southwest Florida activity have been applied for this section. The prospects of a hurricane or tropical storm cutting westward across the Western Indies into Cuba such as Georges of 1998 are essentially non-existent this year based on the 500mb steering pattern and March NAO. Therefore, this leaves us with the last possible major threat for Cuba: a late season monster coming from the western Caribbean Sea. This type of storm is not particularly common, and our research indicates that such an event probably will not occur in 2006. Provided ENSO becomes at least warm-biaed by autumn, any storm that forms in the Caribbean in October or later will have a hard time feeding off optimum conditions to support growth into a major hurricane. It is interesting to note that every late major hurricane that has formed in the western Caribbean Sea has occurred when ENSO was either cool-biased or in a La Nina episode. Neither conditions are expected this year. However, it should be cautioned that if ENSO does somehow slip into cool biased territory later this year, there would be more concern for a late season major hurricane for Cuba. The very warm ATC and western Atlantic troughing in combination with the favorable ENSO would both make such a scenario much more likely.
Nonetheless, that is not to say Cuba will not be impacted by several tropical storms or even minimal hurricanes. The strong Gulf coast ridge and troughing over the western Atlantic, as stated in the Florida section, will be conducive for tropical cyclones in the northwestern Caribbean Sea to track northward towards Cuba and Florida. One to two named storms are forecast to impact Cuba during the season from the Caribbean Sea, any one of them possibly being a minimal hurricane.
Hispaniola, Puerto Rico, and Lesser Antilles
Aside from the rare late-season exceptions such as Lenny in 1999, most tropical cyclone activity that strikes these islands comes from the Mean Development Region. The negative March NAO has promising implications for the Lesser Antilles and Puerto Rico. Out of the 16 recorded years with a negative March NAO, only 1954 and 1964 saw a major hurricane impact any of these islands. Convincingly enough, both of these years featured abnormally strong ridging in the subtropical Atlantic Basin between 20 degrees and 30 degrees latitutde. This enabled the rogue Mean Development Region major storm to get shoved westward towards these islands as opposed to staying adrift at sea. 2006’s springtime steering layers in the subtropical Atlantic Basin are dominated by lower than normal heights and troughing. This is much more in line with the typical negative NAO years. Therefore, a major hurricane landfall in Puerto Rico and the Lesser Antilles can be pretty much ruled out this year. Chances are also low for Hispaniola, but not quite as much so for a special reason. The orientation of the troughing this spring places its base just above Hispaniola. Although tropical cyclones to the south are frequently known to bypass the base of a mean trough during the peak of the season, this is harder to do towards October when the troughing deepens. For instance, the spring Atlantic Basin pattern in 1963 was very similar to this year’s, especially in regards to the location of the trough base. Sure enough, this trough was able to pull Flora, a late September and early October major hurricane, northward while it was in the central Caribbean Sea. Flora slowed down considerably at the point of recurvature but hit both eastern Cuba and Hispaniola before shooting out to sea. If a storm enters the eastern Caribbean Sea late enough in the season, a similar scenario is very possible this year.
Having said that, it does not appear these islands will be immune to tropical storms or even weaker hurricanes. Many of the negative NAO years that also featured a very warm ATC, including 1951, 1958, 1969, 1981, 2001, and 2005, tended to have at least one weak tropical cyclone impact the islands. This will likely be the case this year as well. A minimal hurricane, such as Emily in 2005, is not completely out of the question.
Eastern Yucatan Peninsula and Central America
We are forecasting at least one storm to enter the eastern Caribbean Sea during the peak of the season. Considering such a storm would likely miss the base of the trough situated north of the Dominican Republic, this leaves it nowhere to go but westward. Curving northward towards Cuba and Florida is possible but actually less likely for a westward-tracking Caribbean storm than for in-situ western Caribbean systems. Such a track has happened before, the most notable being Donna in 1960 and Charley in 2004. However, both years featured low 500mb heights over Florida during the spring, signifying the onset of a strong east coast trough but ridging in the subtropical Atlantic to the east. This pattern is not the case this year. While any storm that forms in the northwest Caribbean Sea may be prone to get pulled northeastward by the central Atlantic troughing, the chances are much lower for a storm that has already missed the main connection with the trough and is still chugging westward at a lower latitude. The destiny of this type of storm is logically Central America. Such a scenario is likely to occur this season due to reasons outlined in prior sections. Furthermore, there is always the possibility that a tropical cyclone develops in the southwestern Caribbean Sea far enough under the late season troughing to avoid getting pulled poleward. Based on this information, one to two named storms are forecast to strike Central America or the Yucatan Peninsula this season. The plausible westward-moving system during August or September should encounter favorable enough conditions in the western Caribbean Sea to support intensification into a hurricane, possibly a major hurricane, before landfall.
VII: Monthly Breakdown
The Atlantic Basin hurricane season officially spans six months, beginning on June 1 and ending on November 30. Unlike landfall and regional forecasting, it is very hard to venture out and predict tropical cyclone activity for each month or pair of months by observing the recent patterns alone. Therefore, we looked primarily at climatology and data from analog years for this section of the forecast.
June and July
Generally speaking, the first two months of the Atlantic Basin hurricane season are relatively quiet. On average, one to two named storms form in these months combined. Last year saw the record development of seven named storms and two major hurricanes. Although the insane activity continued throughout the rest of the season, the amount of activity that occurs in June and July does not necessarily have any bearing on how active the remaining four months or the season altogether will be.
There are two major factors towards June and July activity: winter ENSO and June-July ENSO. Our research strongly indicates that years where an El Nino was present the preceding winter tend to have above average tropical cyclone activity in the first two months of the season. However, years with neutral ENSO during the two months themselves is also favorable for more storms. Although the former is difficult to synoptically prove, it is reasonable that neutral ENSO is the most conducive for early activity. During an El Nino year, other factors aside, the late spring and early summer pattern tend to feature an abnormally strong and low jet stream. The strong zonal flow associated with this is not favorable for early tropical cyclone development. In a La Nina summer, there are usually much less frontal features dipping down into the tropics in the first place. This is a problem as well, as many early season storms are spawned in part by remnant frontal boundaries. During a neutral ENSO, fronts are not completely hindered, yet the longwave flow is still amplified enough to allow for tropical development off frontal tail-ends.
This past winter was under a weak La Nina, but a neutral ENSO is currently present as we head into June and July. Thus, we are faced with two conflicting factors. Recent warm ATC years that did not have a winter El Nino and similar spring ENSO conditions include 1979, 1981, 1989, 1996, and 2001. All of these years had between one and three named storms before August. However, this can be narrowed even further. The years in this sample that had early activity in the Mean Development Region, 1979, 1989, and 1996, all had neutral or positive March NAO values, which was not the case this year. Taking this into account, one to two named storms are forecast to develop before August this season.
August and September
Atlantic Basin tropical cyclone activity tends to experience a fairly sharp increase during the month of August, particularly after August 15. August and September are by far the most active months of the season, with the climatological peak of being in early to mid September. In almost all cases, an above average hurricane season equates to above average activity in these two months.
Upon investigating previous active hurricane seasons, we noticed two separate camps concerning tropical cyclone activity during the peak. One group contains years that featured a general range of six to eight named storms in August and September, which is above normal, but not overly so. On the other hand, there are other years that had over ten named storms in the same timeframe, a rather explosive amount of systems for such a duration. This poses the question of exactly how active the peak two months will be this season. We further examined the ten years since 1950 with ten or more named storms and interestingly enough discovered that only six of them were during a warm ATC. Furthermore, all six of these warm ATC years had warm ENSO conditions the preceding winter. The remaining four cool or neutral ATC years had the opposite, a cool winter ENSO. The reasoning for this dichotomy is unclear. A logical hypothesis is that residual El Nino conditions coupled with warm ATC, or residual La Nina coupled with cool ATC, both equally allow more development from frontal-tropical wave interactions. In any event, the March NAO logically also plays a role since many storms during the peak form in the Mean Development Region. This is another restriction on this year’s August and September activity.
The combination of a warm ATC, winter La Nina, and negative NAO highly suggests no more than eight named storms during the peak this year. Other recent years where the first two states were case include 1989, 1996, 1999, and 2001, though 2001 is the only one on the list that also had a negative NAO. It thus comes to no surprise that 2001 had the lowest amount of hurricane and major hurricane activity out of this sample. Taking everything into account, six to eight named storms are forecasted to develop during August and September, with four to five being hurricanes, and around two of those achieving major hurricane strength.
October and November
When October approaches, a noticeable decline in Atlantic Basin tropical cyclone activity is observed. On average, there are two to three named storms in October and November, which is far less than what is typically the case in August and September but slightly more than the first two months. However, in uncommon cases, there can be more activity during these months than around the peak, such as what was observed in last year.
Our research indicates an above average level of storms during the latter portion of the season. There are two primary reasons for this presumption: a very warm ATC and a neutral ENSO. Upon examining all of the past 55 seasons or so, we noticed that the stronger the ATC, the more activity observed in the last two months. This is not surprising considering a strong ATC is favorable for Atlantic Basin activity as a whole. Also, the warmer SSTAs that occur in tandem with a strong ATC increases the likelihood of late season cutoff lows acquiring warm cores and becoming tropical cyclones. Having said that, the relation with ENSO is a bit more interesting. When a moderate or strong El Nino is in place, activity during the last two months is almost always reduced below average due to the stronger subtropical jet stream and associated westerly shear over the basin. Conversely, and to no amazement, more intense tropical cyclones are observed late in the season when a La Nina is present. However, it is found that seasons with neutral or even weakly warm ENSO tend to have a higher amount of storms than what is seen in a La Nina, especially when coupled with a strong ATC. The most logical reason for this is similar to what was outlined in the June to July sub-section. While a La Nina favors the stronger low-latitude systems, there are generally fewer cut-off lows in the subtropics, which often transfer to warm-core tropical cyclones in the late season.
Since a neutral or weak warm ENSO and a warm ATC are both in the forecast this autumn, we expect at least five named storms to form after September, with two to three of those strengthening into hurricanes. It should be noted that the only years that had an ATC comparable to this year’s level coupled with neutral autumn ENSO are 2001 and 2005. Both of these years saw more tropical cyclone activity the last two months than August and September, albeit most of the storms were in the subtropics and did not affect land. A similar case of multiple cutoff tropical cyclones cannot be ruled out this year if the ENSO does not become too warm. It does appear ENSO will be at least warm-biased, and this therefore lessens the chances of a major hurricane during October or November. If ENSO ends up cooler than expected, however, a late major hurricane would be much more likely since one has occurred in every non-El Nino autumn since 1995.
VIII: Summary and Conclusion
The recent upswing in tropical cyclone activity in the Atlantic Basin appears far from being over. A strong ATC combined with neutral ENSO conditions should allow the above normal trend in tropical cyclogenesis to continue into the 2006 season. There are some notable characteristics that stand out in our preseason data. Near average activity is forecast in the Mean Development Region, with most tropical cyclones recurving out to sea. Below normal heights over the central Atlantic may act as a protective barrier of tropical cyclones for the US East Coast. Any tropical systems that manage to undercut the below normal heights and enter the central and western Caribbean Sea may be prone to becoming significant hurricanes. The threat of major hurricane landfalls in the Caribbean is relatively low starting with the Lesser Antilles, but the threat gradually increases with longitude in the central and western Caribbean. Ridging over the southern states and troughing over the central Atlantic should keep any significant, low-latitude tropical cyclones south or east of the US Gulf Coast. However, there is a moderate risk of tropical storm or minimal hurricane impacts in southwest Florida and the Texast coast. The Mexican Gulf Coast faces a similar risk. Finally, our forecast number of systems is presented below. While 2006 is expected to be above average, the level of activity is expected to be far less than that observed in 2005. Last year, extremely warm sea surface temperatures across the entire Atlantic Basin set records and spawned 28 named storms in the process. Sea surface temperatures are above average this year, but they do not rival the anomalies of 2005.
IWIC 2006 Atlantic Basin Hurricane Season Forecast
|Parameter||2006 Forecast||Long Term Average|
Regardless of our exact expectations for 2006, one in a hurricane prone area should always be prepared for a landfall well in advance. Even if 2006 hurricane activity unexpectedly turns out to below normal, it only takes one intense landfalling hurricane to make the season a devastating one. A well-known example of this is the 1992 hurricane season. This was a well below average year by almost all activity measures, yet it was the year of Category Five Hurricane Andrew, which slammed into southern Florida and resulted in over 30 billion dollars of damage.
This seasonal forecast will not be updated during the season, though smaller updates will be posted within our daily tropical weather discussions if necessary. A post-review of the 2006 season will be underway by December 2007. This allows us to analyze how all of our seasonal forecast methods fared through the 2006 season, and better prepare our 2007 forecast. By January 2007, preliminary research on the 2007 Atlantic hurricane season will begin.