Weather456's Tropical Weather Blog

Understanding the Atlantic Hurricane Season: The SST Factor

By: Weather456, 11:13 PM GMT on April 28, 2008

The SST Factor

There are no areas of concern in the tropics today and models are not forecasting any development over the next 7 days. I begin updating visitors to my blog on the tropics, beginning this Thursday, May 1 2008.

One of the necessary conditions for tropical cyclone formation is sea surface temperatures of at least 26.5C at a dept of 50 meters. But why warm temperatures?

Importance of Warm Sea Surface Temperatures

Under normal conditions, more heat and moisture is added to the lower atmosphere when sea surface temperatures are warm. Warm, moist air in the lower levels below cool dry air of the mid-levels is an unstable situation which aids of the development of oceanic thunderstorms.

Sea surface temperatures also promote favorable vertical wind shear through the rising motion of warm air. Warm air rises and flows out in the upper atmosphere, creating an environment favorable for tropical cyclone formation, development and intensification.

Where are the warmest/coolest SSTs found in the Atlantic Ocean?

The warm SSTs are normally found where there is downwelling and along warm ocean currents.

The western Caribbean is a region where there is downwelling. What is downwelling? Downwelling is simply where is water is force towards land and has no where to go but down. This pushes cool waters in the sub surface further downwards promoting to warmer and deeper sea surface temperatures. Some of the most intense hurricanes occur in the Western Caribbean. In the past 6 years there were several storms that attain category 5 statuses in that region of the Atlantic, namely Ivan in 2004, Emily in 2005, Wilma in 2005, Dean in 2007 and some can argue, Felix in 2007.

The North Equatorial-Caribbean Current and the Gulf Stream are the two major warm currents in the tropical North Atlantic as seen in figure 1. The former ocean current flows from the Equator along South America and through the Caribbean Sea while the latter is just a continuation from the Gulf of Mexico, through the Straights of Florida and along the United States east coast.

The North Equatorial-Caribbean Current flows through the areas of downwelling in the Western Caribbean which is a double warming factor for that region.

Famous storms that have intensified over the Gulf Loop current included Katrina and Rita in 2005. Subtropical storm Andrea in 2007 also owes its existence to the Gulf Stream Current.

The cool SSTs are normally found where there is upwelling and along cold ocean currents.

Upwelling occurs where surface water is pushed away from a fix location causing cool waters to come up from the subsurface to replace. Regions of upwelling occur along the Northern Coast of South America and the West Coast of Africa north of 15N. The major cold current in the Atlantic is the Canary current which also occurs near the upwelled region of West Africa, which is a double cooling factor for that region.

What are some of the factors that affect the rate of warming or cooling of sea surface temperatures?

The orientation of the warm and/or cold currents. For example, because of the unusual westward position of the Bermuda-Azores High during the past winter, the Canary current was further north and weaker allowing sea surface temperatures to substantially warm in the East Atlantic. See figure 2. Also, low pressure over the West African landmass increased the onshore flow (water pushing against land) which lead to downwelling and promoted anomalous high sea surface temperatures.

The degree of solar insolation. Of course the sea surface is mainly warmed via insolation from the sun. Several factors affect the amount of solar insolation. They include the amount of cloud cover and dust.

The degree of evaporation. Evaporation removes heat from the sea surface. Evaporation is controlled by wind and atmospheric moisture. The faster the wind speed the higher the rate of evaporation and the lower the atmospheric humidity in the low levels the higher the rate of evaporation. Hurricanes can quickly speed up the rate of evaporation due to the high winds associated with them. During the winter of 2007-2008, above normal tradewinds contributed to the reduction of SSTs in the Central Atlantic and Eastern Caribbean Sea. See figure 2.

Atlantic Multidecadal Oscillation (AMO)

A naturally occuring variability in North Atlantic Sea surface temperatures.

The frequency of weak-category storms - tropical storms and weak hurricanes - is not strongly correlated with the AMO. However, during warm phases of the AMO, the numbers of tropical storms that mature into major hurricanes is significantly greater than during cool phases, at least twice as many. Since the AMO switched to its warm phase, circa 1995, major hurricanes (category 3 or above on the Saffir-Simpson Hurricane Scale) have become much more frequent and this has led to a crisis in windstorm insurance coverage and cost. Based on the typical duration of negative and positive phases of the AMO, the current warm regime is expected to persist at least until 2015 and possibly as late as 2035. However, if part of the recent uptick is related to global warming, the current warm regime may last significantly longer.

Current Factors In Play for 2008

Now we have had below normal sea surface temperatures for much of the tropical Atlantic due to the events of the past winter. However, recent analysis have showed that sea surface temperatures are warming very quickly in those same regions and there are three factors that I have found to contribute to this.

1. Lack of cloud cover due to stable conditions has allowed maximum insolation at the surface.

2. The trade winds have decrease much over the past month due to a more weaken and more centralised subtropical high pressure system.

3. The simple fact, that summer is around the corner.


Figure 1. Schematic of the Atlantic's ocean currents. Red shaded currents are warm and blue/gray shaded currents are cold.


Figure 2. Diagram showing the effects of the Bermuda High over the past winter.

Additional Links and Resources
Current and past Sea Surface Temperature maps
Hurricane Katrina and the Gulf Loop Current
Tracking Caribbean Hurricanes: My Experience
Frequently Asked Questions About the Atlantic Multidecadal Oscillation (AMO)


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Tracking Caribbean Hurricanes: My Experience

By: Weather456, 9:43 PM GMT on April 26, 2008

I have been tracking tropical cyclones since the summer of 2002 and I just wanted to share my experience with you all of the most powerful hurricanes that transit the Caribbean Sea during that time.

Hurricane Isidore

The first Caribbean hurricane I tacked via the internet. Isidore developed near Trinidad and Tobago, which was a bit unusual when I looked at tracks of past hurricanes. I tracked Isidore as it transit the Caribbean and strangely enough, I thought Isidore would of enter and cross the Gulf of Mexico as a hurricane and make landfall on the Gulf coast. Instead, his movement slowed then began to progress on a southward path until it made landfall in the Yucatan. After which, I thought everything was done but the storm meandered across the Yucatan Peninsula, the finally moved northward across the Gulf and eventually made landfall on the Northern Gulf Coast.

Hurricane Charley

I began tracking Charley about two days before it was classified TD 3. I was busy watching the regeneration of TD 2, which became TS Bonnie, when I notice a well define area of thunderstorms organizing east of the Windward Islands. At that time, I thought it would reach TS status before reaching the Islands. When Charley was classified a TS after it passed the islands, I looked the steering environment at the CIMSS and knew Jamaica would be in for a hurricane. Amazingly, as Charley approached the island it jogged southward at the last minute as if a protective shield was surrounding the island of Jamaica. Despite avoiding a direct impact, the storm still came close to Jamaica enough to produce damage. The next in line, I thought, was the Yucatan but then notice that the steering maps updated and a trough was allowing the flow to be more NW-NNW than before. This meant the Caymans and Cuba was the next to take the brunt. Charley pushed through the Caymans and crossed Cuba on the night of August 12 and weakened somewhat. I thought Charley would have weakened more but it was moving too fast and over the narrow peninsula in Western Cuba.

Hurricane Ivan

When Ivan was strengthening just east the Lesser Antilles, I was waiting for him to turn north and take a similar path to Dean later in 2007. Ivan surprised me in many ways when he impacted Grenada. Before then, I did not even know that Grenada was affected by hurricanes. I watched Ivan tore the island apart as he made landfall as a cat 3 hurricane and I knew it would be bad because of what I watched Charley did earlier in the season. Reports that came out of Grenada afterwards were heart breaking. I did not feel the need to track Ivan anymore but I did.

Ivan was now moving on a path similar to Charley but a little south so I new Jamaica was not going to get a direct hit. The steering stayed consistent with the forecast with few wobbles. I did not expect Ivan to reach cat 5 status but he did. In the three years of tracking hurricanes, this was my second cat 5 hurricane. I was always thought and heard meteorologists saying that these monsters were very rare.

Meanwhile, Ivan began to move more northward and eventually cross Charley's path before he reached the island of Jamaica. That made it seemed Jamaica wasn’t so lucky this time around. The storm came dangerously close to the island but moved south of the island just as Charley did. However, Ivan was fluctuating between cat 4 and 5 intensity as it passed the island. The Caymans had it even worst. Ivan came closer to the Caymans and at an even higher intensity, near cat 5. If Grenada suffered a cat 3 storm, I could not image what Jamaica and the Caymans went through.

Ivan finally left the Caribbean through the Yucatan Channel and set his course for the Gulf coast.

Hurricane Jeanne

Jeanne, I did not even know that there was a depression developing offshore my island. LOL. All my attention was drawn to Ivan. Jeanne surprised me in many ways. First, she reached hurricane intensity between Puerto Rico and Hispaniola, which is a narrow channel called the Mona Passage. Second, when Jeanne crossed Hispaniola, I thought it was dead dead dead and that was it. However, to my surprise, she survived and produced torrential rains that caused massive lost to life and property - over 3000 deaths. Very unexpected. Third, regeneration, when some of her energy moved north of the Turks and finally, her loop-to-loop.

Hurricane Dennis

The first hurricane of the 2005 Hurricane Season (at the time). This hurricane stands as the most well documented hurricane ever in my history of tracking. I got four hrs of sleep per day on Dennis and tracked his every movement. He reminded me alot of Charley the year before. TD 4 was ominous looking. It had that look of a severe storm to come. Predictions turned out right. Dennis moved wnw to nw over the next couple of days strengthening to cat 1 intensity quickly. Actually, it was a 70 mph at 5pm advisory then minutes later the Air force recon reported 75 mph winds. Dennis intensified afterwards reaching cat 4 intensity near the Windward Passage. He managed to squeeze between the islands of Jamaica, Hispaniola and Cuba without any interactions with land and made landfall in the Southeast tip of Cuba as a Cat 4 hurricane. I watched as Dennis' eye moved across the tip without even breaking down. Dennis made another landfall hours later as the same cat 4 storm. The storm crossed the island of Cuba over the next 12 hrs and remerged over the Gulf as a cat 1. Though damage reported in Cuba was moderate to heavy there were few deaths attributed to the storm, thus I must commend the Cuban emergency management for their excellent job during Dennis.

Hurricane Emily

Didn’t really like tracking this storm in its early stages for some reason. Dennis took alot of energy from me. At first, I thought Emily was coming my way (Leeward Islands) but it became apparent, she was like Ivan. Emily made landfall on Grenada in the morning as 90 mph hurricane. I went to bed the night before when Emily was just about cat 1 status and woke up to an infrared imagery similar to Ivan. Grenada suffered moderate damage. This was the second hurricane to affect Grenada in less than year. It was becoming more aware that something was wrong. Emily next feat was attaining 155 mph just weeks after Hurricane Dennis reached 150 mph to her north. In addition, the air force reserves reported Emily had reached cat 5 status. I asked myself, how could Emily do this just weeks after Dennis. My explanation was Dennis might of pushed warm water to the south of his track which aided Emily in her intensification. Emily gradually weakened afterwards but maintained her cat 4 status when she made landfall on the Yucatan Peninsula causing heavy damage. Emily was upgraded to cat 5 status in post season analysis which didn’t surprise me because of the recon reports.

Hurricane Wilma

Where to begin, I mean Wilma took me for a ride. I went to bed around 8 pm on the Tuesday night, as Wilma was a cat 1 hurricane. For some strange reason, I woke up around 3 am and turned on the Weather Channel instead of going on the internet. I could remember the meteorologist who was on at that time - Rich Johnson. He stated that Wilma was a cat 5. I was in shock at the level of intensification. The third cat of the season? I thought the world was going to end. When did cat 5's become so popular? Was I missing something? Then I went onto the Jeff Master's blog and heard persons saying that Wilma will never surpass Gilbert's record until news reached my ears that they recorded a pressure of 882 mb. I instantly thought it was an instrument failure. I didn’t believe it until later in the day when they confirmed it. Wilma was truly awesome on satellite imagery, took up almost all the area on the NOAA floaters and the size of her eye caught my attention. It was so tiny compared to past cat 5’s. I knew Wilma could not maintain that intensity for long, it’s just impossible to fathom. Wilma then went through an EWRC, reformed a larger more believable eye, and moved slowly towards the Yucatan. Wilma made landfall on the Yucatan as a healthy cat 4 and lingered over the resort areas for hours. The damage inflicted was unbelievable. News also came out of Jamaica and Honduras where Wilma's rain bands caused flooding and damage.

Hurricane Dean

After a year's break, except for Hurricane Ernesto in 2006, the action ramped up once more, when Dean began organizing and strengthening east of the Islands. I had alot more knowledge now, and a strong and persistent high made me to believe little movement north was expected. Dean gradually organized as it moved quickly westward becoming a hurricane the day before it reached the islands. Dean moved through the island producing at least tropical storm force winds all the way north to Saint Maarten and south to Saint Vincent. The three hardest hit islands were Saint Lucia, Martinique and Dominica. Damage to my island was light, the most damage we had in 7 years from any storm. I began tracking Dean using radar imagery as it passed through the Saint Lucia Channel and enter the Caribbean Sea. Dean’s eye became much more define as it left the islands, and that was a sign of intensification to come. Dean moved on a consistent path towards the west north -west, the most consistent path of any hurricane I tracked. Dean went through a series of EWRCs as it approached Jamaica. Perhaps the most EWRCs of any storm in such a short period. Dean’s outer rain bands passed close to Puerto Rico and Hispaniola as it intensified into a strong cat 4 hurricane. He left modest flooding in those nations due to his fast movement. The next in line, was Jamaica, which seemed popular when it comes to these powerful monsters. Dean’s eye passed dangerous close to Jamaica as it clipped along to the south. Dean also passed well south of the Caymans. Dean intensified to another cat 5, the 4th one in the Caribbean and the 7th overall. Dean then made landfall to the south of the resort areas of the Yucatan in a sparsely populated area as a cat 5, the first such ever landfall in my young life.

Hurricane Felix

This storm was rather short-lived. In one weekend, Felix went from a tropical storm to a cat 5 hurricane. By now, I had become accustom of seeing cat 5 hurricanes so it wasn’t really any big surprise. What surprised me was when the air force recon had to fly out of Felix because of unimaginable turbulence and ice accretion. I didn’t remembered that occuring during Wilma. Felix marked the 8th cat 5 hurricane tracked in 5 years. Had I done it the previous 5 years, only Mitch would be accounted for. Amazing! Felix made landfall as a cat 5 hurricane in Nicaragua causing heavy damage but the most stricking reports coming from that nation was that persons were out fishing not knowing Felix was upon them. Felix made the second cat 5 landfall in my years of tracking.

So basically in the past 6 years I have witnessed more than 50 years worth of records.

Other hurricanes included Hurricane Beta (2005) and Hurricane Ernesto (2006).

Visible Imagery of Hurricane Dean as he passed south of Hispaniola



Hurricane Dennis near his second landfall



Hurricane Ivan in the Southeast Caribbean Sea



Hurricane Felix seen in this MODIS TERRA image on September 03 2007 as it chrurned towards Central America.



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Anomalies in the ITCZ/TC Neoguri

By: Weather456, 10:07 PM GMT on April 23, 2008

The ITCZ from Africa to the Eastern Pacific continues to display anomalous high rainfall through April. If this pattern continues it would enhanced the liklihood of TC development in both basins.



The below image is a GOES-12 infrared imagery showing the location of the intertropical front (ITCZ). The ITCZ actually crosses Northern South America where it is called the Near-equatorial convergence zone or NECZ.



The second image shows a GOES-12 infrared imagery and location of ITCZ one year ago from today. The image reveals a less active ITCZ in 2007 than 2008 in the Eastern Pacific. While in the Atlantic, the ITCZ is at about the same orientation in both years.



The image below is one week's of rainfall accumulation measured by TRMM. The red line represents the moisture gradient, the area where the enviroment would favor tropical wave formation. The greater the moisture gradient between the dry Sahara and moist Guinea Coast, the more frequent occurance of strong tropical waves.



TRMM also revealed the vertical structure of TC NEOGURI on April 17. Notice the very high thunderstorm towers in the eye wall (the region surrounding the eye). These towers aid in the intensification of TCs.



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Understanding the Atlantic Hurricane Season: Technical Terms

By: Weather456, 5:04 PM GMT on April 21, 2008

Technical Terms Simplified

I have compiled and simplified a list of technical terms that we will come across this hurricane season.

Tropical Cyclogenesis: is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere.

Monsoon Trough: a convergence zone, like the ITCZ, that lies between the northeast trades and the monsoon southwesterlies.

Monsoon Southwesterlies: results when the southeast trades cross the equator and are deflected towards the northeast due to the Coriolis force.

African Easterly Wave: a synoptic feature that forms over Eastern Africa and moves westward as convective signatures.

Tropical Upper Tropospheric Trough (TUTT): an upper level trough that develops across the central Atlantic in the summer. It’s different from mid-latitude troughs in that it does not extend into the lower atmosphere and it’s mainly quasi-stationary.

Inverted V signature: a cloud pattern displayed by tropical waves in the Tropical Atlantic that resembles an upside-V.

African Easterly Jet (AEJ): a easterly flowing jet found in the lower part of the atmosphere over Africa that is responsible for the development of tropical waves.

Barotropic Cyclone: where both the temperature and pressure fields increase or decrease towards the center. Example, tropical cyclones.

Baroclinic Cyclone: a cyclone forming within a baroclinic environment and having asymmetric characteristics. Example, extratropical cyclones.

Baroclinic Environment: an environment characterized by vertical wind shear and temperature gradients.

INVEST: an area of disturbed weather persisting for more than 24 hrs outside diurnal variations that has potential to develop and/or threaten land. Invests are assigned by the NHC rotating from 90 to 99. The suffix "L" is assigned to invest in the Atlantic and "E" for eastern Pacific. Example: 90L, 91L, 92L, 93L, 94L, 95L, 96L, 97L, 98L, 99L, 90L, 91L, 92L, and so on.

Diurnal Convective Maximum: the time of the day when instability and convection is highest. It occurs around mid-late afternoon over land and around sunrise over water.

Diurnal Convective Minimum: the time of the day when instability and convection is the least. It occurs just after sunrise over land and around sunset over water.

The Madden-Julian Oscillation (MJO): is an eastward equatorial traveling pattern of anomalous rainfall and convection that is planetary in scale. When the MJO is positive, convection is enhanced and the likelihood tropical cyclone formation is increased.

Tradewind Inversion: an environment over the tropical Atlantic characterized by stable air in the mid-upper levels and stratocumulus or cumulus in the surface layer.

Saharan Air Layer (SAL): a layer of dust originating from West Africa that blows across the Tropical Atlantic.

The Azores High: an area of high pressure found the subtropical Atlantic. Depending its location it is sometimes called the Bermuda High.

Eye wall replacement cycle (EWRC): a process by which an outer eyewall forms, choking and breaking down the inner eyewall. Tropical cyclones often weaken during this process.

Heat of condensation: heat release when water vapor condenses into water droplets

Pressure level: a way of measuring altitude using pressure units.

Lower Level: that part of the atmoshphere between the surface and 600 hpa.

Mid-Level: that part of the atmophsere between 600 hpa and 400 hpa.

Upper Level: that part of the atmopshere above 400 hpa.

Diurnal Cycle (Max and Min) Visualized


Figure 1.The mean diurnal cycle of precipitation features identified from eight years of TRMM PR data over the tropical oceans (a, upper) and land (b, lower) (Image courtesy of Dr. Steve Nesbitt).

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Hurricane Season Outlook Updated

By: Weather456, 9:51 PM GMT on April 06, 2008

2008 Atlantic Hurricane Season Outlook (Updated)

Issued April 06, 2008 by W456

Main Indicators

ENSO
Rainfall Patterns over Western Africa
The North Atlantic Oscillation (NAO)
The Quasi-Biennial Oscillation (QBO)
Sea Level Pressure/500 mb Heights
Miscellaneous Factors such as wind shear and instability
Continuation of above average activity

Overview of Indicators

During El Nino, high SST over the eastern Pacific causes more deep convection there. The resultant outflow aloft enhances upper tropospheric westerlies over the Caribbean and western equatorial Atlantic. Consequently, the 200 mb anticyclonic flow necessary for tropical cyclones to develop is reduced. During Neutrals and weak to moderate La Nina, low SSTs over the eastern Pacific suppress deep convection there. The resultant subsidence enhances lift and weak to moderate upper level easterlies over the Tropical Atlantic Summer, which favors tropical cyclone development.

West Africa represents the birth place of most Atlantic tropical cyclones. It is also the origin of the West African Dust outbreaks known as the Sahara Air Layer. Wetter than normal conditions over Sub-Sahara Africa indicate wetter and cooler tropical waves decreasing the temperature gradient between the sea surface temperature and the 700 mb wave axis and suppressing convection. However, wetter than normal conditions also indicate reduce dust phenomena during the season. Drier than normal conditions produces hotter waves at 700 mb and as they move over the cooler sea surface temperatures, the temperature difference is enough to initiate convection which is needed for cyclogenesis. Though, drier conditions over West Africa means enhanced African Dust.

The Positive NAO index phase shows a stronger than usual subtropical high pressure center and a deeper than normal Icelandic low. The Negative NAO index phase shows a weaker than normal subtropical ridge center and weaker than normal Icelandic low. A stronger than normal ridge lowers SSTs due to increase evaporational cool of winds blowing over the water and due to decrease southerly flow. A weaker than normal ridge implies more ridging in the Central Atlantic and warmer sea surface temperatures over the Atlantic due to increase southerly flow and less evaporation.

During the QBO, Atlantic tropical cyclones are more frequent when 30 mb winds are westerly and increasing, rather than easterly and increasing. There have been 44% more hurricanes and 74% more hurricane days during the west as opposed to east phase of the QBO.

This fifth factor is closely related to the North Atlantic Oscillation. Anomalously high (low) pressure is associated with relatively dry (moist) middle levels, cooler (warmer) midlevel temperatures, and stronger (weaker) 200–850-mb vertical wind shears (John A. Knaff, 1997).

Low (high) vertical wind shear enhances (inhibit) tropical cyclone activity.

The Conditions:

ENSO - A significant La Nina became noticeable from July 2007 and peaked during the winter of 2007-2008. Past La Nina events like in 1988 and in 1999 showed that neutral to weak warm anomalies followed in the years 1989 and 2001 respectively. After which, El Nino developed the years after in late 1991-1992 and late 2002-2003.

Based on these patterns and the weakening trend of La Nina we can make a projection during the summer of 2008, that is, July, August, September, we can begin to see neutral conditions, then weak warm anomalies with an El Nino event in the year 2009.

Using several other analog years where this pattern occurred showed a pattern where the subtropical ridge is centered in the Central Atlantic, consequently, steering most storms North of Caribbean region and along the US East Coast or recurved offshore. Notable storms that made it to land include Hurricane Hugo of 1989. These years also showed at least 2 major hurricanes.

West Africa Rainfall - this pattern remains unchanged with drier than normal conditions over West Africa (Saharan) and wetter than normal conditions over Sub-Sahara and the Guinea Coast and this due to more than active ITCZ. Drier than normal conditions indicate increase African Dust which inhibits TC development but it also means potentially stronger and hotter waves (increase in the low level pressure/moisture gradient between the dry Sahara and the Sub-Sahara/Guinea Coast). The stronger the gradient, the more pronounce the low level monsoon which corresponds to a stronger thermal wind aloft, that is, the African Easterly Jet. These stronger waves have a better chance of surviving and maintaining themselves, especially this year with warm SSTs across the Eastern ATL. It should be noted that dust events may not be a significant factor this year as compared with previous years.

NAO/AO - During the past winter, the NAO and AO was more or less positive and this is indicated by the stronger than normal Subtropical Ridge over the Atlantic. Positive NAO implies stronger ridging, and trades and consequently stronger evaporation leading to a reduction in Atlantic SSTs. This is reflected by looking at any SSTs anomaly chart for March-April. This process however, did not stop SSTs from growing along the Eastern Atlantic where the westward projection of the ridge cause a weaker than normal northerly flow along West Africa and increase southerly flow from the South Atlantic.

During the winter of 1987-1988, the NAO index was more or less positive. That was followed by a negative phase during August-September of 1988. In other years, a persistent positive phase in one season reversed to a persistent negative phase in the following season. Negative values of the NAO implies more ridging in the Central Atlantic and warmer SSTs overall. That is the reason why I think our current ridge should shift more eastward and weaken with time, allowing the current cool sea surface temperatures to recover.

QBO - 50 and 30 mb mb stratospheric winds were in their westerly phase during the early part of 2007 which enhances TC activity, but since then the oscillation has reverse and is currently near the peak of the easterly phase. The easterly phase inhibits TC development. Interestingly enough, the QBO was more or less in the same phase in 1989 (entering peak) and in 1999 and 2004 (leaving peak).

SLP - Pressures have been particularly high across the Subtropics, Caribbean and too a lesser extent the GOM and this can be attributed to the positive NAO and the recent winter. Areas where pressures remained relatively low include the Eastern Atlantic south of 20N, no surprise there. Any changes will be monitored.

Other miscellaneous factors - the central subtropics and tropics have been more stable than normal, while the ITCZ and parts of the Southern Caribbean have been more unstable than normal, with the most prevalent instability in the Eastern Atlantic south of 20N. Wind shear has been below normal over a greater portion of the Eastern Atlantic and across the far West ATL near the US East Coast. Elsewhere is near normal.

In summary, based on the data presented above, it should be an interesting Cape Verde Season.


Figure 1: Warm (red) and cold (blue) episodes based on a threshold of +/- 0.5oC for the Oceanic Niño Index (ONI) [3 month running mean of ERSST.v3 SST anomalies in the Niño 3.4 region (5N-5S, 120-170W)], based on the 1971-2000 base period. For historical purposes cold and warm episodes (blue and red colored numbers) are defined when the threshold is met for a minimum of 5 consecutive over-lapping seasons.


Figure 2: 6-month Accumulated Precipitation % of Normal. Showing drier than normal conditions over West Africa.


Figure 3: The North Atlantic Oscillation Index based on 500 mb height anomalies. Notice the pattern of the negative-positive-negative phases.


Figure 4: QBO 30 mb Index. Gray pointed out that the QBO was in a easterly phase in Jult 2004 and that it would transition to a westerly phase in the following season, that is July 2005. This graph shows the last QBO index was in the westerly phase in 2007 which correlates to an easterly phase in 2008.


Figure 5: Storm tracks during the years 2004, 1989, and 1949.

The Forecast
Based on the above information, adjustments to the normal seasonal average can be made. The 1950-2005 NOAA average is 11.0 named storms, 6.2 hurricanes and 2.7 major hurricanes. Analog years include 2004, 1989 and 1949. The average of those years was 13 named storms, 7 hurricanes and 3 major hurricanes. My current forecast include and addition of 1 storm for ENSO, an addition of 2 storms for African Rainfall, an addition of 2 storms for conditions over the Eastern Atlantic, a subtraction of 1 storm for conditions over the Central Atlantic, and a subtraction of 1 storm for the QBO phase. Lastly, an addition of 1 storm for the continued phase of above normal atlantic activity. The current forecast calls for 15 named storms, 7 hurricanes and 3 major hurricanes.

The next update will be issued May 15 2008.

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The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Weather456's Tropical Weather Blog

About Weather456

With a Bachelors Degree in Environmental Sciences (2009), began tracking tropical storms in 2002 and is now a private forecaster.