Weather456's Tropical Weather Blog

90L Near Depression Status; Watching the Eastern ATL

By: Weather456, 11:25 AM GMT on May 31, 2008

Tropical Invest 90L

Tropical Invest 90L was declared by the NHC yesterday and if anyone was wondering, it’s genesis originated from the energy that broke off from dissipating Alma interacting with a surface trough over the Northwest Caribbean. Initially, the center was rather broad, with a portion over water and another over land. Since then, the circulation has gotten much more define and satellite presentation has improved. Satellite-based winds showed 90L is associated with 32 knot winds, which is enough needed to declare tropical depression and this is supported by my Dvorak estimate which can be found below. However, 90L is very close to land and moving ever so closer. Some development may take place before 90L moves ashore. Regardless of whether a depression forms or not, heavy rains are expected over Belize and the Yucatan, also the Caymans and parts of Cuba.

Movement over the next 24 hrs is pretty simple. A northwestward motion is expected under the influence of a deep layer ridge over the Southwest North Atlantic/SE USA. Over the past 24 hrs, this ridge has shift towards the east which would allow more northwest movement. 90L should emerge over the Bay of Campeche or Gulf of Mexico in 24-48 hrs should it survived the crossing over the Yucatan Peninsula. At that point, another ridge is expected to build over the Southern Plains and force 90L back westward into Mexico. The area will be monitored then, as conditions appear marginally favorable for development.

Goes-12 Infrared Image of 90L



Eastern Atlantic

Elsewhere, the NHC/TPC confirmed the tropical wave that was mentioned on previous updates. The wave is currently moving towards the west and is now along 18W south of 14N. The wave is exhibiting well-define cyclonic turning in the mid-lower levels. An earlier QuikSCAT pass showed no circulation yet at the surface. Clusters of moderate to strong convection surround this well-define feature. Conditions do appear favorable for development but I am just going to watch the wave for signs of development, since it poses no immediate threat to anyone in the near term and because conditions are favorable does not mean the wave will take advantage of this. This feature will be steered westward under the influence of a strong Azores/Bermuda Ridge.

Goes-12/METEOSAT Composite Infrared Image taken 2am EDT



Why so many impressive waves in May?

1. Wetter than average Sub-Saharan Africa

2. Favorable African Easterly Jet

3. Warm waters off the coast of Africa helped much of these waves to about 30W, after which most encountered cooler waters and a somewhat drier environment.

Dvorak Classification of 90L

Curved Band Pattern – 0.40 spiral which gives a T.no. of 2.0 or 30 knots/1009 mb.

Dvorak Enhanced Infrared Image of 90L taken 6am EDT



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Alma Affecting Central America & NW Caribbean

By: Weather456, 1:42 PM GMT on May 30, 2008

Tropical Storm Alma

Tropical Storm Alma made landfall on Thursday near the Nicaraguan colonial city of Leon with winds of 65 mph. The storm was initially forecast to reach hurricane status before reaching land but made landfall without officially doing so. However, based on satellite organization seen on visible imagery near peak intensity, Alma may have reached hurricane intensity.

Nevertheless, the storm dumped very heavy rains across Costa Rica and Nicaragua causing flooding to many communities. Landslides blocked a few highways. In addition, Alma’s winds destroyed houses and knocked out power in Leon, Nicaragua. Alma’s quick intensification offshore caught many forecasters in Central America by surprised with residents scrambling to prepare before the storm hit. Alma’s death toll now stands at one, when a man was electrocuted. This death toll may rise, not necessarily in Nicaragua and Costa Rica but in Honduras and Guatemala, where the weakened storm continues to dump very heavy rains across that mountainous region.

"The wind whipped up the sand, and it lashed your face like sandpaper," said Erasmo Lopez, a fisherman in the coastal hamlet of Poneloya, near where Alma made landfall. "The trees were shaking like crazy, cars were shuddering, and you couldn't even see in front of you."

Despite this, Alma could of been much worst and the damage reports seem to have been isolated.

Alma’s effects are not only being felt across Central America, but associated heavy convection and rains over the Northwestern Caribbean may affect the Caymans, Cuba, Jamaica and even Haiti.

Most models indicate Alma will continue towards the north and emerge over the Gulf of Honduras, then turn towards the west under the influence of a low-mid level ridge to its north. There is some possibility that development may occur over the Northwest Caribbean Sea. Currently, surface observations indicate a small surface circulation in the Gulf of Hondras which is well west of the deepest convection and still being influence by Alma. The area will be monitored and updates posted.

TRMM Measured Precipitation over a week of deluge



Early Morning Visible Image of Tropical Depression Alma and the Strong Area of Energy over the NW Caribbean



Elsewhere, a possible well-define tropical wave is approaching 10W south of 14N moving west near 10-15 knots. Visible imagery and numerical computer analysis continue to show lower mid-level turning along the axis near 5N which appears to have gotten more define over the past 24 hrs. Clusters of moderate to strong convection mainly ahead of the axis. Computer models are hinting the development of a weak low pressure area and conditions are forecast to remain marginally favorable to support development. The chance of this occurring remains at 20%.

MSG-2 Visible Image of the West African Coast





A boy walks on a flooded street in Leon city, some 57 miles (90 km) west of Managua, May 29, 2008. Tropical Storm Alma, the first cyclone of the Americas hurricane season, slammed into Nicaragua's Pacific coast on Thursday, killing one person as winds toppled trees and ripped roofs off flimsy homes. REUTERS/Oswaldo Rivas (NICARAGUA)

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Alma To Bring Moderate to Severe Flooding Threat

By: Weather456, 3:56 PM GMT on May 29, 2008

Tropical Storm Alma

The first tropical storm of the Eastern Pacific season continues to dump heavy rains across parts of Central America from Costa Rica to Honduras/Guatemala., potentially causing life threatening flash floods and mudslides. Rain accumulations over the past 12 hrs have been between 50-100mm and there is a moderate to severe flood threat especially along the Pacific sides the mountains of Costa Rica and Nicaragua.

This morning’s visible imagery showed an impressive feature with a uniform cloud overcast and impressive banding which is being supported by excellent upper level support. Along with visible imagery, infrared imagery and microwave products show a possible eye feature forming with Alma, indicating an intensifying tropical storm. I would expect Alma to reach strong tropical storm/category one hurricane status over the next 6-12 hrs. Movement over the next day or two appears to be northward which should bring it ashore over Central America. After which, Alma is expected to turn more towards the northwest under the influence a building ridge over Mexico.



South of Jamaica

Visible satellite animation along with satellite-derived products at the CIMSS indicated that area of convection to the south of the island of Jamaica is assoicated with a forming mid-level circulation. Currently, there are no signs of a surface circulation associated with this feature, and I'm not suprise given the area at the surface is dominated by the flow assoicated with Alma. However, as Alma moves inland over Central America and weakens, a tropical wave interacts with the area, and enviromental conditions seem marginally favorable for development, its an area to watch, but dont expect much until there are signs of it at the surface.

Eastern Atlantic Development

A tropical wave is located along 6W south of 13N moving west near 12 knots. Satellite analysis of this feature supports mid-level turning along the axis roughly near 5N. Scattered showers and thunderstorms cover much of the wave feature mainly in the southern portion. The CMC, GFS and ECMWF are all hinting possible development of this feature as it merges in about 54-72 hrs time. In the near term this seems possible due marginally favorable environmental conditions, however, as the system moves further west it encounters more hostile conditions and this is reflected in the model runs. In the meantime, the system is expected to move towards the northwest over the next 5-6 days, then more west-northwest under the influence of a building mid-upper ridge near the Azores. Climatology doesn’t normally favor development in this area and environmental conditions are forecast to only be marginally favorable, therefore, I place a 20% chance of development over the next 5 days.

MSG-2 Visible Image



Elsewhere, there are no other areas of concern.

MSG-2/GOES-12 Composite Infrared Imagery at 8am EDT



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Tropical Invest 90E Organizing

By: Weather456, 11:43 AM GMT on May 28, 2008

The NHC has designated the area in the Eastern Pacific as Tropical Invest 90E. An invest is an area of disturbed weather persisting for more than 24 hrs, has the potential to develop and/or affect land. This morning's infrared satellite animation show an organizing system with clusters of thunderstorms developing near the center of circulation. Environmental conditions, such as low vertical shear, poleward outflow, and favorable sea surface temperatures, indicate continue organization of 90E and it is possible to have a depression by Thursday. The steering flow across the area is rather week but by observing the flow around the system can indicate near-term movement, which is supporting eastward movement. This is the movement being observed also in infrared imagery. In the long-term, a mid-upper ridge is expected to build over the Caribbean, allowing a more northward movement and this occurs after 48 hrs.

The development of 90E and its close proximity to the Southwest Caribbean diminishes any chances of something forming there in the near term. There are number of other factors against SW Caribbean develop, like the shear forecast over the next couple of days. I would say the chance of something forming in the Caribbean in the next 48 hrs is 10% with higher chance thereafter.

Very heavy rains and gusty winds are expected along the ithmus of Central America, especially in Costa Rica. These rains have the potential to cause flash flooding and landslides.



Elsewhere, no development is expected over the next day or two. A tropical wave is currently moving through our islands in the Northeastern Caribbean bringing scattered showers and thunderstorms and some breezy winds.

A tropical wave is also near 21W south of 13N moving west near 10-15 knots. This is wave that ermerged off the coast over the past 24 hrs.

There is still one more unconfirmed African Easterly Wave located near 3E south of 15N moving west near 15 knots. The wave is rather strong in terms of convection and should ermerge later this week, where some computer models continue to hint development.

MSG-2/GOES-12 Infrared Imagery at 5am EDT




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The Tropics Come Alive

By: Weather456, 10:33 AM GMT on May 27, 2008

Good Tuesday morning to all!

Well the 2008 Atlantic Hurricane Season official begins on Sunday but that isn’t stopping the slumbering giant from waking up and giving us some preview action of what is suppose to an above average hurricane season. There are 4 areas of interest out there but only two has prospect of development.

Possible Development Later This Week

The first area is a broad area of low pressure that extends from the Eastern Pacific across the isthmus of Central America near Costa Rica/Nicaragua and into the Southwest Caribbean Sea. There are two areas of maximum low level vorticity located in the Southwest Caribbean just offshore Nicaraguan/Costa Rican border and in the Eastern Pacific just west of Nicaragua. The 00Z and 06Z model runs show development of the Eastern Pacific area in the next day or two and develops a weaker low pressure around the Gulf of Honduras (Western Caribbean) that moves into the Yucatan and Bay of Campeche later this weekend as the disturbance moves north from the SW Caribbean Sea. It is rather unusual to have development taking place in both basins but the consensus is that. However, until the disturbance becomes more define, we can’t really give much confidence in the track.

As for now, infrared satellite animation showed no signs of development; rather, it shows signs of vertical wind shearing. However, showers and thunderstorms have been increasing near the analyzed low level circulation seen on last night's ASCAT pass just offshore the Nicaraguan/Costa Rican border. In addition, the situation will be monitored as a westward moving tropical wave in the Central Caribbean interacts with the disturbance and wind shear is forecast to relax over the upcoming days. Regardless of development or not, heavy showers are expected over parts of Central America.

Showers for the Islands

The second area is a tropical wave along 56W and south of 23N moving west near 10-15 knots. The wave is being sheared by westerly winds aloft which is also providing diffluent flow aloft, fueling scattered showers and thunderstorms east of axis south of 15N. This wave will enter the Eastern Caribbean in 24 hrs bringing the prospects of more moisture across the islands, especially to the Leeward Islands.

Central Atlantic Wave

Continuing moving east....we have a tropical wave out there along 30W south of 10N moving west near 10 knots. Infrared satellite animation shows inverted-v signature associated with this wave. The wave is also rather convectively active with moderate to strong clusters of thunderstorms surrounding the axis. Conditions appear only marginally favorable for development over the next 5 days.

Possible Development Again

The last set of areas are across West Africa...two clearly evident tropical waves lie along 12W south of 13N and 11E south of 13N. Showers have diminish along the wave closest to the coast but the GFS, ECMWF, and CMC have been hinting the development of a tropical cyclone around Saturday/Sunday when either of these waves emerges.

GOES-12/MSG-2 Composite Infrared Imagery at 2am EDT



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Tracking Possible Development by Thursday

By: Weather456, 12:04 PM GMT on May 26, 2008

Tracking Possible Development by Thursday

I am currently watching an area of showers and thunderstorms just off the west coast of Central America that appears to be associated with a broad area of low pressure and monitoring for any signs of development. This area is being developed by most models to either advect into the Southern Caribbean Sea or Northwest Caribbean Sea. Because of the model consensus and the latest infrared and shortwave imagery, this situation seems likely over the next few days. Due to the lack of surface observations, visible light and QuikSCAT/ASCAT sea winds, its difficult to pinpoint where the center of this broad area of low pressure is. However, utilizing other tools like the GFS Analysis and satellite derived winds at the CIMSS it appears the area is just south of Guatemala and west of Costa Rica/Nicaragua. Conditions appear only marginally favorable for the development of a tropical system in the Eastern/Western Caribbean but expected to improve over the next 5 days.

Bottom Line:

Most models are in consensus that the initial disturbance will form in the Eastern Pacific (This is highly likely based on the observations presented above), whether it develops into a tropical depression or name storm there, or where will it advect is yet to be seen.

Most models are also in consensus that some sort of advection will occur into the Caribbean Sea and at some point a storm is expected to be in the NW Caribbean Sea.

A tropical wave is along 73W south of 13N moving west near 10 knots. This appears to be wave is enhancing vigorous convection over Western Venezuela/Eastern Colombia.

Elsewhere, there are two possible tropical waves over Africa. One is located over Western Africa along the coast near 7W south of 13N, while the other is located over Central Africa near 20E south of 13N. There appears to be enough evidence to suggest these features are tropical waves. The latter wave is the strongest and the GFS and ECMWF have been hinting some development of this feature when it emerges around June 1.

Goes-12/MSG-2 Composite Infrared Imagery of the Atlantic at 2am EDT



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Hurricane Preparedness Week; The Tropics; Hurricane Journal Part 1

By: Weather456, 1:06 PM GMT on May 25, 2008

Hurricane Preparedness Week


May 25th, which is today, to May 31st, marks Hurricane Preparedness Week 2008, which is centered on five main themes; history (25th), hazards (26th-28th), forecast (29th), preparedness (30th) and action (31st).

National Hurricane Preparedness Week highlights the vital importance of being prepared when natural disasters strike, such as hurricanes. It is not just confined to the United States, but should be implemented elsewhere across hurricane alley, particularly the Caribbean which has suffered great destruction over the past years.

The week begins with the theme “history”, which teaches that a lack of hurricane awareness and preparation are common threads among all major hurricane disasters. By knowing your vulnerability and what actions you should take, you can reduce the effects of a hurricane disaster.

The tropics move forward but we only understand it by looking backwards – Modified from a quote by a Dutch Philosopher.

History has also taught us, that despite our efforts throughout time, there are still inadequacies in preparation and response to hurricanes, and a great example would be the aftermath of Hurricane Katrina in 2005. Despite the National Hurricane center’s efforts to get the word out there, there are still communities across the Caribbean that lack this information and this manifested in 2005 with Hurricane Stan and 2007 along the Mosquito Coast with Hurricane Felix. Furthermore, impoverish nations such as Haiti; still suffer large losses in life till this day. Most recently, Hurricane Noel of last year and Hurricane Jeanne in 2004 caused great loss in life. There should be some system in place in these nations to better get the word out to isolated communities. However, due to a lack of finance, these nations may continue to suffer the brunt of these storms.

There is some progress, however, especially in the Lesser Antilles, where, despite huge financial losses, deaths are minimal. In Cuba, they have an extraordinary preparedness and response system, possibility the best across hurricane alley. I saw this come to past in 2005, during the passage of Hurricane Dennis in 2005 which rampage across the length of the island as a category 3/4 hurricane, affecting millions but killing few. Kudos to them. Another nation that should be applauded for thier efforts is Mexico and hwo they handled Hurricanes Emily and Wilma in 2005 and to some extent Dean in 2007.

I feel over the upcoming years as hurricane forecasting become more accurate, and nations develop more sound economic and social ties, we can begin to reduce the burden of hurricanes.

NOAA’s Hurricane Preparedness Week Website
List of Atlantic Hurricanes in comprehensive categories

The Tropics

Well the models are rather split on the situation with the possible development in the Eastern Pacific and Western Caribbean. The 00Z runs of the CMC and UKMET and the 06Z GFS all showed development taking place in the Eastern Pacific, and moving northward into Guatemala, Central America. On the other hand, the 00Z runs of the ECMWF, NOGAPS and the 06Z GFS all showed some development taking place in the Western Caribbean; moving from Honduras into the Northwest portion and into the Yucatan Peninsula. The general time frame of genesis is around the 28/29th. Based on this, we cannot answer many questions, like where will take place. So we have to rely on other tools to do this. This morning’s infrared imagery showed much of the shower and thunderstorm activity lies in the Eastern Pacific and this is the area that most models agree on that the initial disturbance will originate. In addition, visible satellite animation showed broad cyclonic turning just off the West Coast of Central America.

What the latter models are hinting on is that, the westerly flow in the Eastern Pacific will be enough to cause the disturbed area to enter the Southern Caribbean, but this morning’s satellite derived winds, ASCAT/WindSAT sea winds and surface observations all indicate this flow is weakening. Furthermore, genesis potential and climatology favors development in the Eastern Pacific. This is not to say, that development in the Western Caribbean Sea is impossible, as there continues to be a persistent low level trough/circulation over Panama along with the forecasted upward pulse in the MJO. Also with a weakened westerly flow, it is possible this system moves northward into Central America and emerge and strengthen in the Northwestern Caribbean and any one part of that track seems to be in agreement with most models. However, currently, the intial disturbance will most likely occur in the Eastern Pacific.

Elsewhere, there are no other areas of interest. There remains prevalent dry conditions across much of the Caribbean and tropical Atlantic basins. This should gradually change over the next week or two.

Atlantic Hurricane Journal

A record of data and observations collected over the years tracking hurricanes. Reports before 2002, that is, before the use the internet, where obtain from the Weather Channel. The Journal is quite long so here’s the first part.

Formation Locations in order of number of storms

Caribbean Sea
Gulf of Mexico
Southwest North Atlantic
Tropical Atlantic

Formation Sources

Persistent area of thunderstorms or convection (synoptic in size)
The monsoon trough, NECZ, or ITCZ
A tropical wave
A dissipating frontal boundary or frontal low
An extratropical cyclone/a subtropical cyclone
An upper level low
A surface trough of low pressure (other than NETs)
The interaction between remnant disturbances, tropical waves, upper–mid tropospheric troughs

Ingredients for formation and development

Low wind shear in the vertical
High wind shear in the horizontal or high low level positive vorticity
Warm, deep sea surface temperatures
Tropospheric Humidity in the low to mid levels
Potentially Unstable Atmosphere/Rapid cooling with height
A Threshold Value of Earth's Vorticity (Coriolis parameter)
A low level disturbance (see above)
Upper tropospheric outflow
A large moist air mass extending 6° from the disturbance

Part 2 will deal with ingredients of weakening and signs of developing and weakening systems.


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My Thought's on Noaa's Outlook/The Tropics

By: Weather456, 11:12 AM GMT on May 23, 2008

NOAA Hurricane Outlook

NOAA issued their 2008 Hurricane Season Outlook yesterday morning, predicting a near-normal to above normal hurricane season with a higher chance (65%) of above normal activity. This forecast was based on ongoing conditions that have been conducive to above-normal Atlantic hurricane seasons since 1995 and the lingering effects of La Nina.

Comparisons with my forecast

I issued my hurricane season outlook late last week, predicting an above average hurricane season with 15 named storms, 7 hurricanes and 3 major hurricanes, which falls right between NOAA’s forecast of 12-16 named storms, 6-9 hurricanes and 2-5 major hurricanes. The outlook stated that even though La Nina would dissipate this season, the lingering effects would still impact the peak of the hurricane season and there is very little chance of an El Nino developing by the peak months. In contrast, I forecasted La Nina to continue gradually weakening and expecting neutral-slightly warm conditions this season which would enhance tropical cyclone activity in the Atlantic basin. NOAA also used continuation of above average hurricane season despite 2006 and 2007 (I think it should have said "despite 2006" because 2007 was an active year). However, this was also an indicator used in my forecast which added an addition of one tropical cyclone per year.

The Tropics

Two areas of interest are the Southwest Caribbean and a rather impressive tropical wave for May in the Eastern Atlantic. First, let’s look at the Southwest Caribbean where the GFS has been consistently developing a tropical cyclone there for the past week and the 00Z ECMWF is also hinting low pressure in the Northwest Caribbean, competing with a more developed storm in the Eastern Pacific. The initialization date and track has been inconsistent, with the track jumping around and the initialization date fluctuating between the 26th and 29th. There is more model consensus with developing an area of low pressure in the Eastern Pacific. Currently, we are expecting an upward pulse in the MJO over the area around June 1 and all other conditions appear marginally favorable for development of this system, so its something to watch, but confidence remains low.

Currently, an upper level ridge over the area continues to help fuel showers and thunderstorms over a broad area over Central America and the Eastern Pacific Ocean.

The second area is a tropical wave along 26/27W south of 10N moving west near 10 knots. The wave continues to display excellent cyclonic turning along the axis seen on infrared and visible satellite animations. This morning’s QuikSCAT pass revealed an impressive close circulation near 5.5N-26W with associated rain contiminated winds of 25 knots (black barbs) . However, for this to be classified a tropical depression, that close circulation must remain identifiable for at least 24 hrs and it has to acquire sufficient deep convection, which is difficult to do due to the dry air interacting with the system. Scattered moderate showers are associated with this feature. Conditions appear favorable for development but expected to become unfavorable over the next 5 days. It is interesting to note that both the GFS and ECMWF was hinting a possible low pressure, like the one seen here, last week.

Any updates will be posted in the comment section as usual, if anything changes or this area shows signs of development.

As I promised, the Atlantic Tropical Cyclone Journal will be posted in the next blog entry.

MSG-2 Visible Imagery



QuikSCAT pass taken 1:57am EDT May 23 2008



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The Tropics Today

By: Weather456, 11:29 AM GMT on May 22, 2008

A broad zone of showers and thunderstorms is located over Central America and the Southwest Caribbean Sea and Eastern Pacific between 95W and 75W. This activity appears to be induced by several factors. The most persistent has been upper level outflow enhanced by an upper level ridge across the area along with surface convergence associated with the ITCZ and a passing tropical wave along 80W. The 06Z GFS run is hinting the possible development of a weak low pressure on the Eastern Pacific side of this activity.

There are no other areas of disturbed weather to speak of in the Gulf of Mexico or Caribbean. Dry air remains in place over much of the area, but the islands may get a sprinkle or two today as patches of stratocumulus clouds invades the area.

I continue to monitor an impressive tropical wave that emerged off the coast around Tuesday. The wave is located along 22/23W south of 11N moving towards the west near 6 knots. This morning’s visible satellite animations showed an impressive inverted-V signature with this tropical wave. However, development is not expected as there is too much dry air and upper level winds are too strong ahead of this feature. Scattered showers and thunderstorms along both sides of the axis associated with the ITCZ.

There are no other interesting features to speak of elsewhere. The 06Z GFS model run continues to hint possible development in the Western Caribbean late next week. The GFS has been inconsistent with the track, basically jumping around the place, but, this is common and the model has been pretty consistent with developing something in the SW Caribbean Sea for days now, so it’s an area to watch. There is still not much model agreement, however. As time goes on, confidence may grow, but currently it remains low.

Any updates on any of these systems are posted in the comment section of this blog.

MSG-2 Visible Imagery



NOAA is expected to issue their 2008 Hurricane Season outlook later this morning, which I will give my comments on a later blog entry.

I will also be posting my Atlantic Hurricane Journal on a later blog entry. It is a little file, I kept summarizing most aspects of Atlantic Tropical Cyclones, such as formation sources, location of formation, ingredients for development/weakening, signs of development/weakening, rules of thumbs, just to name a few.


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Tropical Update

By: Weather456, 1:23 PM GMT on May 19, 2008

If you have any concerns, questions or suggestions about tropical weather or the hurricane season, always feel free to leave a comment or drop an email, and I will get back ASAP.

There are two areas of interest out there. The first area is in the Southwest Caribbean and the other is just along the West African Coast.

Scattered showers and thunderstorms are located in the Southwest Caribbean/Eastern Pacific associated with a broad area of low pressure spinning over Panama according to QuikSCAT and satellite animations. Little development is expected in the next 24 hrs since the low pressure area remains over land. In addition, should the area move over the waters of the Southwest Caribbean, there is little time available for development before it moves west into Central America. The 06Z GFS model run shows some of the energy of this system developing in the Eastern Pacific once it crosses Central America.

The second area of interest is an area of vigorous convection located along the West African coast, associated with an emerging tropical wave.

Elsewhere, there are no other areas of interest. There is plenty of dry air remaining in place over the Caribbean and tropical Atlantic Basin. None of the computer models is predicting development in the Atlantic basin through the next 7 days. Remarkably, the long-range GFS model continues to forecast the development of a significant tropical cyclone in the Northwest Caribbean beyond 1 week.

Above: GOES 12 Visible Image taken 8:45am EDT
Bottom: QuikSCAT satellite sea winds pass at 7:17am EDT




METEOSAT-9 Infrared Image



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Tropical Update

By: Weather456, 12:02 PM GMT on May 18, 2008

Significant dry air continues to dominate the Caribbean basin north of 15N, thereby deep convection remains absent. Further south, upper level westerly flow created by an upper ridge over South America is fueling scattered cloudiness and showers from the Southwest Caribbean to the Southern Lesser Antilles. The GFS model has been hinting the development of a weak low pressure area in the Southwest Caribbean by the 22nd, but the model has failed to initialize the system in the last three runs. Conditions do appear favorable for development in this area of the Atlantic, so it’s an area to watch. Any feature forming will move westward into Central America, becoming mainly a rain maker.

A tropical wave has entered the Eastern Pacific along 82W south of 10N moving west near 5-10 knots. This wave is associated with little shower activity but is likely enhancing isolated thunderstorms over Panama.

As one tropical wave exits South America, another has moved ashore near 58/59W south of 10N moving west near 10 knots accompanied by little or no shower activity.

A third tropical wave is along 39W south of 10N moving west near 13 knots based on Hovmoeller diagram estimates. The wave is very evident with most tools used to track tropical waves, including visible satellite animations which shows an excellent inverted-V signature along the ITCZ. Scattered to isolated shower activity accompanies this wave.

A fourth feature is a persistent area of showers and thunderstorms in the far Eastern Atlantic south of 9N between 30W and 15W. This morning’s visible satellite animations showed the possibility of a tropical wave and an area of low pressure forming along 23/24W. Clusters of moderate to strong thundershowers accompany this feature.

A fifth feature is over Western Africa, and there is enough evidence to suggest the presence of an African Easterly Wave. This feature expected to exit the coast by Tuesday.

Outlook for the last 2 weeks of May

The GFS is predicting continued favorable conditions in the Western Caribbean with the Madden-Julian Oscillation (MJO) expected to enhance moisture and convection over the area during the remainder of this month into early June. This coincides with the formation of a tropical storm in the Northwest Caribbean on May 31 that the GFS has been remarkably persisting for 48 hrs now. This is long-range; and I don’t normally look at or rely on models past 1 week for development, but I chose to mentioned it since I was giving an outlook on the last 13 days of May.

GOES-12 Water Vapor image taken 6:45am EDT



GOES-12/METEOSAT Infrared composite image taken 2am EDT



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The Wave Train Continues

By: Weather456, 11:29 AM GMT on May 16, 2008

Hurricane Season Outlook

The wave train is certainly becoming more crowded now with the fourth tropical wave of the season added late yesterday to surface maps.

The first tropical wave is near the Venezuela/Colombian border along 72/73W, south of 12N moving west near 20 knots. The wave axis was determined by looking at few surface observations over Venezuela, which showed a fall in pressure and rise in relative humidity values coinciding with northeast-southeast wind shifts. The wave continues to be associated with westward propagation of moderate to strong showers and thunderstorms. It is likely that the northern portion of this wave may enter the Southwest Caribbean Sea without development occurring.

The second tropical wave is noted along 49/50W south of 11N moving west near 10-15 knots based excellent inverted-V signature and mid-level rotation seen on this morning’s visible satellite animation. In addition, total precipitable water images show a surge in the low level moisture field near the analyzed wave axis and low level derived winds from the CIMSS depict a wide wave length. Clusters of moderate to strong convection cover most of this wave axis.

Another tropical wave is noted near 29W south of 11N moving west near 10-15 knot, but the latest visible satellite animation and TPW images show the wave axis maybe further east near 27W, where the crest of the inverted-V signature is seen. Despite this, the wave remains define with clusters of convection surrounding the wave axis. Additional convection is further east associated with the ITCZ.

A fourth tropical wave maybe emerging off the coast of Africa.

The total number of tropical wave analyzed so far in the first 2 weeks of May 2008 is now 5. The last time we had that many waves by May 16 was in 2005.

Elsewhere, there are no other areas of concern. The GFS model continues to predict a low pressure area will develop in the Southwest Caribbean but moves into the landmass of Central America, which cause me to suspect that it’s the typical Colombian Low that pushes off the coast along the ITCZ now and again.

The GFS long-range model is also predicting the prospect of more moisture for the Caribbean which has been bone dry over the past couple of months.

GOES-12/METEOSAT Infrared Composite image of the Atlantic Ocean taken 2 am EDT/AST



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Hurricane Season Outlook/The Tropics

By: Weather456, 11:21 AM GMT on May 15, 2008

Eastern Pacific Hurricane Season

Today, May 15, marks the start of the 2008 Eastern Pacific Hurricane Season. Based on the latest averages, a typical season sees 15 named storms, of which 9 becomes hurricanes and 4 of those hurricanes attain major hurricane status. The season ends the same time as the Atlantic Hurricane season on November 30.

Currently, there are no threats to the West Coast of Central America or Mexico.

Atlantic Tropics

The third impressive tropical wave of the season rolled off the coast of Africa yesterday morning and is currently along 21/22W south of 12N moving west near 12 knots based on Hovmoeller diagram estimates. The wave axis was analyzed based on low-level derived winds at the CIMSS along with numerical models. This morning’s visible satellite animation showed clusters of moderate to deep convection remain well ahead of the analyzed axis between 23W and 27W. Despite conditions being favorable for tropical cyclone formation, the maximum vorticity center along the axis is below 10N, which will inhibit a circulation from forming. However, this feature should be monitor incase it does move above 10N or show signs of development as the 5-day forecast is showing less significant dust and continued low wind shear just below 14N.

A second tropical wave continues to move west and is now located over Central Venezuela near 64W south of 10N, moving west near 10-15 knots. The wave is very ill-define but is likely enhancing isolated shower and thunderstorm activity over the area.

Elsewhere, there are not other areas of concern; however, the GFS model has been persistently developing an area of low pressure in the southwest Caribbean over the past week now. This morning’s model run at 2 am EDT/AST showed a disturbed area just off the coast of Nicaragua and Costa Rica on the 22nd which is within the more reliable time frame of one week. Any updates on this possible feature will be posted.

Infrared image of the entire Atlantic taken 2 am EDT/AST this morning.



2008 Atlantic Hurricane Season Outlook (Updated)

Issued May 15 2008 by Weather456

The Atlantic hurricane season is expected to be well-above average with 15 named storms, 7 hurricanes and 3 major hurricanes. The areas of most concern are the Northern Caribbean and East Coast of the United States with lesser threats along the Gulf coast, Central America and the Southern Lesser Antilles. There is a high probability that the Cape-Verde Season this year will be exceptionally active, with not so much activity in the Central Caribbean and Gulf of Mexico.

Main Indicators

ENSO (El Nino & La Nina)
Rainfall Patterns over West Sahel and the Subtropical East Atlantic
The North Atlantic Oscillation (NAO)
Mean Sea Level Pressure
Vertical Wind Shear
Sea Surface Temperatures (SSTs)
Continuation of Above Normal Activity since 1995 and New Tools and Technology

Changes

This update will focus more on the factors that have a direct impact on tropical cyclone activity in the Atlantic basin. It excluded the Quasi-Biennial Oscillation (QBO) due to the facts that this factor has become less reliable over the years and it’s not directly a measure of tropical cyclone activity. This update also includes possible tracks of Cape Verde-type Hurricanes.

Overview of Indicators

El Niño-Southern Oscillation (ENSO)

During El Nino, high SST over the eastern Pacific causes more deep convection there. The resultant outflow aloft enhances westerly wind shear 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 rising motion and weak to moderate upper level easterlies over the Tropical Atlantic Summer, which favors tropical cyclone development.

Rainfall Patterns over West Sahel and the Subtropical East Atlantic

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 more convective tropical waves increasing the temperature gradient between the sea surface temperature and the 700 mb wave axis and enhancing convection. 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, this creates a temperature gradient that reduces convection. Drier conditions over West Africa means enhanced African Dust.

The North Atlantic Oscillation (NAO)

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 cooling 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 cooling effects by evaporation.

In addition to the effects on SSTs, weaker trades favor tropical cyclone activity (negative NAO index) while stronger than normal trades suppresses tropical cyclone activity (positive NAO index). This is because, the circulation needed for tropical waves and other disturbances to be classified as tropical depressions are disrupted under too much northeast wind flow.

Overview of Remaining Factors

Tropical cyclone activity is enhanced (suppressed) when mean sea level pressure is below normal (above normal), vertical wind shear is below normal (above normal), and sea surface temperatures are above normal (below normal).

Continuation of Above Normal Activity since 1995 and New Tools and Technology

Since 1995, Atlantic tropical cyclone activity has been in upward swing.

Depiction of new monitoring and analysis technologies (advanced microwave sounding unit tropospheric temperatures [Brueske and Velden, 2003], QuikSCAT [Atlas et al., 2001], and the cyclone phase diagram analyses [Hart, 2003]) have increased Atlantic tropical cyclone counts by about one additional system per year.

Summary of the Conditions:


Table 1: Showing the various indicators, their expected impact during the 2008 Atlantic Hurricane Season.


El Niño-Southern Oscillation (ENSO)

Due to the weakening trend of La Nina over the past few months and the model forecasts presented in figures 1 and 2, it’s fair to say that we will experience a neutral to slightly warm episode this hurricane season. This is not so good, as past experience have shown that neutral years are the most active on record. Table 2 shows the top 10 active hurricane season in the past 58 years. It is clear that neutral to warm episodes occurred in the most active years on record.


Table 2: Showing the top 10 active hurricane seasons between 1950 and the present and the respective ENSO episodes during the months of July, August and September.

Rainfall Patterns over West Sahel and the Subtropical East Atlantic

African easterly waves vary interannually, and these variations have been found to have an impact on the seasonal hurricane frequency. Increases in rainfall over the western Sahel have been associated with more frequent and stronger waves, and with the occurrence of more intense Atlantic hurricanes.

I have notice that convection along the ITCZ, over the Eastern Atlantic and Sub-Saharan Africa has been more active than normal over the past few months. Also, the subtropical Atlantic was wetter than normal over the past month indicating below normal dust outbreaks there. Dust will always be an inhibiting factor, so we look at the frequency and strength of the dust outbreaks by looking at rainfall patterns, which I expect to be below normal levels this season.

The North Atlantic Oscillation (NAO) and Mean Sea Level Pressure (MSLP)

What factors are there to suggest the oscillation is in its negative phase?

1) Negative NAO is reflected with below normal-weak trade winds and drier than normal conditions across the Southeast United States.

2) The index is showing negative values

3) Mean sea level pressure was below normal over the Eastern Subtropical Atlantic for the month of April which further indicates a weaker than normal high pressure system.

The index should remain in the negative phase over the next two weeks as indicated by the ensemble mean NAO forecast. Beyond that, the ECMWF is forecasting lower than normal sea level pressure in the subtropical Atlantic during July, August and September which supports more or less negative values this hurricane season.

Remember, the negative phase enhances the likelihood of tropical cyclone formation and also increases the risk along the Northern Caribbean and East Coast of the United States. The latter was discussed in a previous blog entry.

Vertical Wind Shear

Vertical Wind shear was below normal levels this past month and is expected to remain below normal levels through the main development region (MDR) through the peak months of August, September and October. This clearly supports the ENSO episode we are expected to be in this hurricane season.

Sea Surface Temperatures

When compared to previous years, sea surface temperatures and ocean heat content are the lowest in 2008 and this due to the activity of the past winter which was one of the most active since 1999. Sea surface temperatures are expected to be near normal this hurricane season, which is well above the 26.5 degree threshold needed for tropical cyclone formation, so it should not be a major factor in the number of named storms, but reduces the occurrence of hurricanes and major hurricanes. Sea surface temperature is a powerful factor when determining TC activity. However, this hurricane season maybe more active than mentioned here if sea surface temperatures warm significantly over the next three months.

The Forecast

Based on the above information, adjustments to the normal seasonal average can be made. An average hurricane season based on over 150 years of records is 10.0 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 includes a plus factor for ENSO, Western Sahel rainfall, both the NAO index and its associated MSLP, below normal vertical wind shear. Lastly, an addition of 1 storm for the continued phase of above normal Atlantic activity and increase detection of tropical cyclones in the Atlantic. The current forecast calls for 15 named storms, 7 hurricanes and 3 major hurricanes. I was bit conservative on the issue of the sea surface temperatures since this is major fuel for tropical cyclones.




Figure 1 & 2: Sea surface temperature anomalies for Nino regions 1, 2 and 3.4 (Eastern Pacific and Central Pacific) up until January 2009 from NOAA’s CFS model. The red lines indicate the boundary of typical neutral-episodes conditions.


Figure 3: a) Easterly wave variations during wet versus dry years in Western Sahel, b) 90-day rain accumulations % of normal, showing wetter than normal conditions along Western Sahel and c) TRMM rainfall anomalies over the past 30 days showing average to slightly above normal rains over the Eastern Atlantic Ocean.


Figure 4: Mean sea level pressure (MSLP) anomaly forecast through July, August and September showing a weaker than normal pressures in the vicinity of the Subtropical High pressure belt with normal pressures elsewhere.


Figure 5: Wind shear departure from average (left) and sea surface temperature (SST) departure from average (right) from the May 13, 2008 run of NOAA's CFS model. The model predicts below average wind shear and near average SSTs over the tropical Atlantic for the main development areas and months of August, September and October. However, the model is also forecasting slightly above normal vertical shear in the Gulf of Mexico and where the tropical upper atmospheric trough (TUTT) normally lies. This is the reason why this update is restraining from “upping the numbers” for 2008.


Figure 6: (a) Surface marine observations available in the Atlantic basin around 1200 UTC for a typical day in 1907. These observations were based entirely on ship measurements. (b) Same as Figure 6a but for a typical day in 2007. These data include moored and drifting buoys, QuikSCAT, as well as ship observations. (c) Depiction of new monitoring and analysis technologies (advanced microwave sounding unit tropospheric temperatures [Brueske and Velden, 2003], QuikSCAT [Atlas et al., 2001], and the cyclone phase space analyses [Hart, 2003]) that have increased Atlantic tropical cyclone counts by about one additional system per year.

Possible Tracks of Cape Verde Storms

Forecasting Tracks of Cape Verde Hurricanes using the North Atlantic Oscillation

The final update was scheduled for June 1 2008, but I will cancel it until August 10 2008. Instead, I will just discuss what we can expect for the first 2 weeks of June.

Sources

Climate Prediction ENSO Page
West African Monsoon Page
TRMM Rainfall Measurements
North Atlantic Oscillation Page
ECMWF Seasonal Forecast Page
NOAA Climate Prediction Center

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Broad Area of Showers

By: Weather456, 11:52 AM GMT on May 13, 2008

A broad area of scattered showers and thunderstorms lies between 60W and 30W, south of 15N in association between the interaction of the ITCZ and the southerly-southwesterly flow on the western side of an upper level anticyclone centered in the far Eastern Atlantic at 9N/28W. This southwesterly flow is driving additional bands of mid-level moisture across the Southern Lesser Antilles and the Tropical Atlantic. Visible animation and QuikSCAT showed no associated surface activity below any of the thunderstorm clusters with northeast wind flow dominating the area. An upper trough will dig just north of the Eastern Caribbean, reducing the effects of this upper anticyclone and with the support of surface high pressure and dry air; showers will remain limited to bands of mid-upper level moisture.

A tropical wave is along 55W south of 10N, moving west near 10-15 knots. The wave axis was detected on upper air observations as it passed Cayenne, French Guiana on the 12th of May. The wave’s presence was analyzed this morning based on infrared animation as producing scattered showers and thunderstorms along the coast and inland over Northeastern South America.

Elsewhere, there is not enough evidence to suggest the possibility of a wave along the coast of Africa. Otherwise, there are no other areas of concern and models are not hinting development in the next week.

GOES-12 Water Vapor Imagery taken 7:15 am EDT/AST



Upper-Air Time Series from Cayenne, French Guiana. The vertical axis is the pressure level or height of the atmosphere and the horizontal axis represents the time in days. The red line shows the wave axis in the lower atmopshere based on northeast-southeast wind shifts.



This Thursday, the 15th of May begins the 2008 Eastern Pacific Hurricane Season. In addition, I will issue an update to my April Hurricane Season Outlook and a Final Outlook on June 1 2008. Indicators will include updates on ENSO, the North Atlantic Oscillation (NAO), mean sea level pressure (MSLP), wind shear, sea surface temperatures (SSTs), rainfall patterns and continuation. The Quasi-Biennial Oscillation (QBO) will be excluded from this update.

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Tropical Update

By: Weather456, 3:24 PM GMT on May 12, 2008

Last weekend marked the one year anniversary of Subtropical Andrea, one of the storms I enjoyed forecasting last year.

This Thursday, the 15th of May begins the 2008 Eastern Pacific Hurricane Season. In addition, I will issue an update to my April Hurricane Season Outlook and a Final Outlook on June 1 2008. Indicators will include updates on ENSO, the North Atlantic Oscillation (NAO), mean sea level pressure (MSLP), wind shear, sea surface temperatures (SSTs), rainfall patterns and continuation. The Quasi-Biennial Oscillation (QBO) will be excluded from this update.

The tropics remain quiet with the exception of the ITCZ where clusters of moderate to strong convection stretch from Northeastern South America to West Africa between 10N and 5S. A possible tropical wave is noted near 10W south of 14N based Hovmoeller diagrams, potential vorticity analysis and low level derived winds at the CIMSS. Any updates will be posted.

Meanwhile, upper air observations from Cayenne, French Guiana revealed a tropical wave near 53W-55W south of 10N moving west near 10-15 knots. The wave is less impressive than it was 24 hrs ago in terms of shower activity which is little to non-existent along the wave axis at this moment. This feature may affect the extreme Southern Antilles in 2-3 days.

There are no other areas to speak of and none of the reliable computer models are forecasting development in the next week.

MSG-2 Infrared image taken May 12 at 8 am EDT/AST - Solid line indicate the official tropical wave (wave no. 2) while the dashed line indicate the position of the possible tropical wave.



GOES-12 Visible image taken May 11 at 8 am EDT/AST of tropical wave no. 2 before it moved into South America.



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Three Areas of Disturbed Weather

By: Weather456, 9:25 AM GMT on May 09, 2008

The tropics remain rather quiet this morning with a few areas of showers to speak of, but no development expected. The first area of showers is located over Northwestern South America/Eastern Pacific/SW Caribbean where the monsoon trough is producing moderate to strong squalls over the area. The QuikSCAT pass from last evening showed a pronounce onshore flow indicating a possible low pressure area over Colombia/Panama which is aiding in the development of these thundershowers. Conditions here appear favorable for development but this weak low pressure area will drift westward into the Eastern Pacific.

The second area is a tropical wave south of 7N, approaching 35W moving west near 10 knots. The wave is rather weak and the only tools I am using to track this feature are RGB satellite animations which showed an inverted V signature in the little convection along the ITCZ and total precipitable water (TPW) loops from the CIMSS. Conditions still remain unfavorable for development of this feature.

The third area is along the coast of West Africa where the monsoon trough is producing a broad area of disturbed weather between the Prime Meridian and 20W. This activity will be monitored for signs of a possible tropical wave and any updates will be posted.

Models are still indicating conditions will become favorable for tropical cyclogenesis in the Western Caribbean which is the area we look for development in the mid-late May and June.

RGB Image of the Central Atlantic taken 07:15 am EDT/AST



METEOSAT-9 Infrared Imagery of West Africa taken 07:00 am EDT/AST



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Tropical Wave Number 2

By: Weather456, 10:41 AM GMT on May 08, 2008

The first wave of the season moved into Northeastern South America yesterday with little or no shower activity. This morning's visible animation showed a second tropical wave along 27W-28W south of 10N moving west near 10 knots. This feature is exhibiting inverted V signature within clusters of moderate to strong convection along and within 300 nm east of the wave axis. There is little amplification at the surface as seen on this morning's QUIKSCAT pass. Development of this feature does not seem likely because the area north of 10N where genesis normally occurs, sea surface temperatures are marginal, there is a good amount of African dust/stable air, and zonal wind shear is too high. These conditions are expected to remain in place over the Tropical Atlantic during the next week.

Elsewhere, there are no other areas of interest and none of the reliable computer models are predicting development over the next week. The Western Caribbean is the area of Atlantic where conditions are forecast to progressively become favorable for development over the next 1-2 weeks.

MSG-2 Infrared Imagery taken 5:30 am EDT/AST



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Forecasting Tracks of Cape Verde Hurricanes Using the NAO

By: Weather456, 11:59 AM GMT on May 06, 2008

A weak tropical wave continues to move across the Tropical Atlantic with little or no shower activity. An area of showers that sits off the coast of West Africa appears to be associated with the ITCZ. Any updates will be posted. Models are not forecasting any development in the next week.

Forecasting Tracks of Cape Verde Hurricane Using the NAO

It is rather impossible to forecast the position and behavior of the Bermuda-Azores Ridge three months in advance, but using patterns and oscillations, like the North Atlantic Oscillation and ENSO, we can get an idea of how the high will steer Cape Verde Storms during August and September.

What is the North Atlantic Oscillation?

The oscillation of the pressure difference between the Subtropical Ridge and the Icelandic Low.

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 weak subtropical high and a weak Icelandic low.

Relationship with Atlantic Cape Verde Hurricane Tracks

I looked at some data over the past 58 years and discovered a few things:

The NAO Index during the December before the hurricane season is a better indicator of Cape Verde Hurricane tracks.

When the NAO Index is negative the previous December, this favors the movement of Cape Verde Storms along the Northern Caribbean and the East Coast.

When the NAO Index is positive the previous December, this favors the recurvature of Cape Verde Hurricanes.

Also, a negative phase in December is followed by an abrupt positive phase in January-March and turns negative during the Hurricane season.

Some Evidence

Interestingly enough, we had a negative phase in December 1988 and in the following year, Hurricane Hugo stormed through the Leeward Islands and made landfall in South Carolina. But what is one storm? During the winters 1990-1993, we had more or less positive phases during the December of those years and with the exception of Hurricane Andrew in 1992, none of the Cape Verde Hurricanes made it to land.

Let's look at some more. In December 2003, we had a negative phase in the NAO Index and the result was seen in 2004 where 2 significant Cape Verde Hurricanes made it to Florida, which was, Hurricanes Frances and Ivan.

As active 1995 and 2005 was, all Cape Verde Hurricanes in those years went out to sea. We had positive indices in December 1994 and 2003. Remember, Hurricanes Katrina, Rita and Wilma was not Cape Verde Hurricanes unlike Hurricane Irene, which recurved out to sea. Hurricane Emily is debated but it occurred outside the Cape Verde Hurricane Season.

2008 Hurricane Season

During the past winter, the pattern was similar to 2004 and others where we had a negative phase in December, followed by a positive phase earlier this year. The GFS outlook is also showing megative phases through May. With that in mind, it may be a year where the Cape Verdes Hurricanes are long tracked and break the 2 years of major activity in the United States and almost 10 years for the Leeward Islands.


Figure 1. Mean NAO Index from 1950 to 2005.


Figure 2. The North Atlantic Index through April 9 2008.


Figure 3. Typical tracks of Cape Verde-type hurricanes in both phases of the NAO during the December before the hurricane season.

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Nothing In the Tropics

By: Weather456, 11:32 AM GMT on May 05, 2008

A tropical wave is analyzed along 35W-36W based on the 850 mb vorticity, low-level satellite winds and total precipitable water products at the CIMSS. The wave is moving westward at around 13 knots based on Hovmoeller diagrams accompanied with little or no shower activity mainly along the ITCZ.

Total Precipitable Water Product from the CIMSS



There are no other areas of interest out there and none of the reliable computer models are developing storms over the next 7 days. The long-range GFS forecast has been inconsistently developing features in the subtropics, so I don’t expect anything much from there.

My next blog entry will discuss the question "Can we forecast the Bermuda-Azores High?"

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Tropical Update

By: Weather456, 12:10 PM GMT on May 04, 2008

A tropical wave is pushing across the tropical Atlantic along 24W south of 5N moving west near 10 knots. This wave is less impressive than it was over the past couple of days with its inverted-V signature less defined. Cloudiness and moderate to strong thundershowers lies ahead of the wave axis between 25W and 35W, but this maybe more associated with the ITCZ. This morning’s QuikSCAT pass still revealed the wave presence at the surface but most of its structure at 700 mb is becoming less discernable.

QuikSCAT pass taken 3:46 am AST/EDT


Over the next two weeks the 0600Z GFS is forecasting the development of a low pressure area to the northeast of the Bahamas. The genesis of this particular feature is rather interesting and stems from the interaction of a westward moving area of showers just north of the Caribbean (maybe a surface trough) and the tail end of what appears to be a dying frontal boundary. Environmental conditions at that time seem to favor baroclinic systems, that is, subtropical cyclones. Long-range forecasts are seldom trustworthy, unless the GFS is consistent during model runs. Any updates will be posted.

The GFS is also forecasting the development of a tropical cyclone in the Eastern Pacific Ocean around the beginning of their season which commences May 15. The model has been consistent with this feature and given the time year along wiht favorable conditions, there is a good chance of this happening. It appears that this feature may threaten the West Coast of Central America as with these early season storms.

GFS Model Run - Valid Tuesday May 20 2008


More Waves to Come

The African Easterly Jet, the main feature responsible for the genesis of tropical waves, will established itself near 10N extending from Ethiopia near 30E to the tropical Atlantic near 35W. I expect at least one more tropical wave to emerge from the West African landmass within the next 2-3 weeks.

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First Tropical Waves (2004-2008)

By: Weather456, 12:20 PM GMT on May 03, 2008

A tropical wave along 18W south of 10N continues to move westward at around 12.5 knots based Hovmoeller Diagram estimates. Showers and thunderstorm activity have diminished along this feature and the only satellite-based evidence of the wave axis is a low level rotation seen near 6N-7N. QuikSCAT missed the area this morning but the afternoon pass from yesterday showed a well-define wave. Most computer models are in agreement that wind shear will remain somewhat favorable for the next 72 hrs but in a region of the Eastern Atlantic that is too far south for development. In addition, all computer models are showing the effects of African Dust will linger over the tropical Atlantic over the next 7 days with relative humidity values in the lower atmosphere below 50% (very dry air); so no immediate development expected. The tropical wave will be approaching the Windward Islands in 4-5 days time.

Elsewhere, there are no areas of concerns and none of the computer models are forecasting development over the next 14 days.

METEOSAT-9 Visible Imagery (X Represents rotation)



Relative Humidity in the lower atmopshere (%) Valid Thursday 08 May 2008


First Tropical Waves

This morning I did a search through the Tropical Weather Discussion archive at the FSU and found the dates of the first tropical waves of each year from 2004 to the present. In 2004, several waves were added to the surface map on May 21, with the furthest west wave added near 55W, which means the wave may have existed days before it was recognized by the Tropical Prediction Center (TPC). Now notice how the first tropical waves in 2005, the most active year in history, and 2008 were on the same date. This does not neccessarily mean anything because tropical waves formed later in 2004 and 2007 and both years had a more active Cape Verde Season in terms of number of depressions, intensity and duration of storms. However, it is one interesting observation and We'll have to see how this pans out come Hurricane Season 08.



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First Official Tropical Wave

By: Weather456, 7:21 PM GMT on May 02, 2008

This morning, I posted a blog entry on an area of disturbed weather just off the African Coast. The latest QuikSCAT pass at the time showed little evidence of a tropical wave but other indicators at that time revealed a wave-like feature. Since then the Tropical Prediction Center (TPC) stated that a tropical wave will be added to their 1800 UTC (2 pm EDT) surface analysis chart and potential vorticity charts have showed a more define wave.

The tropical wave is analyzed along 17W south of 10N moving off to the west near 12.5 knots based on 5-day Hovmöller Diagrams. The wave is clearly evident on visible imagery animations as an inverted-V signature and a bit of cyclonic rotation in the lower mid-levels with scattered showers and thunderstorms within 300 nmi either side of the wave axis. Current conditions seem conducive for development and may remain so over the next 24 hrs before the system encounters easterly zonal shear and a good amount of African Dust. If the system shows signs of development more updates will be posted.

This is not the first wave to be analyzed, but the first added to the surface maps of the TPC.

315K (700mb) Potential Vorticity and Winds



METEOSAT-9 Visible Imagery



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Showers Off West Africa

By: Weather456, 11:46 AM GMT on May 02, 2008

This morning's Goes-12 Infrared imagery show showers have diminish across the Southwest Caribbean and there are no other areas of disturb weather out there. None of the reliable computer models are forecasting development over the next 7 days. However, an area of showers and thunderstorms lies just offshore the coast of Africa in association with a possible tropical wave. This morning's QuikSCAT pass of the area and visible imagery animations revealed this activity to be associated with the monsoon trough/ITCZ but there are other factors that suggest otherwise. Over the past 7 days, this feature has been associated with the westward movement of a vorticity (spin) maximum in the lower mid-levels and causing wave-like signatures in the African Easterly Jet. Clusters of moderate to strong convection from the Equator to 7N between 10W and 15W with slight rotation detected near 5N. We do not normally look here for development in May but since conditions seem conducive for tropical cyclone formation and forecast to improve further, the area will be monitored incase it shows sign of development and updates will be posted.

METEOSAT-9 Visible Image


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Tropics in May: The Transition Month

By: Weather456, 5:26 AM GMT on May 01, 2008

The Tropics In May: The Transition Month

The 2008 Atlantic Hurricane Season official begins one month from today, June 1 2008. This date conventionally defines the commencement of the period when most tropical cyclones form in the Atlantic basin during the year. However, past experience have shown that storms do occur before June 1, and most recently we had the development of Subtropical Storm Andrea which developed last year on May 9th.

During the month of May there were numerous tropical depressions, tropical storms and subtropical storms. There have been 3 hurricanes in the month of May since records began and one major hurricane, which was Hurricane Able in 1951, the earliest major hurricane recorded.

In the past 6 years, only one system formed in May and that was previously mentioned Subtropical Storm Andrea in 2007.


Figure 1. The occurrence of tropical cyclones of tropical storm status and above in May from 1950 to the present.

Why are tropical cyclones rare in May?

The conditions that favor tropical development are not particular in place as yet.

1. Tropical cyclones require humidity above 70% between the surface and mid-levels (700-500 average). During May, only the lower troposphere contains high levels of humidity as the mid-levels remain dry.

2. Vertical wind shear is quite high especially around the subtropical jet which stretches from East Coast of the United States to the Northern part of West Africa between 20N-30N. The westerlies across the Eastern Caribbean and Tropical Atlantic below 20N still dominate.

3. Tropical cyclones form from a number of sources including the famous tropical wave and ITCZ. During May these disturbances are absent, that is, the ITCZ too far south and tropical waves (AEWs) are too weak in amplitude (not extending to the surface). Also in May, formation sources rely much on fronts and mesoscale disturbances, like surface troughs.

Over recent years, sea surface temperatures have been well above the requirement for tropical cyclogenesis in the main development areas of the Caribbean and Gulf of Mexico during the month, so this factor is rather discounted. However, sea surface temperatures in the Tropical Atlantic greatly influence development there.

Current Sea Surface Temperatures


Figure 2. Systematic diagram of environmental conditions in place during May. The two main areas of development are area (1) which involves tropical cyclones and area (2) which involves subtropical cyclones. Notice how area (1) lies under an upper ridge and over warm SSTs, while area (2) lies within baroclinic environment (wind shear and temperature gradients).

What Changes Occur During the Course of May?

1. The dept of atmospheric humidity increases as land/sea surface gains more heat from the sun.

2. Ridging becomes more established causing the subtropical jet to shift northwards and weaken. For example, the monsoon ridge shifts from its winter-spring position over the Equatorial Pacific and South America towards its summer position over Mexico and the Four Corners. As this occurs, the jet that flows on its northern flank shifts northward. The jet weakens as the temperature gradient* also weakens. This creates favorable upper winds for areas west of 70W.

However, westerlies in the tropical Atlantic remain particular high even as June commences, that’s why the Western Caribbean and Gulf of Mexico favors TC development in the early part of the season.

3. The ITCZ shifts northward (above 10N in some places) and tropical waves (AEWs) become more frequent and amplified.


Figure 3. Major changes occurring during May into early June (the transition months). Arrows show movement of features over the period. The two main areas of development are area (1) which involves tropical cyclones and area (2) which involves subtropical cyclones. Notice how area (1) lies under an upper ridge, over warm SSTs aided by disturbances (AEWs), while area (2) lies within baroclinic environment (wind shear and temperature gradients).

Where do developments normally occur in May?

Development normally occurs in the Western half of the Atlantic due to more favorable conditions such as warmer sea surface temperatures, low-moderate wind shear, higher heat and moisture content and due to the fact that non tropical systems are more keen to become hybrid (subtropical) during the transition months. Refer to figures 1 and 2.

When do developments normal occur in May?

Storms mainly occur in late May. Six of the eight tropical systems (tropical storm status and above) that formed in May occurred after May 15 and this is reflected in figure 4.


Figure 4. Illustrating activity per 100 years between May and December. Notice the slight peak in late May.

Can we expect any development in May 2008?

Currently, there is area of showers and thunderstorms in the Southwest Caribbean Sea associated with the Colombia Low and ITCZ. No development expected in the next 48 hrs.

Due to climatological reasons, there is very slim chance of TC development in May. None of the reliable computer models are forecasting development over the next 2 weeks. The 00Z long-range GFS forecast is showing a rather active ITCZ which I will follow up in upcoming blog entries. I do not expect any sustainable development here. However, based on sea surface temperatures, relative humidity, vertical wind shear and climatology, there are two areas that seem conducive for tropical (1) and subtropical development (2) over the next two weeks.

I evaluated area (1) based on warm SSTs, high humidity, low wind shear, the presence of the ITCZ and an anticyclone/ridge aloft.

I evaluated area (2) based the baroclinic and blocking environment.


Figure 5. Areas that are forecasted to appear conducive for the potential development of tropical and/or subtropical systems during the period May 1 to May 15 2008. Any thing developing would take a track similar to the ones presented here.

Subtropical Storm Andrea


The circling clouds of an intense low-pressure system sat off the southeast coast of the United States on May 8, 2007, when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this image. By the following morning, the storm had developed enough to be classified as a subtropical storm, a storm that forms outside of the tropics, but has many of the characteristics—hurricane-force winds, driving rains, low pressure, and sometimes an eye—of a tropical storm.

Although it arrived several weeks shy of the official start of the hurricane season (June 1), Subtropical Storm Andrea became the first named storm of the 2007 Atlantic hurricane season. The storm has the circular shape of a tropical cyclone in this image, but lacks the tight organization seen in more powerful storms. By May 9, the storm’s winds reached 75 kilometers per hour (45 miles per hour), and the storm was not predicted to get any stronger, said the National Hurricane Center.

Though Subtropical Storm Andrea was expected to remain offshore, its strong winds and high waves pummeled coastal states, prompting a tropical storm watch. The winds fueled wild fires (marked with red boxes) in Georgia and Florida. The wind-driven flames generated thick plumes of smoke that concentrated in a gray-brown mass over Tampa Bay, Florida. Unfortunately for Georgia and Florida, which are experiencing moderate to severe drought, Subtropical Storm Andrea was not predicted to bring significant rain to the region right away, according to reports on the Washington Post Website.


*The jet results from the temperature gradient between the warm tropics and the cool temperate zone which is around 30N. As this gradient weakens as summer nears, so does the jet.


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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.