Weather456's Tropical Weather Blog

Strong Tropical Wave over Africa Poses to Become the Season's Second Depression

By: Weather456, 1:37 PM GMT on June 30, 2008

A very strong and impressive African Easterly Wave (AEW) is located over Africa along 11-12W south of 15N moving west near 10-15 knots. Recent infrared images showed classic convective signitures and makes it one the most impressive tropical waves of 2008. The products at the CIMSS showed strong vorticity in the lower atmosphere, which is not unusual since the system is embedded within the monsoon trough. It is difficult to ascertain whether a surface circulation exists, but there is excellent cyclonic curvature along the wave axis as seen in visible animations. Most global models (ECMWF) develop this wave around the next couple of days, which seems reasonable. The existence of a favorable African Easterly Jet (AEJ), along with very low wind shear, modest-high humidity levels and modest-high SSTs in its path, this seems possible. With a sprawling Azores-Bermuda ridge expected by most models, any development would take a west/west northwest movement in the near term. This is troubling for the islands since there is no frontal trough expected for more than a week, thus making re-curvature slim within 168 hrs or 1 week. We still cannot put 100 per cent confidence in the global models; since there is always the chance of a forecast “bust” but I’m inclined to believe them based on the data at hand and the extremely excellent model agreement.

There is another wave along 1W south of 15N just to the east of this feature.

There are no other areas of disturbed weather in the remainder of the tropics.

This morning's infrared imagery of the wave over Africa. The wave the models are developing is the one nearest the coast.

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Tropical Update: Models All Over The Place

By: Weather456, 1:41 PM GMT on June 28, 2008

Good Saturday morning to all!

Before I get into the deep tropics, sometime next weekend, I will post an entry on the Tropical Upper Tropospheric Trough (TUTT), looking at its formation, nature climatology and role it plays in the weather across the Caribbean, tropical cyclogenesis and cyclosis.

There remains a weakening upper level circulation in the Northwestern Gulf of Mexico which has become less distinct on water vapor loops over the past 24 hrs. Nonetheless, this feature continues to generate scattered showers in the diffluent flow to its east from Isthmus of Tehuantepec (the area between Mainland Mexico and the Yucatan Peninsula) to the Gulf coast waters of Louisiana and Alabama. This morning’s products at the CIMSS showed some decent vorticity in the lower levels in the Bay of Campeche, but further analysis showed it to be associated with a passing tropical wave. Upper winds have slackened off a bit over the area but I'm still not expecting development because the expanding deep layer ridge, which is now over Northern Florida, will drive anything westward into Mexico/Texas and upper winds will increase more over the next 72 hrs over the area. Some increase in moisture along the Mexican/Texan Gulf coast is expected as this feature moves west.

Scattered showers and isolated thunderstorms will affect the islands today as a westward moving tropical wave along 64W interacts with the tropical upper tropospheric trough (TUTT). No tropical development expected here. This wave maybe further east that stated here, like 60W due the excellent inverted V signature in the ITCZ convection.

GOES-12 Water Vapor Image

A central Atlantic tropical wave along 36W south of 12N moving off towards the west near 10 knots. The wave displayed an impressive round of convection during the over night hours mainly due to the diurnal maximum, which would place it in pre-genesis stage. This morning's RGB imagery loop showed a decent mid-lower level circulation along the wave axis near 10N. However, products at the CIMSS showed one broad area of convergence and spin in the lower atmosphere. RGB imagery also showed the wave is surrounded my widespread decks of stable air stratocumulus clouds, mainly induced by a layer of Saharan dust, which should keep any development slow or limited. The area will be monitored for signs of a low-level closed circulation (LLCC) as the lone model, UKMET, still wants to develop this feature. This feature will continue to off towards the west, where its future remains uncertain.

The 00Z CMC has backed off from the Eastern Atlantic disturbance and has focus on developing two areas closer to home. One is an area off the coast of the SE United States on the 3rd and the second is just east of islands on the 1st, most likely the central Atlantic wave discussed above. The former does seem possible but I’m skeptical about the latter due to unfavorable wind shear that is expected to remain the Caribbean basin. The 06Z GFS still wants to develop the African disturbance on the 3rd which is very consistent but lacks model support.

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Tropical Update: Central Atlantic Disturbance

By: Weather456, 2:01 PM GMT on June 27, 2008

Good Morning All!

The tropics remain rather quiet today but with a few interesting areas I’m watching. There remains an upper level low pressure area in the Northwestern Gulf, helping to generate some scattered showers, over Central America, the Yucatan and the central Gulf as a passing tropical wave interacts with the area. As for now, there remains no signs of a surface circulation and environmental conditions remain unfavorable for development. The area will be monitored. This upper low and its associated tropical wave is expected to move either west into Mexico or Texas under the influence of westward moving deep layer ridge currently over the Northern Bahamas. As it does this it should spread some moisture along the Gulf Coast from SW Louisiana to Mexico.

GOES-12 Water Vapor Image of the Upper Level Low over the Northwestern Gulf of Mexico.

The other area is an area of convection along the ITCZ near 40W-30W south of 10N. To my eyes, QuikSCAT caught a partial glimpse of what maybe a surface circulation with winds of 15-25 knots. Visible animation also supported this with lower level turning near 35.5W/7N. The wave is under favorable vertical wind shear, warm sea surface temperatures and even has the backing of the 00Z UKMET. Despite a layer of Saharan Dust to its north, the area will be monitored as it meanders slowly within a weak steering environment.

GOES-12 Visible Image of the central Atlantic disturbance.

Otherwise, the GFS continues to forecast the development of a tropical depression off the Coast of Africa by Wednesday.

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Tropical Update: Gulf of Mexico and the Eastern Atlantic

By: Weather456, 12:15 PM GMT on June 26, 2008

A broad area of scattered showers and thunderstorms is situated over the Southeastern Gulf region from the Yucatan Peninsula, NW Caribbean Sea and Central America to the Southern Florida Peninsula. Water vapor loop indicates this feature is associated with the diffluent flow ahead of an upper level circulation over the Western Gulf aided by a passing tropical wave over the Northwestern Caribbean. Surface buoy reports indicated that surface pressures are slightly falling maybe due to the rising air brought on by the upper diffluence. Despite this, pressures remain relatively high and wind shear is around 25 knots over the area and not expected to relax much over the next 24 hrs. The area will be monitored for future signs of surface signatures and development, but so far, QuikSCAT showed only easterly 20+ knot winds. The GFS has the bulk of this moisture moving towards the northwest under the influence of an expanding deep layer ridge near the Bahamas.

An Eastern Atlantic tropical wave is located along 20W south of 15N based on the CIMSS low-level wind product which showed an excellent lower-level circulation. This area could be watch over the next 48 hrs, while upper winds remains favorable.

The other feature worth mentioning is a tropical wave currently over far Eastern Africa that the GFS has been consistently developing in the Eastern Atlantic around next Wednesday. While environmental conditions are expected to be favorable at the time, there isn’t much model consensus. This could all change, so the best we can do is watch and wait.

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Tracking Tropical Cyclones Part 1: Center Fix

By: Weather456, 12:44 PM GMT on June 22, 2008

The tropics remain relatively quiet this morning. A tropical wave is along 41W south of 16N moving off towards the west-northwest with broad lower mid-level turning and accompanied by scattered moderate to isolated strong convection. Water vapor animation showed upper winds to its north is beginning to interact with this feature and this is expected to continue over the next 72 hrs, so development of this feature seems unlikely. None of the models are foreseeing development over the next 5 days.

Tropical Cyclone Center Fix

The center of a tropical cyclone is defined as the vertical axis or core of the tropical cyclone. It is usually determined by three major variables – cloud patterns, wind and pressure.

Center fix is the process of locating a tropical cyclone using data collected from various tools.

Center Fix Tools
Satellite Imagery
Surface Observations
Satellite-based instruments (QuikSCAT)
Microwave Imagery
Aircraft Reconnaissance
Radar Imagery
Numerical Computer Models

Satellite Imagery

This is by far the most popular and universal, useful tool used for center fix. There are two centers displayed by a tropical cyclone seen in satellite images – the infrared center (which is the center of the intense convection seen on infrared imagery) and the visible center (which is the vortex or focal point of all curve band clouds seen on visible imagery). The main advantage of satellite imagery is that it is available in a timely fashion. GOES-EAST images are updated every 30 minutes, 15 minutes on some websites. However, this tool is subject to misinterpretations. Both the infrared and visible centers can be off by several miles from the geographic center, especially if you have a large dense overcast (blob of convection) obscuring the surface pattern. Sometimes, to the untrained eye, the mid-level circulation is mistaken for the low level circulation which can be off by miles, especially in developing or sheared tropical cyclones. The Dvorak tool utilizes this tool, with center fix being the first step in the technique.

This tool measures the cloud variable.

Surface Observations

This tool utilizes reports made by buoys, ships and land-based stations. Usually the center will be located near the lowest reported pressures, the greatest pressure falls and the focal point of wind shifts being reported. This tool is useful because it’s an actually measure of the tropical cyclone’s center and quality control is maintained (easy to spot a faulty report). The major problem with this is that it is only useful in the Caribbean, Gulf of Mexico and the SW North Atlantic. There very few stations located in the tropical Atlantic and even parts of the Caribbean Sea.

This tool measures the wind and pressure variable.

Satellite-Based Instruments

The use of QuikSCAT, ASCAT, WindSAT and AMSU in measuring winds over the ocean. The center of a tropical storm is easy identifiable as the focal point of the counterclockwise circulation. These tools have been very useful over the years especially in regions where they are few synoptic reports, like stated recently. The disadvantage is that QuikSCAT and others only produce 2 useable passes per day and they are 2-3 hours late. In addition, because they are polar-orbiting satellites they do not overlap between 45N/S and thus you have blank swaths. Sometimes these swaths are over the area of interest.

This tool measures the wind variable.

Microwave imagery

These are sensors onboard satellites in orbit that transmit and receive signals towards/from earth to peer into a storm to understand its structure. There are various sensors including the Precipitation Radar (PR) and SSMI onboard the TRMM and DMSP satellites, respectively. There are much more sensors, with each measuring different channels. The process is quite complex so I’m not going into detail to cause confusion. These images can be found at the NRL Navy Website and the center is usually clearly identifiable. The disadvantages of this tool are the same for satellite-based instruments since these are also on polar orbiting satellites. However, they are updated much faster and because they are so many sensors, images are available at most every 6 hrs. Also, because these sensors see below the clouds, it eliminates the errors caused in satellite imagery.

This pattern measures the cloud variable.

Aircraft Reconnaissance

The best and most accurate method of locating a tropical cyclone is by direct observation from aircraft. Another useful tool that is confined to west of 40W, where airplanes travel into storm systems and accurately measure the storm’s structure and help in center fix.

This tool measures the wind and pressure variables.

Radar Imagery

A tool that has a high refresh rate but is only beneficial to areas near land.

Numerical Models

If all else fails, use computer models to get handle on the center. It’s not really useful for present day center fix as it usually outdated. However, it is useful for predicting future center positions.

This tool measures the wind and pressure variables.

Visible center of Tropical Storm Florence

Infrared center of Tropical Storm Ernesto

Using surface observations to locate the precursor of Arthur on May 30 2008

QuikSCAT of a cyclone in the Southern Indian Ocean

Multiplatorm satellite winds of TC Sidr in the Northern Indian Ocean

Microwave Imagery of TC Ivan

Radar Loop of Tropical Storm Ernesto

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Rainy Day In the Islands; Tropical Update

By: Weather456, 10:58 PM GMT on June 20, 2008

Good evening all!

Showers associated with the area mentioned yesterday across the SW North Atlantic appear to have diminished and models have backed off developing any feature in that area. As such, development is not expected there over the next 48 hrs.

The nest feature of interest is a tropical wave pushing through the Eastern Caribbean along 65/66W south of 20N based on 18Z surface charts and 850 mb vorticity product by the CIMSS. Visible imagery showed some level of tuning along this feature but recent surface convergence charts indicate this may not be at the surface. This wave continues be sheared by strong westerly winds aloft leaving much of the shower and thunderstorm activity east of the wave axis across the Lesser Antilles. Overcast skies, with embedded showers and isolated thunderstorms have dominated the weather across the islands for much of the day. Satellite based radar along with land-based radar suggest that the bulk of showers fell along the Windward Islands today, with additional showers spreading across the Leewards, Virgins and possible Puerto Rico. Future development of this wave remains uncertain.

The GFS continues to show shear relaxing in 96-120 hrs and this supported by the CIMSS 24 hour shear tendenacy chart but still not to levels I would call favorable, maybe marginally favorable. By this time, the wave may already make its way into Central America/Mexico Yucatan due to deep easterlies it is embedded within. The situation will be monitored and the wave should not be totally written off until then.

Regardless of development, the wave will continue off towards the west and should be south of Haiti in 24 hrs. Showers are expected to stray over Puerto Rico and Eastern Hispaniola later tonight and early on Saturday.

Latest GOES-12 visible image

The third feature is a tropical wave approaching 30W. I am amazed by the impressiveness of these May/June waves and it is giving me the goose-bumps of what might happen in July, August and September. Tonight’s infrared animation showed the wave is associated the mid-level turning with scattered showers surrounding the wave and extending southwestward along the ITCZ. I am not expecting much of this wave in the next 72 hrs, but the area will be monitored as it may have a chance beyond then.

Otherwise, I don't see development taking place over the next 5 days.

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Tropical Update

By: Weather456, 9:15 PM GMT on June 19, 2008

An elongated band of showers stretching across the southwest North Atlantic is associated with a frontal boundary pushing off the Eastern Seaboard. There are some indicators that this frontal band is dissipating into a broad surface trough of low pressure. Models continue to develop two lows from this disturbance. The northern most low has the most model consensus and expected to be extra-tropical in nature as it races off towards the northeast. The second, and most southernmost low is being forecast by the NAM and based on environmental conditions and the low latitude, something tropical or subtropical may form. The area will be monitored.

Goes-12 visible image and surface observations overlaid.

The second area of interest is a tropical wave east of the Lesser Antilles along 56W based on TPW and PV fields. The wave continues to be embedded within a mid-upper level dry airmass and being sheared by 30-40 knots of westerly shear leaving much of shower activity limited and displaced east of the wave axis. This environment is expected over the next 24 hrs; at least, therefore development does not seem possible. This wave is expected to bring scattered showers and isolated thunderstorms to the islands as early as Friday morning. It is interesting to note that these westerlies disrupted the mid-level circulation associated with the wave, much like blow really hard on a spinning top.

When can we expect this hostile environment to end?

It was forecast to happen, the MJO was forecast to be in a downward pulse in the latter part of June and it is.

My best bet is when the Madden-Julian Oscillation (MJO) returns to the region in about 2 weeks time, around the July 4 weekend even stronger that the week with Alma/Alberto. At this time, wind shear and other environmental variables are expected to be favorable in the climatological favored areas of early July. This does not mean development will occur, but should any of those strong waves roll off Africa, I see no reason why one shouldn’t develop between 40W and the Central Caribbean. All eyes open when the MJO returns.

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Tropical Update

By: Weather456, 5:53 PM GMT on June 18, 2008

Hello all in blogosphere. Well before I get into the tropics, the theme of Sunday’s blog will be about tropical cyclone center fix. It will address the various tools use to center and position tropical cyclones. Sometime later this hurricane season I will also discuss the Dvorak Technique to tropical cyclone intensity. If you would like, I have also posted a previous entry on how to detect and track tropical waves.

The tropics remain in typical June state. The most impressive feature out there continues to be a tropical wave roughly along 44W based on visible imagery, TPW and 850 mb vorticity. Visible imagery showed some increase in shower activity behind the wave axis mainly along the ITCZ with a low level swirl at 12.6N/47.5W moving off towards the west around 14 knots. Now this feature will to be watch despite the relatively hostile environment. It has persisted and fought its way through some of the driest air in the Atlantic without completely dissipating and this maybe due to the moisture field associated with the ITCZ as seen on TPW charts and there are hints the circulation is getting closer to the surface based on the current circulation and 925 hpa vorticity. There is still no model support with this system. The system is expected to continue to move off towards the west at a good clip south of a deep layer ridge and should reach the islands by Friday with little development expected due to expected unfavorable upper winds.

Based satellite signatures, PV charts and products from the CIMSS, a tropical wave is estimated to be along 11W south of 11N. Recent visible imagery showed this wave possesses an impressive mid-level circulation just as the one mentioned above when it rolled Africa, a few days ago.

Lastly, most models are showing a cut off low forming in the SW North Atlantic around the 19th. Now this area will be monitored and anything forming would likely cut off from a dying frontal boundary. Sometimes these can aquire subtropical or tropical characteristics. The area will be monitored as conditions appear even favorable for something tropical to develop. Anything forming will likely pull off towards the NE.

Elsewhere, none of the models are indicating development over the next 7 days.

Goes-12 Visible Image and Surface Observations overlaid.

Tropical Depression 07W (Frank)

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By: Weather456, 1:16 PM GMT on June 15, 2008

The tropics remain rather quiet this morning. The upper low in the Bay of Campeche appears to have dissipated over the Isthmus of Tehuantepec.

A broad area of low pressure in conjunction with the ITCZ and a passing tropical wave is situated across the SW Caribbean, helping to enhance some showers and thunderstorms. This is the same tropical wave that helped fuel some strong thunderstorms across Hispaniola and Puerto in the past 24 hrs. Since then, activity has diminished as the wave continues west. Some lingering moisture along with daytime heating may enhance a few showers later today across the area.

Water vapor imagery showed a relatively dry environment across much of the Tropical Atlantic in association with a layer of Saharan dust.

Last, but not least, a tropical wave located along 20W/21W moving off towards the west near 10-15 knots. I'm going to watch this tropical wave to see if it retains convection during the diurnal minimum. If so, this wave may be in its pre-genesis stage, based on favorable atmospheric conditions and an INVEST may be assigned to it. Satellite imagery showed excellent cyclonic turning in the mid-lower levels with most of the convection ahead of the wave axis which is an indicator of AEJ. The wave has maintained impressive organization over the past 24 hrs relative to the diurnal cycle. Though, its a bit far south, so much development not expected for now, it's still attached to the ITCZ and there is no evidence of a low-level closed circulation as yet. However, being that far south means it remains in the favorable region of SSTs and low shear, so the area will be monitored as it continues off towards the west.

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Tropical Update

By: Weather456, 2:31 PM GMT on June 14, 2008

The mid-upper level circulation across the Western Gulf of Mexico continues to move towards the west-southwest under the influence of a mid-upper level anticyclone stationed over Northern Mexico. Satellite images along with Mexican Radars showed scattered showers associated with this feature mainly on the southern portion. There is still no evidence of a surface circulation or low associated with this feature.

We have a complex upper level pattern across the tropics this morning that is reponsible for inducing westerly flow across the Northern Caribbean. Southwesterly winds across the Caribbean basin proudced by an upper anticyclone at 15N/40W, while a series of upper level circulations at 30N-35N is producing westerly winds to their south. Vertical wind shear increased 10 knots from 40 kts to 50 kts in the Central Caribbean as a result. These upper winds are also helping to fuel thunderstorms across Hispaniola, Puerto Rico and the extreme northern Leewards.

A rather impressive wave may have exited the coast of Africa in the past 6 hrs based recent analysis. Most of the action in this wave remains in the middle to lower atmopshere. Moderate to strong convective bands surrounds this feature. Conditions in the near term seem conducive for development so the area will be monitored, keeping in mind it's rather far south for much development.

As for the models, most dropped the TRP ATL storm and now the NAM (Green) is predicting development on Tuesday 17 June. There isn't any model agreement with this feature yet but some consistency in the model run. Unfavorable winds are forecast to be over the area south of Western Cuba at that time.

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Tropical Invest 91L

By: Weather456, 3:56 PM GMT on June 11, 2008

A tropical wave along 57/58W moving off to the west at 10-15 knots was declared an invest by the NHC this morning. Visible satellite imagery along with microwave imagery showed the vorticity center of the wave located near 9.5N-57.5W moving towards the west-northwest. This morning's QuickSCAT pass showed no closed low level vortex but recent visible satellite animations revealed a nice cyclonic turning associated with this feature with little to moderate convection. Vertical wind shear remains 5-10 knots over the disturbance but with a sharp increase to its north and west. The forecast indicates that this low wind shear "bubble" may accompany this wave into the Southeastern Caribbean over the next 36 hrs, but not much development is expected due to restricted outflow induced by these upper winds and its close proximity to South America. The area will be monitored and updates posted. Regardless of development, moderate showers with isolated thunderstorms will pass across the Southern Anntilles and Venezuela.

Elsewhere, an African Easterly wave continues to move across West Africa and is somehwere near 10W south of 15N and should exit over the next 24-36 hrs.

Invest 91L southeast of the Lesser Antilles

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Watching An Area Offshore Mexico

By: Weather456, 11:05 AM GMT on June 09, 2008

An area of disturbed weather is located near 16.0N/101.1W just south of Acapulco, Mexico based on infrared satellite animations, surface observations and GFS 06Z analysis. Recent satellite animations showed increase convective banding and consolidation of thunderstorms near the center of circulation, despite its proximity to land. Conditions appear conducive for some additional slow development over the next 24-48 hrs. None of the reliable computer models really picked up on this system. Future movement is rather uncertain due to the weak steering flow but over the past 12 hrs, a westward drift was noted. Regardless of development, heavy rains are expected along the Mexican Riviera.

Goes-12 infrared image of the area of disturbed weather in the Eastern Pacific.

Elsewhere, across the Atlantic, no development is expected over the next 48 hrs. Wind shear remains unfavorable for development across a good portion of the Atlantic. Any development would most likely occur into next week (after the 16 June) when wind shear is expected to decrease substantially to marginal values.

A possible tropical wave is indentified as approaching 0E. Further analysis would be needed to confirm this feature. In addition, if this wave is confirmed, it would be the most northern wave of the season.

Tracking Tropical Waves

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The Tropics; Tracking Tropical Waves

By: Weather456, 1:04 PM GMT on June 08, 2008

The Tropics

A very broad upper level cyclonic circulation envelopes the area between 90W and 65W, centered on an upper level low near 25N/76W. Scattered cloudiness is seen rotating in and around this feature. Meanwhile, this broad circulation is producing strong westerly flow south of 20N with upper level diffluent producing scattered showers and thunderstorms from the Bay of Campeche, across Central America and into the Caribbean Sea to 70W. Additional mid-high level cloudiness extends from 70W across the Lesser Antilles and into the tropical Atlantic. As mentioned in the previous update, daytime heating and the passage of a tropical wave may enhance additional thunderstorms over these landmasses.

An area of disturbed weather located south of Mexico is being monitored as it as has the potential to develop into a tropical depression. Satellite imagery, surface observations and QuikSCAT indicated a broad circulation centered near to the Mexican coast at 15.5N/95.8W with winds around 20-25 knots. Much of the convective bands remain in an arc south and west of the analyzed center but has shown improvements on the first few frames of visible imagery. Due its proximity to land, not much development expected over the next 24 hrs.

Elsewhere, development is not expected over the next 24 hrs. The 06Z NAM is showing something developing near the coast of Mexico in the Bay of Campeche on the 11th, so this area will be monitored. Also a tropical wave out in the central Atlantic displayed recent increase in shower activity, but no development expected.

GOES-12 infrared image of the disturbed weather area south of Mexico

Tracking Tropical Waves

What is an African Easterly Wave and how does it differ from a tropical wave?

Simply Answer: They are basically the same features. An African Easterly Wave (AEW) is just one of the three conceptual models of tropical waves.

A tropical wave is defined as a westward propagation of convective signature forming within the instability of the African Easterly Jet (AEJ) over Central-Eastern Africa with maximum intensity in the mid-lower levels and associated with wind shifts, pressure and moisture fluctuations.

Conceptual Models of Tropical Wave

African Wave Model
Frank's Inverted "V" Model
Riehl's Easterly Wave Model

Characteristics of the African Wave Model

These waves form over the Ethiopian Highlands of Eastern African within the instability associated with the African Easterly Jet (AEJ) and the topography of the area.

They track westward as convective signatures and are affected by diurnal variations.

They reach maximum intensity at 500-700 mb, before they reach the coast. Afterwards, they acquire low level-surface signatures 800 mb-surface. They are rather difficult to detect east of 10E, before August. They are also associated with two vorticity maxima north and south of the axis. One is associated with the wave and the other is associated with the low level monsoon over West Africa.

Maximum surface convergence and bad weather ahead of the disturbance while divergence and calm weather near and rear of the axis.

As they pass stations near the coast, a produce wind shift and fall in pressure is associated. They are also associated with jet max in the AEJ at 600 mb.

Characteristics of Frank's Inverted "V" Model

The cloud bands associated with these waves are aligned parallel or near parallel to the lower level winds. The lower level winds are southeast behind the axis and northeast ahead. Hence the cloud bands take on an upside (inverted “V”) signature.

Typically found between the Cape Verdes and the Lesser Antilles

Characteristics of Riehl's Easterly Wave Model

Often referred to as the “classical” model as it was the first model developed for the Eastern Caribbean.

In a typical case, the area west of the wave axis is characterized by subsidence and fair weather, while areas of convergence and disturbed weather occur east of the axis.

Northeasterly winds are felt ahead of the axis and as it passes, winds veered towards the southeast.

Now we know the characteristics of the three tropical wave models, we can use the following tools to detect and track them.

Satellite Imagery including Hovmoeller Diagrams
Sea winds
Satellite-derived wind products at the CIMSS
Upper air time observations
Surface Observations
Numerical Models

Some examples:

Tropical Wave with Inverted-V signature
Tropical Wave with Inverted-V signature
Hovmoeller Diagrams showing westward propagation of convective signature
QuikSCAT Sea winds
Low level-derived winds
Twin vorticity centers associated with a wave over West Africa
Total Precipitable Water (TPW) surge
Upper air cross section from Barbados showing the passage of a recent tropical wave
Surface Observations from Point, Grenada showing the passage of a recent tropical wave
Numerical Models – 700 mb Potential Vorticity (PV), AEJ axis, winds and troughs

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Tropical wave to bring showers to the Antilles

By: Weather456, 4:58 PM GMT on June 06, 2008

A tropical wave is quickly racing towards the west at 15-20 knots and is now along 56W. A surge of moisture associated with this feature was noted on water vapor imagery. Despite this, development is not expected as upper level winds are too strong to support TC formation. In addition, recent visible imagery showed the system is moving so quickly westward that, there were no apparent signs of westerly winds south of the system that would indicate development. If that wasn’t enough, water vapor imagery also showed an upper trough across the Caribbean, producing unfavorable southwesterly upper winds across the area. This pattern is expected to last into the next 3 days so much development in the near term is not expected. Development or not, showers are expected over the Lesser Antilles, especially the Windward Isles, Trinidad and Tobago and Barbados over the next 24 hrs.

Diffluent flow associated with an upper trough and a series of embedded TUTT cells that digs into the Caribbean is helping to fueling scattered showers and thunderstorms over Central America and the SW Caribbean. This activity is aided by low level lift induced by a passing tropical wave along 79W and the Colombian Low.

Additional moisture surges from there northeastward across Puerto Rico and Hispaniola. Daytime heating over these landmasses should aid in the development of more showers later this afternoon. It would be interesting to see how the tropical wave interacts with this feature. Normally, the case for additional showers results but this is not reflected in the forecast.

Another area of disturbed weather is located just south of Guatemala and SE Mexico. The area will be monitored and updates posted.

The last area I would like to touch on is that upper low in the SW North Atlantic near 25N/68W. The upper low has already induced a surface trough to its east which is very common. Sometimes these surface troughs help produce vigorous convection which is beginning of subtropical development. The area will be monitored as it moves towards the west and conditions appear conducive for subtropical development.

GOES-12 Water Vapor Image at 11:45am EDT

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Tropical Update

By: Weather456, 12:53 PM GMT on June 04, 2008

Before, I get into the tropics, viewers to my blog have always asked me how do I detect a tropical wave and how do you find the center of a tropical storm like Arthur. I will try to put together a blog this weekend to discuss these areas.

The Tropics

Showers and thunderstorms continue over the Southeast Mexico, Guatemala and Belize in association with an area of low pressure centered over the Tehuantepec Peninsula at 18.4N/93.3N. This area should be monitored as it moves towards the west-northwest or northwest over the next day or two. Conditions appear only marginally favorable for development, as water vapor satellite animation showed dry air north of this feature across the Gulf of Mexico.

GOES 12 Infrared Image with surface observations overlaid.

The only other area to speak of in the Atlantic is a tropical wave in the Central Atlantic along 38/39W south of 15N moving towards the west. The wave is associated with a weak low pressure area attached along the axis near 10N. This is the weak low pressure area being forecast by most models a few days ago, so I’m not surprise. This is the second tropical wave this season to acquire a surface low, very unusual. Despite this, the wave remains embedded within a dry environment and thus showers remain limited or non-existent. Little expected over the next 24 hrs as it continues westward.

A possible tropical wave maybe along Western Africa.

Overview of the Atlantic Basin

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91E Near Depression Status

By: Weather456, 11:46 AM GMT on June 03, 2008

The Tropics

Tropical Invest 91E is nearing tropical depression status this morning, with last night's QuikSCAT pass showing a well-define circulation with atleast 25 knot winds and satellite imagery showing increase convection associated with this feature. Development is possible as the area moves slowly towards the north. Development or not, heavy rains are expected over Southeastern Mexico over the next day or two.

The 00Z UKMET and 06Z NOGAPS and GFS are developing an area of low pressure that is situated on the Western Coast of the Yucatan and moves it towards the northwest across the Bay of Campeche. Little is expected from this area.

Eslewhere, development is not expected over the next 24 hours. The tropical wave in the Central Atlantic continues to be associated with little or no shower activity.

Tropical Storm Arthur

Tropical Storm Arthur's death toll now stands at 7, with 5 dead and another 2 suspected dead across Southern Belize.

Read of the voices of the people affected by Arthur.


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The Tropics

By: Weather456, 12:08 PM GMT on June 02, 2008

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Tropical Storm Arthur weakened to a tropical depression on Sunday after soaking the Yucatan Peninsula with up to 6 inches of rain, but still threatened to cause dangerous flooding and mudslides in Mexico, Belize and Guatemala.

Reports indicated ports were closed in Cozumel, Isla Mujeres and Chetumal, Mexico. Television pictures from Chetumal showed vehicles driving through flooded streets, and strong waves splashing over the sea wall, even as heavy rains and winds battered the town.

Emergency officials said floodwater levels would remain high with the rain, as all drains led to the bay, which was also rising in the stormy weather. In northern Belize, the National Emergency Management Organization warned of possible flooding around the Azul Hondo River.

Currently, satellite imagery showed lingering moderate to strong convection remains over the Northwest Caribbean, bringing additional rains to Belize, Honduras and the Caymans. Along with satellite imagery, surface observations indicate something may try to develop once more in the Gulf of Honduras. This area will be monitored.

Now if that wasn’t enough, a surface trough extends from the remnant low of Arthur into the Eastern Pacific just south of Mexico, where some slow development may occur over the next day or two. It is already an unusual storm, and if that area becomes 02E Boris then it would be an even weirder storm. If Boris does form, it would reduce the chance of anything forming in the Gulf of Honduras, a similar situation that happened late last week. Can you say Dejavu? Development or not, more heavy rains for SE Mexico, Guatemala and El Salvador.

Infrared image showing the remnants of Arthur, with energy in the Gulf of Honduras (right) and in the Eastern Pacific (left). Similiar to what went down a few days ago.

A tropical wave is moving along 54W south of 14N moving west near 10-15 knots. Visible imagery and numerical models indicates some turning in the mid-levels but this remains weak. Furthermore, a recent QuikSCAT pass indicated no signs of a circulation at the surface with northeast trades dominating the area. Scattered cloudiness and showers are ahead of the wave axis from 12N to inland over South America. Currently, conditions seem favorable for development with warm SSTs, humidity and favorable upper level winds with outflow. However, the wave will encounter more hostile conditions in the SE Caribbean, so its future remains uncertain. Development or not, the wave will bring some showers to our friends in the Southern Windward Islands and Trinidad.

Showers remain limited with the tropical wave in the Eastern Atlantic. There remains a well-define cyclonic turning within the lower levels. The wave will be monitored for signs of increase convection. Otherwise, little development expected over the next day or two.

Goes-12/MGS-2 Atlantic Infrared Image taken 6am EDT

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June Outlook; The Tropics

By: Weather456, 12:10 PM GMT on June 01, 2008

The 2008 Atlantic Hurricane Season begins today and runs until November 30. A typical Atlantic hurricane season sees about 11 named storms, of which 6 become hurricanes and of those 6 hurricanes, 3 attain major hurricane status of winds above 111 mph. The forecasts issued by most major forecasting centers is for this season to be near-normal to above average, with the most recent outlook issued by NOAA. As for me, I am expecting an above average hurricane season, especially an active Cape Verde season, with 15 named storms, 7 hurricanes and 3 major hurricanes. We have already seen one tropical storm so far. The climatological peak of the season occurs around September 10.

June Climatology and Outlook

June normally sees three tropical cyclones per 5 years, that is, over a five year span; we see three tropical cyclones forming in June. However, over the past five years we have seen 5 tropical cyclones forming in June indicating some exceptions. This is the time of the year when sea surface temperatures and instability increases across the tropics. The mid-latitude retreat and become less active allowing a more favorable environment for tropical cyclone formation.

In June, we typically look at the Western Caribbean and Gulf of Mexico for development, as conditions are ripe for TC formation. We have warm sea surface temperatures, ample moisture and heat topped off with an overhead ridge allowing favorable shear. Westerlies remain over the Tropical Atlantic which makes it difficult for systems to form out there.

Any system forming here, normally tracks northward or northeastward, these are very consistent paths, especially over the past 5 years.

I am expecting one storm to form in June 2008. This is based on an MJO pulse that is expected to end around June 12 and return around the early part of July, below normal wind shear, near normal to above average sea surface temperatures and climatology.

June zones of origin and tracks

Tropical Storm Arthur

Tropical Storm Arthur formed yesterday afternoon just offshore Belize. The storm initially moved ashore in the mid-morning but then the center relocated offshore and based on organization and surface reports, the NHC began issuing advisories. The storm later made landfall in Belize as a 40 mph tropical storm. Despite this, the storms still maintained tropical storm status while over land due its impressive satellite presentation and surface reports.

This morning Arthur remains inland over Mexico as a 40 mph Tropical Storm and reemergence into the Bay of Campeche is becoming more unlikely as he moves westward and he sould weaken to tropical depression status later today. However, there is still always the possibility that Arthur may enter the Bay as he is located towards the southwest of a ridge over the SE USA/Western Atlantic. The main threat from Arthur will continue to be he’s heavy rains which could cause up to 10 inches over parts of Belize, Mexico and Guatemala, potentially causing life-threatening flash floods and mudslides, especially in higher terrain.

Want more drama? This morning’s infrared imagery showed a mid-level circulation just offshore Belize near the deepest convection. This area will be closely monitored as surface observations are hinting cyclonic flow and this would not be the first time Arthur surprised us.

The formation of Arthur made the first time ever that a named storm formed in two consecutives Mays – the other being Andrea from last year. This is also the first tropical storm in May since Arlene in 1981.

GOES-12 Infrared Image of Tropical Storm Arthur

Eastern Atlantic Wave

A tropical wave is along 26W south of 10N moving west near 15 knots. The wave continues to display excellent mid-lower level turning seen on satellite imagery but a recent QuikSCAT pass indicated that a circulation is clearly at the surface near 6N. Showers have diminished significantly along this feature from 24 hrs ago but conditions remain favorable for development. Scattered low clouds surrounds the wave axis. Whether or not this develops, it will remain embedded in deep westerly flow south of an expanding subtropical ridge which would put it near 50W in one week’s time.

GOES-12/MSG-2 Composite Infrared Image taken 2am EDT

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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.