INVEST 98L – CARIB HEATS UP (KIND OF) – STRONG WAVE WEST AFRICAN COAST

By: Steve Gregory , 5:11 PM GMT on August 28, 2014

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INVEST 98L – NO THREAT

A weak and broad surface Low pressure system is centered about 120NM east of Brownsville, TX, and is drifting W/SW at about 5Kts. As occurred yesterday morning, there’s been another early morning burst of convection near and to the northeast of the Low itself. This system originated in the NW GOM at the ‘tail end’ of a very strong mid-level Dry Line that surged south/southwest from the SE US across Florida and into the northern Gulf of Mexico (GOM) 3 days ago – triggering a line of very strong T-storms along and ahead of it. While the dry line boundary has essentially faded away, relatively dry air continues to dominate many areas of the Gulf due in large part to subsidence under a large, high level anti-cyclone. The small surface circulation and ‘ball’ of convection that is 98L is still within an area of high wind shear that exists around the periphery of the large high level anti-cyclone. While the shear has dropped off from yesterday, it is still unfavorable for development, with speeds averaging over 20Kts. While the shear may drop-off enough to allow the system to develop some, it is extremely unlikely the system could become a Tropical Depression before it moves inland overnight or by FRI AM.

CARIBBEAN MOISTENS UP WITH COMPLICATED SET OF DISTURBANCES

There are 2 significant Wave structures in the CARIB, with an Inverted TROF in the western CARIB that WAS at the southern end of the T.W. system that gave birth to CRISTOBAL last weekend. A second elongated tropical wave that was INVEST 97L a couple days ago before NHC dropped it, is in the vicinity of the far eastern CARIB. A relatively high moisture environment dominates much of the CARIB in association with the above 2 systems and a moisture surge from the deep tropics that was pulled northward over the past couple days to the east of a small and weakening upper air LOW.

Wind shear remains high over the northern portions of the eastern CARIB where the bulk of the Tropical Wave ('EX' 97K) is located, but is lower in the southern CARIB. Most every Global model run since yesterday morning has consistently moved the eastern CARIB wave westward across the CARIB towards the Yucatan over the weekend, and then begins to develop the system by the time it moves into the southern GOM/Bay of Campeche next week. This evolution seems reasonable given the expected lower shear environment, very warm waters in the western CARIB and GOM – along with a more anti-cyclonic flow in the upper air. Since the system is relatively shallow and embedded in the easterly trades, the track forecast shown by the Global models, at least for the next 2-3 days, seems quite reasonable as well. One final note: while none of the models show the system developing into a cyclone prior to reaching the Yucatan Peninsula, I believe there is a low but significant (10%) chance that it could become a Depression before leaving the CARIB.

MODERATELY STRONG DISTURBANCE EMERGING OFF WEST AFRICAN COAST

What was one of the season's strongest ‘African Waves’ has emerged off the West African coast, but appears to have weakened significantly over the last 24 hours. However, this may be a function of the huge drop-off in convection as the system moved over the cooler water off the west African coast and the SAL becoming entrained in the northern portion of the system. There still appears to be a mid level rotation, but the low level circulation appears to have opened at the southern end.

This weakening of the circulation field appears to have been picked up by the Global models which all week had forecast the system to become a cyclone within 1-2 days after moving off the African coast, and then tracked it to the West/Northwest before turning the system northward in the central Atlantic – and ultimately out sea. However, with the weakening of this system, the models are now forecasting this relatively shallow system to remain caught up in the easterly Trades, moving it towards the CARIB late next week, with only minor indications of development.

FYI: At this time, it appears well defined Tropical Waves will be coming off the West African coast every 4-7 days for the next 3 weeks – but with none of the models forecasting major intensification of any of them at this time. This MAY turn out to be correct in a few cases as the convectively suppressed portion of CCKW's will be over the MDR starting late next week.



Fig 1: Early morning imagery shows the small Low pressure area in the far NW GOM (INVEST 98L) with its small area of strong convection – an active inverted TROF / Tropical Wave in the western Caribbean and an elongated Tropical Wave (once called INVEST 97L) in the eastern CARIB region. These 2 waves are generally westbound at 15-20 Kts. Finally, what was an especially strong and convectively active Tropical Low and Wave structure while moving across Africa, has weakened significantly during the last 30 hours, with very little shower/convection - especially for that portion of the system off the African coast. (This weakening is primarily due to cooler SST’s and African dust / dry air associated with the Saharan Air Layer (SAL) which extends from near the wave structure westward into the central Atlantic (the approximate leading edge of which is delineated by the dashed line). Until such time that the T.W. can escape or mix out this dry and dusty air – little development can be expected.



Fig 2: The above wind shear analysis over the GOM shows INVEST 98L within a high shear environment that surrounds the large upper air anti-cyclone (High pressure) centered over the central Gulf region. No significant development is expected before the system moves inland overnight.



Fig 3: The early morning VIS/IR image of the Caribbean depicts the extensive cloud cover associated with the two Tropical Waves (T.W.) and upper air flow of moisture laden air from the deep tropics of northern South America (SOAMER). The T.W. over the eastern CARIB has become an East-West orientated wave that is moving west, but has shown a slight northward component drift over the last 48 hours, and for the most part, remains within a high shear (30 Kts) environment (See Fig 5 below), inhibiting any development. That said, the system is expected to reach the western CARIB in a few days and should encounter a bit more favorable overall environment for development as it approaches the Yucatan.



Fig 4: The above Water Vapor (WV) image of the Caribbean region shows something we have rarely (if ever) seen this season: a lot of moisture! Within this environment we find a fairly complicated set of T.W.s’, an upper Low /inverted TROF (not identified) and a flow of moisture into the CARIB from the south. The dry air north of the eastern CARIB originated from the SAL, but is not showing any significant movement at this time.



Fig 5: High level winds over the GOM and CARIB show 2 significant features: a large anti-cyclonic flow pattern over much of the GOM (again, something we have not often seen this season) and a small but developing anti-cyclonic flow in the south central CARIB between the small (and dissipating) upper Low in the west-central CARIB and the tropical wave in the eastern CARIB. This ‘setup’ is often seen in the tropics – but only sometimes does it directly lead to the formation a large and strong anti-cyclone that ultimately helps in the development of a tropical cyclone.

The next update will be SUNDAY UNLESS CONDITIONS WARRANT AN EARLIER UPDATE

Steve Gregory

* NOTE: My regularly scheduled Weather Updates is 3-Days per week on Sundays, Tuesdays and Thursdays – except when we have or expect active/significant Tropical Cyclones in the Atlantic Basin – in which case Updates will be at least once daily. In addition, if a strong cyclone is expected to impact the US mainland – I will be posting my own detailed forecast charts.

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