Tropical Depression could form later this week and threaten the Leeward Islands
A tropical wave with an associated area of low pressure in the Eastern Atlantic is increasing the potential to become the next tropical depression of the season. Satellite imagery along with surface analysis charts revealed a broad area of low pressure centered near 1009 millibars at 9N-34W. Much of the convection is currently west of the low-pressure area due to easterly shear from a superimposed upper anticyclone to its northeast. Despite this, convection has increase in both spatial and intensity and has become more concentrated and organized around the low-pressure area. An ASCAT pass from last night along with RGB imagery indicated that this low-pressure area lies along the monsoon trough with west winds noted south of the system. However, there is little indications that the low-pressure area itself is closed but it should have little troubling closing off since it is not embedded in the classic “NE-SE” ITCZ, which is confluent but rather the “NE-SW” monsoon trough which is cyclonic. Upper winds, with the exception of the super-imposed anticyclone, appear conducive for development of a depression over the next few days. A depression should be declared when the low-pressure area manages to close off and convection becomes more concentrated around it, which I think, should not take more than 2 days.
Figure 1. Visible image of the Central Atlantic showing 91L's relation to the islands.
SHIPS appear to be handling the environment around 91L very well, indicating 14 knots of shear from 31 degrees (NNE) which is being observed on satellite imagery. Should we follow the SHIPS guidance, upper winds should continue to lower over the next 3 days, giving way to favourable conditions for gradual intensification and should 91L be declared a depression, could become the 2nd hurricane of the season. Through 3-5 days, shear begins to drastically increase due to encounter with the Tropical Upper Tropospheric Trough (TUTT), which will be located north of the Leeward Islands. The LGEM is the best model when it comes to intensity due to a change in the environment and so this is the guidance I am going with to have a potentially category 1 hurricane approaching the islands in 4 days, weakening as it encounters the TUTT.
Currently the steering flow is predominately westward, with a very large weakness in the subtropical ridge between 60W and 90W (30 degrees of weakness, wow). Most model guidances indicate that this weakness should influence 91L to move more west-northwest to northwest once it takes off. The exact timing of both features and the angle at which 91L leaves 10N will indicate how close the system gets to the extreme northeast Leeward Islands, the Virgin Islands and Puerto Rico. The weakness will not be strong enough to recurve 91L and it is likely to continue northwestward towards the Bahamas and the USA but this is beyond 1 week so to pinpoint a potential landfall that far on the spatial and temporal (time) scales will not be reliable. The ECMWF develops 91L and takes it on Georges-type track with probably a similar intensity. A feature that far south approaching 40W should be monitored closely.
Figure 2. Current computer model runs of 91L's track.
Interests in the Northeast Caribbean should continue to monitor the progress of 91L.
I postponed my August Outlook until a later date.
Weather456
Reader Comments
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Its not an ULL or TUTT cell, which would drift west to SW. This is the semi-permanent TUTT axis which rarely leaves its position north of the islands.
The weakness is too large to the north for the high to rebuild with such influence. The high is also not sprawling as in 2007, its more centralised.
It is 2010 when 20 named storms are being predicted....expect anything at this point, but hope for the best.
Great update -- timely and to the point.
Best Regards
that is because, semi-permanent does not mean fix in location and intensity. Just like the Azores High is named after the geographic location the high is predominately found (near the azores islands, it varies in intensity and position but basically located around the same geographic region. The TUTT is like this, and even though it is located north of the islands most of the time, it varies in intensity and position. It can be located near the East Coast or in the central Atlantic, but these motions varies month to month hence why we say semi-permanent. Not all tropical cyclones encounter the same variations of the TUTT. The TUTT might be weak or near absence allowing for cyclones to intensify all the way like Frances, Isabel and many others. It might be located in the E Atlantic and hinder development like Irene 2005, until the system reaches the W Atlantic. it might dig deep into tropical atlantic and hinder development like Karen 2007. it all depends on location of the TUTT and this case, the TUTT has been mostly located north of the islands and is suppose to be located there based on the GFS and some other models. the ECMWF foresees a weak trough, another variation.
No it would not be safe to say so. Notice that all three systems this year - Alex - TD 2 and Bonnie were all steered into the GOM. At the very edge of most model cycles like the GFS and ECMWF, 91L ends up in the GOM.
TIA
sheri
WTNT32 KNHC 290303
TCPAT2
BULLETIN
TROPICAL STORM EARL ADVISORY NUMBER 15
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL072010
1100 PM AST SAT AUG 28 2010
...EARL MOVING WESTWARD TOWARD THE LEEWARD ISLANDS...NEW WATCHES AND
WARNINGS ISSUED...
SUMMARY OF 1100 PM AST...0300 UTC...INFORMATION
-----------------------------------------------
LOCATION...16.6N 55.4W
ABOUT 520 MI...835 KM E OF THE NORTHERN LEEWARD ISLANDS
MAXIMUM SUSTAINED WINDS...65 MPH...100 KM/HR
PRESENT MOVEMENT...W OR 280 DEGREES AT 20 MPH...32 KM/HR
MINIMUM CENTRAL PRESSURE...989 MB...29.21 INCHES
WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY...
THE GOVERNMENTS OF ANTIGUA AND BARBUDA HAVE ISSUED A HURRICANE
WARNING FOR ANTIGUA...BARBUDA...MONTSERRAT...ST. KITTS...
NEVIS...AND ANGUILLA.
THE GOVERNMENT OF THE NETHERLANDS ANTILLES HAS ISSUED A HURRICANE
WATCH FOR ST. MAARTEN...SABA...AND ST. EUSTATIUS.
THE GOVERNMENT OF ANTIGUA AND BARBUDA HAS ISSUED A HURRICANE WATCH
FOR THE BRITISH VIRGIN ISLANDS.
A HURRICANE WATCH HAS BEEN ISSUED FOR PUERTO RICO AND THE U. S.
VIRGIN ISLANDS...INCLUDING THE ISLANDS OF CULEBRA AND VIEQUES.
SUMMARY OF WATCHES AND WARNINGS IN EFFECT...
A HURRICANE WARNING IS IN EFFECT FOR...
* ANTIGUA...BARBUDA...MONTSERRAT...ST. KITTS...NEVIS...AND ANGUILLA
A HURRICANE WATCH IS IN EFFECT FOR...
* ST. MAARTEN...SABA...ST. EUSTATIUS
* BRITISH VIRGIN ISLANDS
* U.S. VIRGIN ISLANDS
* PUERTO RICO INCLUDING THE ISLANDS OF CULEBRA AND VIEQUES
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* SAINT MARTIN AND SAINT BARTHELEMY
* ST. MAARTEN...SABA...ST. EUSTATIUS
A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA. A WARNING IS TYPICALLY ISSUED
36 HOURS BEFORE THE ANTICIPATED FIRST OCCURRENCE OF
TROPICAL-STORM-FORCE WINDS...CONDITIONS THAT MAKE OUTSIDE
PREPARATIONS DIFFICULT OR DANGEROUS. PREPARATIONS TO PROTECT LIFE
AND PROPERTY SHOULD BE RUSHED TO COMPLETION.
A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA. A WATCH IS TYPICALLY ISSUED 48 HOURS
BEFORE THE ANTICIPATED FIRST OCCURRENCE OF TROPICAL-STORM-FORCE
WINDS...CONDITIONS THAT MAKE OUTSIDE PREPARATIONS DIFFICULT OR
DANGEROUS.
A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED
STATES...INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE
MONITOR PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE
FORECAST OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE
THE UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.
DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
AT 1100 PM AST...0300 UTC...THE CENTER OF TROPICAL STORM EARL WAS
LOCATED NEAR LATITUDE 16.6 NORTH...LONGITUDE 55.4 WEST. EARL IS
MOVING TOWARD THE WEST NEAR 20 MPH...32 KM/HR...AND THIS GENERAL
MOTION IS EXPECTED TO CONTINUE WITH A GRADUAL DECREASE IN FORWARD
SPEED ON SUNDAY. A GRADUAL TURN TOWARD THE WEST-NORTHWEST IS
EXPECTED SUNDAY NIGHT AND MONDAY. ON THE FORECAST TRACK...THE
CENTER OF EARL COULD PASS NEAR OR OVER THE NORTHERN LEEWARD ISLANDS
SUNDAY NIGHT OR MONDAY.
MAXIMUM SUSTAINED WINDS ARE NEAR 65 MPH...100 KM/HR...WITH HIGHER
GUSTS. STRENGTHENING IS FORECAST DURING THE NEXT 48 HOURS...AND
EARL IS EXPECTED TO BECOME A HURRICANE ON SUNDAY.
TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 160 MILES...260 KM
FROM THE CENTER.
ESTIMATED MINIMUM CENTRAL PRESSURE IS 989 MB...29.21 INCHES.
HAZARDS AFFECTING LAND
----------------------
WIND...HURRICANE CONDITIONS ARE EXPECTED OVER THE NORTHERN LEEWARD
ISLANDS BY SUNDAY NIGHT. HURRICANE CONDITIONS MAY SPREAD OVER THE
VIRGIN ISLANDS AND PUERTO RICO MONDAY AND MONDAY NIGHT.
STORM SURGE...STORM SURGE WILL RAISE WATER LEVELS 1 TO 3 FEET ABOVE
NORMAL TIDE LEVELS...ACCOMPANIED BY LARGE AND DANGEROUS BATTERING
WAVES...NEAR THE CENTER OF EARL.
RAINFALL...EARL IS EXPECTED TO PRODUCE TOTAL RAINFALL ACCUMULATIONS
OF 3 TO 5 INCHES OVER MUCH OF THE NORTHERN LEEWARD ISLANDS...WITH
POSSIBLE MAXIMUM ISOLATED AMOUNTS OF 8 INCHES.
NEXT ADVISORY
-------------
NEXT INTERMEDIATE ADVISORY...200 AM AST.
NEXT COMPLETE ADVISORY...500 AM AST.
$$
FORECASTER BEVEN
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