096
WTNT43 KNHC 261431
TCDAT3
TROPICAL DEPRESSION EIGHT DISCUSSION NUMBER 4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082009
1100 AM AST SAT SEP 26 2009
SATELLITE IMAGERY THIS MORNING INDICATES THAT TROPICAL DEPRESSION
EIGHT HAS BECOME A LITTLE LESS ORGANIZED...WHICH IS LIKELY TO DUE
TO 20 KT OF WEST-SOUTHWESTERLY VERTICAL WIND SHEAR INDICATED BY
ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN. THE
CIRCULATION HAS BECOME ELONGATED ALONG A NORTHEAST-SOUTHWEST
AXIS...WITH TWO OR THREE EMBEDDED VORTICITY CENTERS NOTED. IN
ADDITION...THE CONVECTIVE BANDING IS FARTHER FROM THE CENTER THAN
24 HR AGO. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN
30 AND 25 KT...SO THE INITIAL INTENSITY REMAINS 30 KT.
THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 295/12. THE LARGE-SCALE
MODELS FORECAST A LARGE-DEEP LAYER TROUGH AND ASSOCIATED SURFACE
COLD FRONT CURRENTLY OVER THE WESTERN ATLANTIC TO MOVE EASTWARD
DURING THE NEXT 48-96 HR. THIS SHOULD CAUSE THE SUBTROPICAL RIDGE
OVER THE EASTERN ATLANTIC TO WEAKEN...AND SHOULD ALLOW THE TROPICAL
DEPRESSION TO TURN NORTHWARD AND THEN RECURVE INTO THE WESTERLIES
ON THE SOUTH SIDE OF THE TROUGH. MOST OF THE TRACK GUIDANCE CALLS
FOR THIS SCENARIO...AND THE NEW TRACK FORECAST IS FOR RECURVATURE
AFTER 72 HR. AN ALTERNATE SCENARIO IS SHOWN BY THE ECMWF...WHICH
FORECASTS THE DEPRESSION TO WEAKEN WITH THE REMNANT TROUGH MOVING
WESTWARD IN THE LOW-LEVEL FLOW.
THE INTENSITY FORECAST IS A LITTLE PROBLEMATIC. THE DEPRESSION IS
NOT ONLY ENCOUNTERING MODERATE SHEAR...BUT IS ALSO OVER A TONGUE OF
COLDER SEA SURFACE TEMPERATURES AND HAS AN AREA OF DRY AIR JUST TO
ITS WEST. WHILE THE SHEAR IS FORECAST TO CHANGE LITTLE DURING THE
NEXT 48-60 HR...THE SSTS INCREASE ALONG THE FORECAST TRACK. THE
MOST LIKELY SCENARIO IS THAT THE COMBINATION OF SHEAR AND DRY AIR
WILL CAUSE THE DEPRESSION TO GRADUALLY DECAY OVER THE NEXT
48-72 HR. ON THE OTHER HAND...WITH THE INCREASING SSTS...THE
CYCLONE COULD STRENGTHEN IF AND WHEN THE SHEAR DECREASES...AS
SUGGESTED AFTER 72 HR BY THE SHIPS...LGEM...AND GFDL MODELS. THE
NEW INTENSITY FORECAST GOES WITH THE WEAKENING SCENARIO...CALLING
FOR THE DEPRESSION TO DECAY INTO A REMNANT LOW IN ABOUT 72 HR.
HOWEVER... THERE IS LOWER CONFIDENCE IN THIS SCENARIO THAN THERE
WAS YESTERDAY.
THE DYNAMICAL MODELS SHOW THE DEPRESSION OR ITS REMNANTS SURVIVING
LONGER THAN THEY DID 24 HR AGO...SO A 96 HR FORECAST POINT HAS BEEN
ADDED. THE SYSTEM IS EXPECTED TO DISSIPATE OR BE ABSORBED INTO THE
BAROCLINIC TROUGH BY 120 HR.
FORECAST POSITIONS AND MAX WINDS
INITIAL 26/1500Z 18.0N 34.8W 30 KT
12HR VT 27/0000Z 19.1N 36.4W 30 KT
24HR VT 27/1200Z 20.5N 37.8W 30 KT
36HR VT 28/0000Z 21.9N 38.9W 25 KT
48HR VT 28/1200Z 23.0N 40.0W 25 KT
72HR VT 29/1200Z 24.5N 40.5W 20 KT...REMNANT LOW
96HR VT 30/1200Z 26.0N 37.0W 20 KT...REMNANT LOW
120HR VT 01/1200Z...DISSIPATED
FORECASTER BEVEN
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