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...FOR INTERGOVERNMENTAL USE ONLY...
TROPICAL STORM SEBASTIEN DISCUSSION NUMBER  12
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT MON OCT 23 1995
 
SOME CONVECTION HAS RETURNED NEARER TO THE CENTER...HOWEVER
SEBASTIEN REMAINS IN A STRONG SHEARING ENVIRONMENT AND THUNDERSTORMS
ARE NOT ORGANIZING AROUND THE LOW-LEVEL CIRCULATION CENTER.  THE
LATEST AVN MODEL FORECAST FOR 200 MB SHOWS THE UPPER LOW NORTH OF
PUERTO RICO WEAKENING WITH A LARGE ANTICYCLONE CENTERED OVER CUBA
REMAINING DOMINANT.  HOWEVER THE AVN MODEL HAS A WELL-KNOWN BIAS
TOWARDS WEAKENING CYCLONES IN UPPER-LEVEL TROUGH NEAR THE ANTILLES.
IF THE UPPER LEVEL LOW HOLDS TOGETHER IT COULD MAINTAIN
SOUTHWESTERLY SHEARING OVER SEBASTIEN...AND WEAKEN IT.  ANOTHER
POSSIBILITY IS THAT SEBASTIEN COULD GET SHEARED MORE FROM THE NORTH
DUE TO THE LARGE UPPER-LEVEL ANTICYCLONE TO ITS WEST.  THE OFFICIAL
INTENSITY FORECAST CALLS FOR SOME DECREASE IN WINDS...BUT THE STORM
COULD EASILY WEAKEN MUCH FASTER.
 
MOTION HAS BEEN SOUTHWESTWARD...NEAR 235/10.  ALL OF THE PURELY
DYNAMICALLY-BASED TRACK PREDICTION MODELS QUICKLY TURN SEBASTIEN TO
THE WEST AND NORTHWEST.  THE AVN DEVELOPS A BROADER-SCALE CYCLONIC
CIRCULATION TO THE SOUTHWEST OF SEBASTIEN.  THIS LARGER LOW IS
FORECAST BY THE MODEL TO TRACK WESTWARD ACROSS THE GREATER ANTILLES
WITH SEBASTIEN MOVING NORTHWESTWARD ON THE NORTHEASTERN PERIPHERY OF
THE LOW.  THE LATEST OFFICIAL FORECAST SHOWS A MORE WESTWARD MOTION
DEVELOPING WITHIN 12 TO 24 HOURS FOLLOWED BY A TURN TO THE WEST-
NORTHWEST AND NORTHWEST...SIMILAR TO THAT FROM THE PREVIOUS
ADVISORY.

MOST OF THE STRONG WINDS ARE OVER THE NORTHERN SEMICIRCLE OF THE
STORM...WHICH SHOULD MINIMIZE THE EFFECTS OF SEBASTIEN ON THE
ISLANDS OF THE NORTHEAST CARIBBEAN.  HOWEVER...A TROPICAL STORM
WATCH IS BEING POSTED FOR THE U.S. AND BRITISH VIRGIN ISLANDS...IN
CASE SEBASTIEN DOES NOT TURN MORE WESTWARD AND DOES NOT WEAKEN.  
 
PASCH
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     23/0900Z 20.0N  60.9W    50 KTS
12HR VT     23/1800Z 19.7N  62.5W    45 KTS
24HR VT     24/0600Z 19.8N  64.4W    40 KTS
36HR VT     24/1800Z 20.3N  66.0W    40 KTS
48HR VT     25/0600Z 21.0N  67.5W    40 KTS
72HR VT     26/0600Z 23.0N  70.0W    40 KTS
 
 
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