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SUBTROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   1
NATIONAL WEATHER SERVICE MIAMI FL
5 PM EDT FRI SEP 20 2002
 
SATELLITE IMAGERY INDICATES THAT THE NON-TROPICAL LOW THAT FORMED
EAST-SOUTHEAST OF BERMUDA HAS DEVELOPED ORGANIZED CONVECTION...A
SIGN OF TROPICAL CHARACTERISTICS.  THERE IS ALSO EVIDENCE OF
ANTICYCLONIC OUTFLOW AT UPPER LEVELS.  ON THE OTHER HAND...A TRMM
OVERPASS AT 1601Z INDICATED THAT THE MAXIMUM WINDS WERE AT LEAST
100 N MI FROM THE CENTER...WHICH IS RATHER UNLIKE A TROPICAL
CYCLONE.  THE LOW IS THUS UPGRADED TO A SUBTROPICAL DEPRESSION
INSTEAD OF A TROPICAL DEPRESSION.

THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 035/4.  THE CYCLONE IS
TRAPPED BETWEEN THE SUBTROPICAL RIDGE TO THE EAST...A DEEP-LAYER
RIDGE TO THE NORTH...AND A DEEP-LAYER CYCLONIC CIRCULATION CENTERED
SOUTHWEST OF BERMUDA.  LARGE-SCALE MODELS INDICATE THAT THE
EVOLUTION OF THESE FEATURES WILL BOUNCE THE DEPRESSION AROUND THE
SUBTROPICAL ATLANTIC IN A RATHER ERRATIC FASHION...AND THE VARIOUS
TRACK GUIDANCE MODELS RESPOND TO THIS WITH A VARIETY OF LOOPS AND
HAIRPIN TURNS.  IT IS UNCLEAR AT THIS POINT WHICH MODEL HAS THE
BEST HANDLE ON THE SYSTEM...SO THE FORECAST TRACK WILL CALL FOR THE
CYCLONE TO MOVE NORTH-NORTHEASTWARD FOR 36-48 HR...FOLLOWED BY A
WESTWARD TURN AS IT ENCOUNTERS THE RIDGE TO THE NORTH.  THIS TRACK
IS IN BEST AGREE WITH THE BAM MODELS.

CONDITIONS CURRENTLY APPEAR FAVORABLE FOR DEVELOPMENT...AS THE
CYCLONE SHOWS GOOD OUTFLOW IN A LIGHT SHEAR ENVIRONMENT AND
CONVECTION IS INCREASING NEAR THE CENTER.  THUS...IT IS RATHER
PUZZLING THAT THE GLOBAL MODELS DO NOT WANT TO DEVELOP THIS 
LARGE-SCALE HYBRID CYCLONE.  THE INTENSITY FORECAST IS BASED ON
THE PREMISE THAT THE CURRENT DEVELOPMENT WILL CONTINUE...WITH THE
CYCLONE GRADUALLY BECOMING MORE TROPICAL WITH TIME.  SHOULD THE
SHEAR REMAIN AS LIGHT AS FORECAST BY THE SHIPS MODEL...THE SYSTEM
WOULD HAVE A GOOD CHANCE TO ACQUIRE FULL TROPICAL CHARACTERISTICS
SOMETIME DURING THE FORECAST PERIOD.
 
FORECASTER BEVEN
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     20/2100Z 28.4N  51.7W    25 KTS
12HR VT     21/0600Z 29.2N  51.4W    30 KTS
24HR VT     21/1800Z 31.0N  50.9W    35 KTS
36HR VT     22/0600Z 32.6N  50.5W    40 KTS
48HR VT     22/1800Z 33.5N  50.5W    45 KTS
72HR VT     23/1800Z 33.5N  52.0W    50 KTS
 
 
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