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TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   7
NATIONAL WEATHER SERVICE MIAMI FL
11 AM EDT THU OCT 07 1999
 
THERE HAS BEEN LITTLE CHANGE IN THE OVERALL STRUCTURE OF TROPICAL
DEPRESSION TWELVE DURING THE LAST 24 HOURS.  THE CENTER REMAINS
EXPOSED WEST OF THE DEEP CONVECTION DUE TO PERSISTENT SHEAR.  DVORAK
SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN AT 35 KT AND
30 KT RESPECTIVELY. THUS...THE INITIAL INTENSITY WILL REMAIN 30 KT.

VISIBLE IMAGERY SHOWS THE LOW LEVEL CENTER IS A LITTLE FARTHER EAST
THAN INDICATED DURING THE NIGHT...SO THERE IS A SLIGHT RELOCATION OF
THE INITIAL POSITION. THE INITIAL MOTION IS 300/3. WATER VAPOR
IMAGERY INDICATES A LARGE MID/UPPER LEVEL LOW CENTERED NEAR
26N45W...MOVING SLOWLY EAST. VISIBLE IMAGERY AND SHIP REPORTS
SUGGESTS THAT THE LOW HAS CREATED A WEAKNESS IN THE LOW LEVEL RIDGE
NORTH OF THE DEPRESSION...LEAVING IT IN VERY WEAK STEERING FLOW. THE
UKMET MODEL...WHICH SEEMS TO HAVE THE BEST HANDLE ON THE UPPER
LOW...INDICATES IT WILL BE 24 TO 36 HOURS BEFORE THE LOW WEAKENS AND
MOVES FAR ENOUGH TO THE EAST FOR ITS INFLUENCE ON THE DEPRESSION TO
DIMINISH. ACCORDINGLY...THE OFFICIAL FORECAST TRACK CONTINUES THE
SLOW WEST-NORTHWEST MOTION FOR 36 HOURS...WITH SOME INCREASE IN
FORWARD SPEED AFTER THAT.

CIRRUS CLOUD MOTIONS SHOW AN ANTICYCLONE OVER THE DEPRESSION.
HOWEVER...WATER VAPOR IMAGERY CLEARLY SHOWS WEST-SOUTHWEST FLOW
ASSOCIATED WITH THE UPPER LOW UNDERCUTTING THE ANTICYCLONE AND
CAUSING SHEAR. THE UKMET...AVN...AND NOGAPS ALL INDICATE THE SHEAR
SHOULD DIMINISH AFTER 24 TO 36 HOURS...WHICH WOULD ALLOW THE
STRENGTHENING INDICATED IN THE INTENSITY FORECAST. GIVEN THE MODELS
TRACK RECORD ON FORECASTING THE SHEAR AFFECTING THIS SYSTEM... IT
WOULD NOT BE SURPRISING TO SEE IT PERSIST LONGER THAN THAT. IT IS
ALSO POSSIBLE THE DEPRESSION COULD WEAKEN BEFORE THE SHEAR CAN
DECREASE. HOWEVER...THE CIRCULATION IS 400-500 NM WIDE AND THEREFORE
IS UNLIKELY TO DISSIPATE WITH THE CURRENT AMOUNT OF SHEAR.
 
BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     07/1500Z 15.9N  46.9W    30 KTS
12HR VT     08/0000Z 16.1N  47.3W    30 KTS
24HR VT     08/1200Z 16.4N  47.9W    30 KTS
36HR VT     09/0000Z 16.7N  48.5W    35 KTS
48HR VT     09/1200Z 17.0N  49.5W    40 KTS
72HR VT     10/1200Z 18.0N  52.0W    50 KTS
 
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