[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 212134
TCDAT1
TROPICAL DEPRESSION SIX DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT THU JUL 21 2005

AIR FORCE RECON AIRCRAFT HAS BEEN INVESTIGATING THE STRONG TROPICAL
WAVE JUST EAST OF THE BAHAMAS AND HAVE FOUND WINDS THAT SUPPORT AT
LEAST A 30-KT TROPICAL DEPRESSION...AND ADVISORIES HAVE BEEN
INITIATED ON TD-6. CONVECTION HAS DECREASED SOMEWHAT NEAR THE
CENTER OVER THE PAST COUPLE OF HOURS...BUT THIS IS TYPICAL FOR THIS
TIME OF THE DAY. UPPER-LEVEL OUTFLOW HAS CONTINUED TO IMPROVE
DURING THE DAY...INCLUDING THE RESTRICTED SOUTHWEST QUADRANT.

THE INITIAL MOTION ESTIMATE IS 300/11...BASED ON THE PAST 3 HOURS OF
SATELLITE POSITIONS. THE FORECAST TRACK IS SOMEWHAT PROBLEMATIC.
THE GLOBAL MODELS TAKE THE SURFACE TO 850 MB CIRCULATION NORTHWARD
AND THEN RECURVE IT NORTHEASTWARD THROUGH THE SUBTROPICAL RIDGE.
HOWEVER...ALL OF THE MODELS GENERALLY AGREE THAT THE MID-LEVEL/500
MB CIRCULATION WILL MOVE INTO THE RIDGE AXIS AND STALL IN ABOUT 48
HOURS AS A SHORTWAVE TROUGH MOVES NORTHEAST OF THE DEPRESSION AND
ALLOWS THE RIDGE TO BUILD BACK IN TO THE WEST. AFTER THAT...SOME OF
THE MODELS MEANDER THE SYSTEM SLOWLY EAST OR SOUTHWARD...WHILE THE
GFS AND GFDL MODELS TAKE THE MID-LEVEL CIRCULATION SOUTHWESTWARD
TOWARD FLORIDA. THE MEDIUM AND DEEP BAM MODELS ACTUALLY TAKE THE
DEPRESSION SLOWLY WESTWARD AFTER 48 HOURS ACROSS CENTRAL FLORIDA.
GIVEN THE LARGE SPREAD IN THE MODEL GUIDANCE...WHICH TYPICALLY
SUGGESTS SLOW MOTION...THE FORECAST TRACK SHOWS A GRADUAL TURN TO
THE NORTH AFTER 24 HOURS AND THEN TURNS THE CYCLONE SLOWLY EASTWARD
AND MAKING A SMALL CLOCKWISE LOOP WELL EAST OF THE CENTRAL FLORIDA
COAST.

THE WATER AHEAD OF THE DEPRESSION IS QUITE WARM...AROUND 29C...AND
THE SHEAR IS FORECAST REMAIN LOW WITH A FAVORABLE OUTFLOW PATTERN
FOR THE NEXT 36 TO 48 HOURS. THEREFORE...THE DEPRESSION COULD REACH
HURRICANE STRENGTH WITHIN 48 TO 72 HOURS. NOTE...THE AIRCRAFT HAS
JUST REPORTED AT 2114Z FLIGHT-LEVEL WINDS OF 48 KT...WHICH WOULD
NORMALLY SUPPORT UPGRADING THE DEPRESSION TO A TROPICAL STORM.
HOWEVER...CONVECTION HAS BEEN WEAKENING AND WE WOULD PREFER TO SEE
DEEP CONVECTION PERSIST BEFORE UPGRADING THIS SYSTEM TO A TROPICAL
STORM.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      21/2100Z 25.3N  75.4W    30 KT
 12HR VT     22/0600Z 26.3N  76.8W    35 KT
 24HR VT     22/1800Z 27.5N  77.5W    40 KT
 36HR VT     23/0600Z 28.5N  77.6W    45 KT
 48HR VT     23/1800Z 29.0N  77.0W    50 KT
 72HR VT     24/1800Z 29.0N  76.5W    60 KT
 96HR VT     25/1800Z 28.5N  76.0W    60 KT
120HR VT     26/1800Z 28.0N  76.5W    60 KT

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