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TROPICAL STORM DEAN DISCUSSION NUMBER 3
NATIONAL WEATHER SERVICE MIAMI FL
11 PM EDT WED AUG 22 2001
WITHOUT RECON OR VISIBLE SATELLITE IMAGERY...THE LOCATION IS ALMOST
IMPOSSIBLE TO PINPOINT. RADAR FROM SAN JUAN IS INCONCLUSIVE...AND A
RECENT QUIKSCAT PASS SHOWS DEAN AS AN OPEN WAVE. THE INITIAL
POSITION...MOTION...AND SHORT-TERM FORECAST ARE BASED ALMOST
EXCLUSIVELY ON CONTINUITY FROM LAST-LIGHT SATELLITE TRENDS.
THE INTENSITY FORECAST IS PARTICULARLY CHALLENGING TONIGHT. DEAN IS
CURRENTLY EXPERIENCING FAIRLY STRONG WESTERLY SHEAR...AS A RESULT OF
A SHARP UPPER TROUGH OVER THE SYSTEM COUPLED WITH STRONG LOW-LEVEL
EASTERLIES...AND MAY IN FACT NO LONGER HAVE A CLOSED CIRCULATION.
HOWEVER...OVER THE NEXT 24 HOURS DEAN SHOULD BE MOVING UNDER AN
UPPER ANTICYCLONE THAT WILL PROVIDE A FLOW MORE IN STEP WITH THE
LOWER STEERING WINDS. UNLIKE THE RECENT MODEL RUNS WITH
CHANTAL...IN WHICH A FAVORABLE UPPER ENVIRONMENT WAS FORECAST TO
DEVELOP BUT NEVER REALLY DID...THE UPPER HIGH IS ALREADY IN PLACE
HERE AND DEAN MERELY NEEDS TO MOVE INTO THE RIGHT PLACE. HOWEVER...
THERE IS ALSO A DISTINCT POSSIBILITY THAT THE SYSTEM WILL NOT
SURVIVE LONG ENOUGH TO TAKE ADVANTAGE. THE OFFICIAL INTENSITY
FORECAST...WHICH MAKES DEAN A HURRICANE IN 36 HOURS...PRESUMES THAT
THE SYSTEM WILL SURVIVE THE NEXT 12.
TRACK GUIDANCE IS DIVERGENT...AND WITH THE COMPLICATED UPPER FLOW
PATTERNS THE FUTURE PATH OF DEAN WILL BE STRONGLY DEPENDENT ON THE
INTENSITY. A DEEPER SYSTEM COULD MOVE MORE TO THE WEST UNDER THE
UPPER HIGH AND RECURVE RELATIVELY SLOWLY...WHILE A WEAK SYSTEM IS
MORE LIKELY TO HIT THE TROUGH OFF THE EAST COAST SOONER AND
ACCELERATE NORTHEASTWARD. IN EITHER CASE...DEAN IS NOT EXPECTED TO
BE A THREAT TO THE U.S. MAINLAND.
FORECASTER FRANKLIN
FORECAST POSITIONS AND MAX WINDS
INITIAL 23/0300Z 20.2N 68.1W 50 KTS
12HR VT 23/1200Z 21.5N 70.2W 50 KTS
24HR VT 24/0000Z 23.1N 72.3W 55 KTS
36HR VT 24/1200Z 25.1N 72.5W 65 KTS
48HR VT 25/0000Z 27.0N 72.0W 70 KTS
72HR VT 26/0000Z 30.5N 69.5W 75 KTS
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