Tropical Storm SERGIO

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TROPICAL STORM SERGIO DISCUSSION NUMBER  23
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   EP212006
100 AM PST SUN NOV 19 2006
 
SERGIO REMAINS A SHEARED TROPICAL STORM WITH AN EXPOSED LOW-LEVEL
CENTER AND CONVECTION CONFINED TO THE NORTHEASTERN QUADRANT. DVORAK
DATA-T NUMBERS FROM ALL AGENCIES HAVE RISEN TO 3.0... CORRESPONDING
TO 45 KT. HOWEVER...MUCH OF THE INCREASE IN COLD CLOUD TOPS APPEARS
TO BE DUE TO AN EXPANDING CIRRUS CANOPY AND NOT TO ANY SUBSTANTIAL
INCREASE IN CONVECTION...SO THE ADVISORY INTENSITY REMAINS 40 KT.
WEAKENING REMAINS THE EXPECTATION SINCE THE SOUTHWESTERLY VERTICAL
SHEAR IS FORECAST TO INCREASE DURING THE NEXT COUPLE OF DAYS. OCEAN
TEMPERATURES ARE NEAR 29 CELSIUS...HOWEVER...SO SERGIO WILL
PROBABLY WEAKEN GRADUALLY. THE NEW OFFICIAL INTENSITY FORECAST IS
SIMILAR TO THE PREVIOUS ADVISORY AND THE LATEST SHIPS GUIDANCE.
 
EVEN THOUGH A MID-LEVEL RIDGE IS IN PLACE TO THE NORTHWEST OF
SERGIO...WHICH IS TRYING TO FORCE THE CYCLONE WESTWARD OR
WEST-NORTHWESTWARD...THE STRONG UPPER-LEVEL SOUTHWESTERLIES ARE
IMPEDING THAT MOTION.  THE LOW-LEVEL CENTER HAS MOVED VERY LITTLE
DURING THE PAST FEW HOURS...PERHAPS DUE WESTWARD AT ABOUT 2 KT. 
DURING THE NEXT 24-36 HOURS...MOST OF THE GLOBAL MODELS FORECAST
THE RIDGE TO NOT MOVE MUCH...SO THE MOTION OF SERGIO DURING THAT
PERIOD WILL PROBABLY DEPEND ON HOW MUCH CONVECTION IT MAINTAINS AND
THEREFORE HOW MUCH INFLUENCE THE UPPER-LEVEL SOUTHWESTERLIES HAVE. 
THE OFFICIAL FORECAST PRESUMES ENOUGH CONVECTION WILL CONTINUE AND
CALLS FOR A CONTINUED CRAWL TO THE WEST THROUGH 36 HOURS. 
THEREAFTER IT IS ASSUMED THE CONVECTION WILL HAVE DECREASED AND THE
WEAKENING CYCLONE WILL BE STEERED A LITTLE FASTER BY THE LOW- TO
MID-LEVEL EASTERLIES TO THE SOUTH OF THE RIDGE THAT IS FORECAST TO
BUILD EASTWARD.  SINCE THE TRACK FORECAST HINGES SO MUCH ON HOW
LONG THE CONVECTION HANGS ON...AND SINCE THERE IS SUCH DISPARITY IN
THE TRACK MODEL GUIDANCE...IT IS HIGHLY UNCERTAIN.  THE NEW
OFFICIAL TRACK FORECAST IS SOUTH OF AND SLOWER THAN THE PREVIOUS
ADVISORY.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      19/0900Z 15.6N 105.2W    40 KT
 12HR VT     19/1800Z 15.7N 105.6W    35 KT
 24HR VT     20/0600Z 15.8N 106.3W    30 KT
 36HR VT     20/1800Z 15.9N 107.2W    30 KT
 48HR VT     21/0600Z 16.2N 108.5W    25 KT
 72HR VT     22/0600Z 16.5N 111.0W    20 KT...REMNANT LOW
 96HR VT     23/0600Z...DISSIPATED
 
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FORECASTER KNABB
 
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