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Tropical Storm HELENE (Text)

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TROPICAL STORM HELENE DISCUSSION NUMBER  16
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082006
500 AM EDT SAT SEP 16 2006
 
A SERIES OF MICROWAVE OVERPASSES REVEALS THAT HELENE HAS A
WELL-DEFINED CIRCULATION...BUT WITH DEEP CONVECTION THAT DOES NOT
PERSIST IN THE SAME QUADRANT FOR VERY LONG. THE MOST RECENT IMAGE
FROM AMSR-E AT 0420 UTC INDICATED THAT DEEP CONVECTION WAS PRESENT
ONLY TO THE NORTHEAST OF THE CENTER...AND GOES IMAGERY SINCE THEN
SHOWS THAT THE CONVECTION HAS WRAPPED AROUND TO THE NORTHWEST SIDE.
MEANWHILE...THERE IS VERY LIMITED DEEP CONVECTION IN THE
SOUTHEASTERN SEMICIRCLE. EVEN THOUGH DVORAK T-NUMBERS REMAIN AT 4.0
USING AN EMBEDDED CENTER PATTERN...CORRESPONDING TO AN INTENSITY OF
65 KT...THE PERCOLATING BEHAVIOR OF THE CONVECTION LEADS ME TO
LEAVE THE ADVISORY INTENSITY AT 60 KT FOR NOW. NEVERTHELESS...THE
ENVIRONMENT APPEARS CONDUCIVE ENOUGH FOR HELENE TO BECOME A
HURRICANE LATER TODAY AND SUBSEQUENTLY STRENGTHEN DURING THE NEXT
FEW DAYS. THE SHIPS AND GFDL BOTH FORECAST GRADUAL INTENSIFICATION
FOR ABOUT 48 HOURS. THEREAFTER THE SHIPS LEVELS OFF NEAR 80 KT...
BUT THE GFDL ANTICIPATES HELENE LATER APPROACHING MAJOR HURRICANE
STATUS. THE NEW OFFICIAL INTENSITY FORECAST IS A BLEND OF THOSE TWO
MODELS EARLY ON...THEN FOLLOWS THE GFDL...AND IS VERY CLOSE TO THE
PREVIOUS ADVISORY.

DUE TO THE PULSATING CONVECTIVE PATTERN...I WOULD NOT HAVE A VERY
GOOD IDEA WHERE THE LOW-LEVEL CENTER IS WITHOUT THE BENEFIT OF THE
MICROWAVE IMAGERY...WHICH INDICATES THE INITIAL MOTION IS
ESSENTIALLY UNCHANGED AT 300/13.  ALL OF THE TRACK GUIDANCE SHOWS A
MOTION BETWEEN WEST-NORTHWEST OR NORTHWEST FOR THE NEXT 3-4 DAYS
INTO A WEAKNESS IN THE SUBTROPICAL RIDGE OVER THE CENTRAL ATLANTIC. 
THAT WEAKNESS IS CURRENTLY ASSOCIATED WITH HURRICANE GORDON AND
SHOULD BE REINFORCED BY A SHORT-WAVE MID- TO UPPER-LEVEL LOW THAT
WILL MOVE EASTWARD FROM THE EAST COAST OF THE UNITED STATES DURING
THE NEXT FEW DAYS.  THAT FEATURE WILL PROBABLY NOT BE ENOUGH TO
TURN HELENE NORTHWARD...BUT THE LONGWAVE TROUGH CURRENTLY OVER THE
WESTERN UNITED STATES PROBABLY WILL...WHEN IT GETS INTO THE WESTERN
ATLANTIC IN SEVERAL DAYS.  NEARLY ALL OF THE DYNAMICAL MODELS TURN
HELENE NORTH-NORTHWESTWARD AHEAD OF THAT TROUGH ON DAY 5...AND THAT
TURN IS NOW INTRODUCED IN THE OFFICIAL FORECAST...WHICH IS
OTHERWISE VERY SIMILAR TO THE PREVIOUS ADVISORY.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      16/0900Z 17.9N  44.7W    60 KT
 12HR VT     16/1800Z 18.8N  46.1W    65 KT
 24HR VT     17/0600Z 19.7N  47.6W    75 KT
 36HR VT     17/1800Z 20.6N  49.1W    80 KT
 48HR VT     18/0600Z 21.4N  50.4W    85 KT
 72HR VT     19/0600Z 23.0N  53.0W    95 KT
 96HR VT     20/0600Z 25.0N  56.0W    95 KT
120HR VT     21/0600Z 28.0N  58.0W    90 KT
 
$$
FORECASTER KNABB
 
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