Tropical Storm KAREN

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TROPICAL STORM KAREN DISCUSSION NUMBER  13
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122007
1100 PM EDT THU SEP 27 2007
 
A LARGE BURST OF DEEP CONVECTION FORMED NEAR THE CENTER OF KAREN
TONIGHT.  HOWEVER...THUNDERSTORMS ARE ALREADY DIMINISHING AND
SOUTHWESTERLY WIND SHEAR CONTINUES TO HINDER ANY DEVELOPMENT.
DVORAK SATELLITE ESTIMATES ARE A LITTLE HIGHER THAN EARLIER...BUT
ARE STILL BELOW THE PREVIOUS INITIAL INTENSITY.  WINDS WILL BE KEPT
AT 50 KT FOR THIS ADVISORY.
 
THE INITIAL MOTION ESTIMATE OF 295/12 IS A BLEND OF THE EXTRAPOLATED
MOVEMENT FROM THE LAST AVAILABLE VISIBLE IMAGES AND A RECENT SSMIS
MICROWAVE PASS.  A SMALL RIGHTWARD BEND TO THE TRACK IS EXPECTED
OVER THE NEXT COUPLE OF DAYS AS LOW TO MID-LEVEL RIDGING TO THE
NORTH OF THE SYSTEM SLOWLY WEAKENS.  THE RIDGE IS EXPECTED TO BE
THE WEAKEST IN ABOUT 48-72 HOURS DUE TO A MIDDLE-LATITUDE TROUGH IN
THE CENTRAL ATLANTIC AND COULD STEER THE STORM MORE TO THE
NORTH-NORTHWEST AT THAT TIME.  HOWEVER...THIS TROUGH IS NOT EXPECTED
TO BE STRONG ENOUGH TO TURN THE STORM OUT TO SEA AND ALL MODELS
BUILD RIDGING TO THE NORTH OF THE STORM...FORCING A WEST-NORTHWEST
TURN AT THE END OF THE FORECAST PERIOD.  THE OFFICIAL FORECAST IS
SHIFTED WESTWARD WITH THIS PACKAGE IN BEST AGREEMENT WITH THE
GFS/ECMWF MODELS AND IS ON THE LEFT SIDE OF THE GUIDANCE ENVELOPE. 
IF THE STORM WEAKENS MORE THAN EXPECTED...THIS FORECAST MAY HAVE TO
BE ADJUSTED FARTHER TO THE WEST..ESPECIALLY IN THE SHORT-TERM.
 
KAREN SHOULD BE IN AN ENVIRONMENT OF STRONG WESTERLY SHEAR CAUSED BY
THE MID-OCEANIC TROUGH FOR AT LEAST THE NEXT 48-72 HOURS. THIS
PATTERN FAVORS AT LEAST SLOW WEAKENING DURING THAT TIME. HOWEVER...
IF THE STORM SURVIVES THE SHEAR...GLOBAL MODELS SUGGEST THAT AN
OPPORTUNITY FOR RESTRENGTHENING COULD OCCUR IN THE LONG-RANGE AS
THE TROUGH WEAKENS AND IS REPLACED BY RIDGING. RELIABLE INTENSITY
GUIDANCE RESPONDS TO THIS CHANGE IN SHEAR AND SHOWS AN INCREASE IN
STRENGTH BEYOND 3 DAYS. IN FACT...THE GFDL/HWRF MODELS ACTUALLY
MAKE KAREN A HURRICANE IN 5 DAYS. DUE TO LARGE UNCERTAINTY AT THE
LONGER-RANGES...THE OFFICIAL FORECAST WILL ONLY BE SLIGHTLY
INCREASED...AND COULD BE CONSERVATIVE IF THE STORM ENDURES THE NEXT
48 HOURS OR SO.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      28/0300Z 14.8N  49.5W    50 KT
 12HR VT     28/1200Z 15.7N  50.9W    45 KT
 24HR VT     29/0000Z 16.9N  52.5W    45 KT
 36HR VT     29/1200Z 18.0N  53.8W    40 KT
 48HR VT     30/0000Z 19.2N  54.8W    40 KT
 72HR VT     01/0000Z 21.0N  56.0W    40 KT
 96HR VT     02/0000Z 22.5N  57.5W    45 KT
120HR VT     03/0000Z 23.5N  59.5W    50 KT
 
$$
FORECASTER BLAKE
 
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