Tropical Storm KAREN

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TROPICAL STORM KAREN DISCUSSION NUMBER   6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122007
500 AM EDT WED SEP 26 2007
 
AFTER LOOKING RATHER ANEMIC YESTERDAY...KAREN IS FINALLY STARTING TO
PRODUCE PRODUCE DEEP CONVECTION NEAR THE CENTER AND MICROWAVE DATA
SUGGEST THE EARLY STAGES OF AN INNER CORE.  SATELLITE INTENSITY
ESTIMATES HAVE RISEN AND THE INITIAL INTENSITY IS INCREASED TO 45
KT.
 
KAREN IS MOVING RIGHT ALONG THE PREVIOUS FORECAST TRACK WITH AN
INITIAL MOTION OF 280/11.  THE CYCLONE IS EXPECTED TO GRADUALLY
TURN NORTHWESTWARD OVER THE NEXT COUPLE OF DAYS AS A SMALL WEAKNESS
DEVELOPS IN THE DEEP-LAYER SUBTROPICAL RIDGE OVER THE CENTRAL
ATLANTIC.  THEREAFTER...THE GLOBAL MODELS DIFFER IN THEIR FORECAST
OF A LARGE MIDDLE-UPPER LEVEL TROUGH EXPECTED TO DEVELOP OVER THE
WESTERN ATLANTIC.  THE NOGAPS...GFDN...AND HWRF MODEL SHOW THIS
TROUGH REMAINING IN PLACE LONG ENOUGH TO KEEP THE CYCLONE MOVING
NORTHWESTWARD THROUGHOUT THE ENTIRE FORECAST PERIOD.  CONVERSELY...
THE UKMET QUICKLY LIFTS THE SAME TROUGH NORTHWARD BEYOND 72 HOURS
ALLOWING THE CYCLONE TO TURN WESTWARD UNDER THE INFLUENCE OF A
DEVELOPING WEST ATLANTIC RIDGE.  THE GFS IS BETWEEN THESE TWO
SOLUTIONS AS IT SHOWS THE TROUGH SIMILARLY LIFTING NORTHWARD BUT
WITH A MUCH WEAKER RIDGE IN ITS WAKE.  THIS PATTERN WOULD KEEP THE
CYCLONE MOVING GENERALLY NORTHWESTWARD...ALBEIT VERY SLOWLY.  THE
OFFICIAL FORECAST LEANS TOWARD THE GFS SOLUTION BASED ITS PAST
PERFORMANCE AND IS A LITTLE TO THE LEFT OF THE DYNAMICAL MODEL
CONSENSUS.  THIS FORECAST IS ALSO CLOSE TO THE FSU SUPERENSEMBLE.
 
WITH THE RECENT DEVELOPMENT OF CONVECTION NEAR THE CENTER AND
GENERALLY FAVORABLE ENVIRONMENTAL CONDITIONS...KAREN SHOULD
STRENGTHEN DURING THE NEXT DAY OR TWO.  THEREAFTER...THE INTENSITY
FORECAST BECOMES HIGHLY PROBLEMATIC DUE TO THE POSSIBLE DEVELOPMENT
OF UNFAVORABLE UPPER-LEVEL WINDS.  THE GLOBAL MODELS SHOW STRONG
WESTERLY FLOW IMPINGING UPON THE CYCLONE IN A DAY OR TWO.
HOWEVER...NEITHER THE GFDL NOR THE HWRF MODELS RESPOND TO THE SHEAR
AND CONTINUE TO STRENGTHEN KAREN THROUGHOUT THE FORECAST.  IN
FACT...THE HWRF MODEL MAKES KAREN A CATEGORY 2 HURRICANE BY DAY 5.
SINCE SUCH ROBUST DEVELOPMENT SEEMS QUESTIONABLE GIVEN THE EXPECTED
INCREASE IN VERTICAL SHEAR...THE OFFICIAL FORECAST IS BASED MORE ON
THE SHIPS AND LGEM MODELS WHICH SHOW KAREN REACHING MINIMAL
HURRICANE STRENGTH IN A DAY OR TWO FOLLOWED BY SLIGHT WEAKENING.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      26/0900Z 11.2N  42.1W    45 KT
 12HR VT     26/1800Z 11.9N  43.9W    55 KT
 24HR VT     27/0600Z 13.0N  46.1W    60 KT
 36HR VT     27/1800Z 14.3N  48.0W    65 KT
 48HR VT     28/0600Z 15.6N  49.4W    65 KT
 72HR VT     29/0600Z 18.0N  51.5W    60 KT
 96HR VT     30/0600Z 20.0N  53.0W    55 KT
120HR VT     01/0600Z 22.5N  55.0W    55 KT
 
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FORECASTER RHOME
 
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