Tropical Storm KAREN

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TROPICAL STORM KAREN DISCUSSION NUMBER   3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122007
1100 AM EDT TUE SEP 25 2007

KAREN IS EXHIBITING SOME BANDING FEATURES ON VISIBLE SATELLITE
IMAGERY BUT THE DEEP CONVECTION IS NOT THAT STRONG OR CONCENTRATED
NEAR THE CENTER AT THIS TIME.  DVORAK INTENSITY ESTIMATES FROM TAFB
AND SAB REMAIN AT 35 KT SO THE CURRENT INTENSITY IS UNCHANGED FROM
THE PREVIOUS ADVISORY.  THE ENVIRONMENT AHEAD OF THE STORM IS
QUESTIONABLE FOR STRENGTHENING.  KAREN'S UPPER-LEVEL OUTFLOW IS
BEING RESTRICTED OVER THE NORTHWEST QUADRANT DUE TO A NEARBY
UPPER-LEVEL CYCLONIC SHEAR AXIS.  WITHIN THE NEXT FEW DAYS...
GLOBAL MODELS DEPICT A COUPLE OF UPPER-LEVEL TROUGHS OVER THE
CENTRAL SUBTROPICAL ATLANTIC THAT SHOULD CAUSE INCREASING
SOUTHWESTERLY VERTICAL SHEAR OVER THE TROPICAL CYCLONE.  THE SHIPS
MODEL...USING THE GFS FORECAST...SHOWS THE SHEAR INCREASING TO 20 KT
BY 60 HOURS AND MORE THAN 30 KT NEAR THE END OF THE PERIOD.  IN
SPITE OF THIS SEEMINGLY HOSTILE ENVIRONMENT...SHIPS PREDICTS KAREN
TO BECOME A HURRICANE BY 48 HOURS AND TO MAINTAIN HURRICANE
STRENGTH ALMOST TO THE END OF THE FORECAST PERIOD.  THE LATEST RUN
OF THE GFDL MODEL ALSO INTENSIFIES THE TROPICAL STORM INTO A
HURRICANE AFTER 72 HOURS.  IT IS DIFFICULT TO ACCEPT THIS GUIDANCE
IN THE FACE OF SUCH STRONG PREDICTED SHEAR.  THEREFORE THE OFFICIAL
FORECAST...AS IN THE PREVIOUS PACKAGE...STRENGTHENS KAREN TO JUST
BELOW HURRICANE STATUS AND THEN INDICATES WEAKENING AFTER DAY 3.

INITIAL MOTION...290/13 THE SAME AS IN THE PREVIOUS ADVISORY.  KAREN
CONTINUES ON THE SOUTH SIDE OF A LOW- TO MID-LEVEL SUBTROPICAL
RIDGE.  THIS SCENARIO SHOULD PERSIST FOR THE NEXT DAY OR SO. 
AFTERWARDS...A MID- TO UPPER-TROPOSPHERIC CYCLONE SHOULD CREATE A
WEAKNESS IN THE RIDGE.  IF THIS OCCURS...KAREN SHOULD GRADUALLY
TURN NORTHWESTWARD AND THEN NORTH-NORTHWESTWARD.  THERE CONTINUES
TO BE A FAIR AMOUNT OF SPREAD IN THE TRACK GUIDANCE MODELS.  THE
HWRF AND GFS TRACKS ARE NEAR THE RIGHT SIDE OF THE GUIDANCE
ENVELOPE AND THE U.K. MET MODEL TRACK IS NEAR THE LEFT SIDE.  THE
OFFICIAL FORECAST IS CLOSE TO THE DYNAMICAL CONSENSUS...NOT FAR
FROM THE GFDL TRACK...AND FAIRLY SIMILAR TO THE PREVIOUS NHC TRACK.

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      25/1500Z 10.8N  38.7W    35 KT
 12HR VT     26/0000Z 11.2N  40.7W    40 KT
 24HR VT     26/1200Z 11.8N  43.5W    45 KT
 36HR VT     27/0000Z 12.5N  46.1W    55 KT
 48HR VT     27/1200Z 13.9N  48.3W    60 KT
 72HR VT     28/1200Z 17.2N  51.2W    60 KT
 96HR VT     29/1200Z 20.0N  53.0W    55 KT
120HR VT     30/1200Z 22.5N  54.0W    55 KT
 
$$
FORECASTER PASCH
 
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