Tropical Storm KAREN

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TROPICAL STORM KAREN DISCUSSION NUMBER  17
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122007
1100 PM EDT FRI SEP 28 2007
 
DETERMINING THE ACTUAL CENTER OF KAREN HAS BEEN A SIGNIFICANT
CHALLENGE THIS EVENING.  IT APPEARS THAT THE EARLIER EXPOSED
CENTER...NOW MOVING NORTHWESTWARD...HAS BECOME JUST ANOTHER SWIRL
AROUND AN ELONGATED LOW-LEVEL CIRCULATION FARTHER TO THE SOUTHWEST. 
A RECENT QUIKSCAT PASS SUPPORTS THIS ASSESSMENT...SHOWING A LARGE
AREA OF LIGHT WINDS SOUTHWEST OF THE OLD CENTER.  THUS...THE CENTER
IS RELOCATED WESTWARD TO THE LIGHTER WIND AREA AS SUGGESTED BY
QUIKSCAT AND NIGHT VISIBLE SATELLITE IMAGES.  OVERALL..THE SYSTEM
REMAINS VERY DISORGANIZED DUE TO FEROCIOUS WESTERLY SHEAR...WITH
THUNDERSTORMS MOSTLY IN THE EASTERN SEMICIRCLE.  EVEN WITH THE
DISHEVELED SATELLITE APPEARANCE...QUIKSCAT SUPPORTS AN INTENSITY OF
AT LEAST 35 KT...AND THE INITIAL INTENSITY WILL BE HELD AT 35 KT.
 
STRONG WESTERLY OR SOUTHWESTERLY SHEAR IS FORECAST TO CONTINUE FOR
ABOUT THE NEXT 48 HOURS AND SHOULD INDUCE WEAKENING OF KAREN. THE
INTENSITY FORECAST FOR THIS TIME SHOWS THE SYSTEM BARELY HANGING ON
AS A TROPICAL CYCLONE....AND DISSIPATION IS POSSIBLE. 
THEREAFTER...THE GFS AND UKMET MODELS CARVE OUT A PIECE OF THE
MID-OCEANIC TROUGH INTO A CUT-OFF LOW THAT MOVES SOUTHWARD NEAR AND
THEN TO THE SOUTH OF WHATEVER REMAINS OF KAREN. THIS EVOLUTION
COULD PLACE THE SYSTEM IN A MORE FAVORABLE LOCATION FOR SOME
STRENGTHENING WITH MORE EASTERLY FLOW ALOFT TO THE NORTH OF THE
CUT-OFF LOW. THE GFDL RESPONDS TO THESE CONDITIONS BY MAKING KAREN
A HURRICANE IN ABOUT 4 DAYS. HOWEVER...OTHER MODELS KEEP MODERATE
TO STRONG SHEAR OVER THE SYSTEM FOR A LONGER PERIOD OF TIME AND
THIS SOLUTION WOULD FAVOR DISSIPATION.  UNTIL THERE IS SOME CLARITY
ON IF THE SYSTEM WILL SURVIVE THE SHEAR...I'M INCLINED TO MAKE
LITTLE CHANGE TO THE LONG-TERM FORECAST AND KEEP THE CYCLONE AS A
TROPICAL DEPRESSION.  OBVIOUSLY...FORECAST CONFIDENCE IS EXTREMELY
LOW TONIGHT.
 
TO SMOOTH OUT THE IRREGULAR MOTION DURING THE PAST 24 HOURS... A
LONG-TERM MOVEMENT WILL BE USED AS THE INITIAL MOTION...ESTIMATED
AT 295/8.  A TRACK BETWEEN WEST-NORTHWEST AND NORTHWEST IS EXPECTED
FOR THE NEXT FEW DAYS AS KAREN MOVES AROUND THE SOUTHWESTERN SIDE
OF A LOW TO MID-LEVEL RIDGE OVER THE CENTRAL ATLANTIC OCEAN. 
THEREAFTER A TURN TO A MORE WESTWARD MOTION IS POSSIBLE DUE TO
RISING HEIGHTS TO THE NORTH OF KAREN.  NUMERICAL GUIDANCE HAS
BEEN MUCH TOO FAR TO THE RIGHT WITH THIS SYSTEM AND...GIVEN THE
CONTINUED WEAK INTENSITY OF THE STORM...THE NHC FORECAST WILL LIE
ON THE LEFT SIDE OF THE GUIDANCE ENVELOPE SIMILAR TO THE 18Z GFS. 
THE LARGE WESTWARD SHIFT AT 4 AND 5 DAYS IS MOSTLY DUE TO THE
CENTER REFORMATION...AND NOT A HUGE CHANGE IN THE MODEL GUIDANCE.  
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      29/0300Z 16.2N  51.9W    35 KT
 12HR VT     29/1200Z 17.0N  53.2W    35 KT
 24HR VT     30/0000Z 18.2N  54.9W    30 KT
 36HR VT     30/1200Z 19.5N  56.3W    25 KT
 48HR VT     01/0000Z 20.7N  57.6W    25 KT
 72HR VT     02/0000Z 23.0N  60.0W    30 KT
 96HR VT     03/0000Z 24.5N  62.5W    30 KT
120HR VT     04/0000Z 25.5N  64.5W    30 KT
 
$$
FORECASTER BLAKE
 
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