000
WTNT42 KNHC 290845
TCDAT2
TROPICAL STORM KAREN DISCUSSION NUMBER  18
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122007
500 AM EDT SAT SEP 29 2007

ONCE AGAIN...LOCATING THE PRIMARY LOW-LEVEL CIRCULATION OF KAREN
CONTINUES TO BE QUITE A CHALLENGE THIS MORNING.  BOTH CONVENTIONAL
AND MICROWAVE IMAGERY SUGGEST THAT THE CIRCULATION HAS BECOME
ELONGATED WITH SEVERAL SMALLER SWIRLS EMBEDDED WITHIN THE LARGER
CIRCULATION.  AS A RESULT...THE INITIAL POSITION IS BASED MOSTLY ON
AN EXTRAPOLATION FROM THE PREVIOUS ADVISORY.  KAREN REMAINS IN AN
EXTREMELY HOSTILE UPPER-LEVEL ENVIRONMENT WITH 40-45 KTS OF
WESTERLY SHEAR...WHICH IS SIGNIFICANTLY DISRUPTING CONVECTIVE
ACTIVITY.  DVORAK DATA T-NUMBERS AND CURRENT INTENSITY ESTIMATES
ARE 30 KT AND 35 KT RESPECTIVELY FROM BOTH TAFB AND SAB...AND THE
INITIAL INTENSITY IS KEPT GENEROUSLY AT 35 KTS.

THE INTENSITY FORECAST REMAINS PROBLEMATIC.  THE STRONG WESTERLY
SHEAR IS EXPECTED TO PERSIST FOR AT LEAST THE NEXT 24 HOURS...AND
IT IS DIFFICULT TO SAY WHETHER OR NOT KAREN WILL SURVIVE SUCH AN
UNFAVORABLE ENVIRONMENT.  BOTH THE GFDL AND HWRF MODEL GUIDANCE
SUGGEST THAT THE SHEAR WILL LESSEN IN ABOUT 72 HOURS...PROVIDING A
MORE FAVORABLE ENVIRONMENT FOR THE CYCLONE TO STRENGTHEN.  IN
FACT...THE GFDL TAKES KAREN TO 80 KT IN FOUR DAYS AND THE HWRF
MODEL INTENSIFIES KAREN TO AROUND 100 KT BY DAY 5.  SINCE SO MUCH
UNCERTAINTY EXISTS IN KAREN SURVIVING THE NEXT 24 TO 36 HOURS...I
WILL MAINTAIN CONTINUITY AND FORECAST KAREN AS A DEPRESSION THROUGH
FIVE DAYS.  THIS INTENSITY FORECAST IS ALSO ALIGNED WITH THE LGE
MODEL.

AN 18-24 HOUR MOVEMENT OF THE SYSTEM YIELDS AN ESTIMATED MOTION OF
300/10.  KAREN IS FORECAST TO CONTINUE TO MOVE ON A GENERAL WEST-
NORTHWESTWARD TO NORTHWESTWARD TRACK AROUND THE SOUTHWESTERN
PERIPHERY OF A LOW-MID LEVEL RIDGE.  IT APPEARS FROM THE MODEL
GUIDANCE THAT THE MID-LATITUDE TROUGH CURRENTLY LOCATED JUST OFF
THE EAST COAST OF THE UNITED STATES WILL BYPASS KAREN IN THE NEXT
COUPLE OF DAYS. THEREAFTER...THE RIDGE TRIES TO BUILD BACK IN
STEERING KAREN ON A MORE WEST-NORTHWESTWARD TO WESTWARD TRACK.  THE
OFFICIAL TRACK FORECAST IS ESSENTIALLY AN UPDATE OF THE PREVIOUS
PACKAGE AND REMAINS CLOSE TO THE GFS SOLUTION...BUT SOUTH OF THE
MODEL CONSENSUS. 


FORECAST POSITIONS AND MAX WINDS

INITIAL      29/0900Z 16.6N  52.9W    35 KT
 12HR VT     29/1800Z 17.4N  54.3W    30 KT
 24HR VT     30/0600Z 18.5N  55.7W    25 KT
 36HR VT     30/1800Z 20.0N  57.0W    25 KT
 48HR VT     01/0600Z 21.0N  58.2W    25 KT
 72HR VT     02/0600Z 23.0N  60.5W    30 KT
 96HR VT     03/0600Z 24.5N  63.0W    30 KT
120HR VT     04/0600Z 25.5N  65.0W    30 KT

$$
FORECASTER MAINELLI