000
WTNT42 KNHC 250843
TCDAT2
TROPICAL STORM KAREN DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL122007
500 AM EDT TUE SEP 25 2007
 
SATELLITE IMAGERY INDICATES THAT TROPICAL DEPRESSION TWELVE HAS
CONTINUED TO GRADUALLY ORGANIZE...WITH INCREASED CONVECTIVE BANDING
OVER A LARGE AREA SURROUNDING THE CENTER.  SATELLITE INTENSITY
ESTIMATES FROM TAFB AND SAB ARE AGAIN 35 KT...SO THE DEPRESSION IS
UPGRADED TO TROPICAL STORM KAREN.

THE INITIAL MOTION REMAINS 285/14.  KAREN IS ON THE SOUTH SIDE OF A
LOW/MID-LEVEL RIDGE...WHICH SHOULD STEER THE CYCLONE FOR THE NEXT
36-48 HR.  AFTER THAT...THE LARGE-SCALE MODELS AGREE ON THE
DEVELOPMENT OF A MID/UPPER-LEVEL TROUGH TO THE NORTHEAST OF THE
LESSER ANTILLES...WHICH WOULD TURN KAREN NORTHWESTWARD OR
NORTHWARD.  THERE IS SOME SPREAD IN THE GUIDANCE...WITH THE
GFDL...HWRF...ANND ECMWF SHOWING A NORTHWARD MOTION BETWEEN
50W-53W...WHILE THE UKMET AND GFS ENSEMBLE MEAN SHOW A
NORTHWESTWARD MOTION TO WEST OF 56W BY 120 HR.  SINCE THERE IS
OFTEN A TENDENCY FOR THE DYNAMICAL MODELS TO HAVE A NORTHWARD BIAS
IN THE TROPICAL ATLANTIC...THE NEW FORECAST TRACK IS ALONG THE
SOUTH EDGE OF THE MODEL GUIDANCE FOR THE FIRST 72 HR...AND THEN TO
THE LEFT OF CENTER OF THE GUIDANCE ENVELOPE THEREAFTER.

THE INTENSITY FORECAST...IF ANYTHING...HAS BECOME MORE PROBLEMATIC. 
KAREN IS CURRENTLY IN AN ENVIRONMENT OF LIGHT NORTHWESTERLY VERTICAL
WIND SHEAR...WHICH SHOULD CONTINUE FOR 24-36 HR.  AFTER THAT...AN
UPPER-LEVEL TROUGH IS FORECAST TO DEVELOP VERY CLOSE TO THE STORM
TRACK...FOLLOWED BY THE DEVELOPMENT OF THE MID/UPPER-LEVEL TROUGH
MENTIONED ABOVE.  THE LARGE-SCALE MODELS FORECAST THE FIRST TROUGH
TO CAUSE BOTH STRONG UPPER-LEVEL DIFLUENCE AND SHEAR OVER KAREN...
AND IT IS UNCLEAR AT THIS TIME WHETHER THIS SYSTEM WOULD CAUSE THE
STORM TO INTENSIFY OR WEAKEN.  THE SECOND TROUGH IS FORECAST TO
CAUSE SIGNIFICANT SHEAR WITHOUT STRONG DIFLUENCE...WHICH SHOULD
CAUSE WEAKENING.  THE SHIPS...GFDL...AND HWRF MODELS ALL FORECAST
KAREN TO BECOME A HURRICANE BY 60-72 HR.  GIVEN THE UNCERTAINTY IN
THE STRENGTH OF THE SHEAR...THE INTENSITY FORECAST CALLS FOR KAREN
TO PEAK JUST BELOW HURRICANE STRENGTH...FOLLOWED BY WEAKENING DUE
TO SHEAR FROM THE SECOND TROUGH.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      25/0900Z 10.4N  38.0W    35 KT
 12HR VT     25/1800Z 10.8N  40.0W    40 KT
 24HR VT     26/0600Z 11.3N  43.0W    45 KT
 36HR VT     26/1800Z 11.8N  45.7W    55 KT
 48HR VT     27/0600Z 12.9N  47.9W    60 KT
 72HR VT     28/0600Z 16.0N  51.0W    60 KT
 96HR VT     29/0600Z 19.0N  53.0W    55 KT
120HR VT     30/0600Z 22.0N  54.5W    55 KT
 
$$
FORECASTER BEVEN