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| #152382 (Received by flhurricane at: 4:46 AM 25.Sep.2007) 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 |
