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Tropical Storm KAREN Forecast Discussion Number 7
2013-10-04 22:40:18| National Hurricane Center (Atlantic)
Issued at 400 PM CDT FRI OCT 04 2013 000 WTNT42 KNHC 042040 TCDAT2 TROPICAL STORM KAREN DISCUSSION NUMBER 7 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122013 400 PM CDT FRI OCT 04 2013 AFTER A BURST OF DEEP CONVECTION NEAR AND EAST OF THE CENTER EARLIER THIS AFTERNOON...THE LOW-LEVEL CENTER OF KAREN IS AGAIN EXPOSED WEST OF THE WARMING CLOUD TOPS. DATA FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE THAT THE INTENSITY OF KAREN REMAINS 45 KT...AND A CENTER DROP FROM THE PLANE REPORTED A PRESSURE OF 1003 MB. MODERATE SHEAR AND DRY AIR SHOULD RESULT IN LITTLE OVERALL INTENSITY CHANGE DURING THE NEXT 24 HOURS...EVEN THOUGH SHORT TERM CHANGES IN CONVECTIVE ORGANIZATION COULD RESULT IN SOME SHORT TERM FLUCTUATIONS IN INTENSITY. THE GLOBAL MODELS ARE NOW IN BETTER AGREEMENT ON THIS EVOLUTION...AND SHOW THE MID-LEVEL CIRCULATION WEAKENING OR DISSIPATING ENTIRELY IN THE NEXT DAY OR TWO. IN PARTICULAR THE GFS IS WEAKER WITH ITS FORECAST OF KAREN AFTER DATA FROM THE NOAA GULFSTREAM-IV JET...WHICH SHOWED 200-MB WINDS WEST OF KAREN STRONGER THAN PREVIOUSLY ANALYZED...WERE INCORPORATED INTO THE 12Z ANALYSIS. AFTER 24 HOURS...KAREN COULD STRENGTHEN A LITTLE DUE TO AN INCREASE IN UPPER-LEVEL DIVERGENCE AHEAD OF A MID-LATITUDE TROUGH...BUT SIGNIFICANT STRENGTHENING IS NOT EXPECTED. AN ALTERNATIVE SCENARIO IS THAT KAREN COULD BECOME COMPLETELY DECOUPLED FROM THE DEEP CONVECTION AND WEAKEN. THE INITIAL MOTION OF KAREN HAS VARIED DEPENDING ON HOW CLOSE THE CONVECTION WAS TO THE CENTER. AFTER MOVING MORE NORTHWARD EARLIER THE AFTERNOON THE CENTER HAS WOBBLED WESTWARD AS IT BECAME EXPOSED. SMOOTHING THROUGH THESE SHORT TERM VARIATIONS...THE INITIAL MOTION ESTIMATE IS 335/06. THE GLOBAL MODELS ARE NOW IN A LITTLE BETTER AGREEMENT ON THE HEADING OF THE STORM OVER THE NEXT COUPLE OF DAYS...IN PARTICULAR THE GFS HAS SHIFTED TO THE LEFT AND IS NOW CLOSER TO THE ECMWF. HOWEVER...THERE REMAIN LARGE DIFFERENCES IN FORWARD SPEED WITH THE GUIDANCE CONTINUING TO TREND SLOWER OVERALL. THE NHC FORECAST SHOWS A TURN TOWARD THE NORTH BY 24 HOURS AND THEN A SHARP NORTHEASTWARD TURN BY 36 HOURS AS KAREN FEELS THE INFLUENCE OF THE AFOREMENTIONED TROUGH. THE NHC TRACK HAS BEEN ADJUSTED TOWARD THE SLOWER MULTI-MODEL CONSENSUS...BUT IS FASTER AND TO THE LEFT OF THE ECMWF AND GFS AFTER 24 HOURS. CONFIDENCE IN THE DETAILS OF THE TRACK FORECAST...AND THEREFORE THE DISTRIBUTION OF IMPACTS ALONG THE COAST...REMAINS LOW. GIVEN THE NEW INTENSITY FORECAST...THE HURRICANE WATCH HAS BEEN CHANGED TO A TROPICAL STORM WATCH FROM THE MOUTH OF THE PEARL RIVER TO DESTIN FLORIDA...AND DISCONTINUED ELSEWHERE. THE 12Z HWRF RUN SHOWED CONSIDERABLY LESS INTENSIFICATION WITH KAREN COMPARED TO PREVIOUS RUNS AFTER ASSIMILATING DATA FROM THE FROM THE NOAA P-3 TAIL DOPPLER RADAR. THIS MARKS THE FIRST TIME DOPPLER RADAR DATA HAVE BEEN ASSIMILATIED INTO AN OPERATIONAL HURRICANE MODEL IN REAL TIME. FORECAST POSITIONS AND MAX WINDS INIT 04/2100Z 25.9N 90.3W 45 KT 50 MPH 12H 05/0600Z 26.9N 90.6W 45 KT 50 MPH 24H 05/1800Z 27.9N 90.7W 45 KT 50 MPH 36H 06/0600Z 28.8N 89.6W 50 KT 60 MPH 48H 06/1800Z 29.9N 87.7W 50 KT 60 MPH 72H 07/1800Z 33.0N 82.5W 25 KT 30 MPH...POST-TROP/EXTRATROP 96H 08/1800Z...ABSORBED $$ FORECASTER BRENNAN
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