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Tropical Storm KAREN Forecast Discussion Number 5
2013-10-04 10:52:17| National Hurricane Center (Atlantic)
Issued at 400 AM CDT FRI OCT 04 2013 000 WTNT42 KNHC 040851 TCDAT2 TROPICAL STORM KAREN DISCUSSION NUMBER 5 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122013 400 AM CDT FRI OCT 04 2013 KAREN IS SHOWING THE AFFECTS OF 20-25 KT OF WESTERLY VERTICAL WIND SHEAR. THE MAIN CONVECTIVE AREA IS NOW LOCATED ABOUT 100 N MI EAST OF THE CENTER...WITH ONLY OCCASIONAL SHOWERS AND THUNDERSTORM ACTIVITY NEAR THE CENTER. REPORTS FROM AIR FORCE RESERVE AND NOAA HURRICANE HUNTER AIRCRAFT INDICATE THAT THE CENTRAL PRESSURE HAS RISEN TO 1002 MB...AND THAT BOTH THE FLIGHT-LEVEL AND SFMR WINDS HAVE DECREASED. BASED ON THE AIRCRAFT DATA...THE INITIAL INTENSITY IS REDUCED TO A POSSIBLY GENEROUS 50 KT. THE INITIAL MOTION IS 325/9. KAREN IS CURRENTLY BEING STEERED BY A LOW/MID-LEVEL RIDGE OVER THE SOUTHEASTERN UNITED STATES. THIS RIDGE IS FORECAST TO WEAKEN AS A LARGE DEEP-LAYER TROUGH MOVES EASTWARD INTO THE CENTRAL UNITED STATES. THIS EVOLUTION SHOULD CAUSE KAREN TO TURN NORTHWARD AND EVENTUALLY NORTHEASTWARD...WITH THE BIG QUESTION BEING THE TIMING OF THE NORTHEASTWARD TURN. THE UKMET...NAVGEM...AND CANADIAN MODELS DELAY THE TURN UNTIL AFTER KAREN MAKES LANDFALL IN SOUTHERN LOUISIANA. THE GFS SHOWS THE SHARPEST TURN...AND IT CALLS FOR THE CENTER TO MAKE LANDFALL ON THE NORTHWESTERN FLORIDA PENINSULA. THE GFDL...HWRF...AND ECMWF ARE BETWEEN THESE EXTREMES WITH LANDFALL FORECASTS FROM THE MOUTH OF THE MISSISSIPPI RIVER TO THE WESTERN FLORIDA PANHANDLE. OVERALL... THE GUIDANCE IS SLOWER THAN IT WAS 6 HOURS AGO...AND THE GUIDANCE ENVELOPE HAS SHIFTED A LITTLE TO THE WEST DURING THE FIRST 48 HOURS. THE NEW FORECAST TRACK REFLECTS THESE TRENDS WITH A SLIGHT WESTWARD SHIFT AND A SLOWER FORWARD SPEED. GIVEN THE SPREAD OF THE GUIDANCE...THE FORECAST OF THE LANDFALL POSITION IS LOW CONFIDENCE. THE INTENSITY FORECAST IS RATHER PROBLEMATIC. FIRST...KAREN IS LIKELY TO INTERACT WITH A SMALL UPPER-LEVEL TROUGH DURING THE NEXT 24 HOURS...WHICH MIGHT PROVIDE SOME SUPPORT FOR RENEWED CONVECTION. SECOND...THE SHEAR MAY DIMINISH IN THE WAKE OF THE TROUGH...WHICH COULD ALLOW FOR INTENSIFICATION...ESPECIALLY IF KAREN TURNS NORTHEASTWARD WHILE STILL OVER WATER. THE ECMWF SHOWS ABOUT 10 MB OF DEEPENING AFTER RECURVATURE...AND THE GFS...HWRF...AND GFDL MODELS SHOW MORE. BASED ON THESE MODELS AND CURRENT TRENDS...THE INTENSITY FORECAST IS MODIFIED TO SHOW LITTLE CHANGE IN STRENGTH THROUGH 24 HOURS...FOLLOWED BY INTENSIFICATION TO NEAR HURRICANE STRENGTH BY 48 HOURS. AFTER LANDFALL...KAREN SHOULD WEAKEN AND EVENTUALLY BECOME EXTRATROPICAL AS IT MERGES WITH THE DEEP-LAYER TROUGH. SINCE THE CURRENT FORECAST IS SLOWER THAN THE PREVIOUS FORECAST...NO CHANGE IN WATCHES AND WARNINGS IS NECESSARY AT THIS TIME. THIS WILL BE REVISITED AS KAREN PROGRESSES NORTHWARD ACROSS THE CENTRAL GULF OF MEXICO. FORECAST POSITIONS AND MAX WINDS INIT 04/0900Z 24.9N 89.8W 50 KT 60 MPH 12H 04/1800Z 25.9N 90.1W 50 KT 60 MPH 24H 05/0600Z 27.0N 90.4W 50 KT 60 MPH 36H 05/1800Z 28.0N 90.4W 55 KT 65 MPH 48H 06/0600Z 29.1N 89.5W 60 KT 70 MPH 72H 07/0600Z 32.5N 84.5W 35 KT 40 MPH...INLAND 96H 08/0600Z 37.5N 77.5W 25 KT 30 MPH...POST-TROP/EXTRATROP 120H 09/0600Z...DISSIPATED $$ FORECASTER BEVEN
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