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ZCZC MIATCDAT1 ALL TTAA00 KNHC DDHHMM TROPICAL STORM ARLENE DISCUSSION NUMBER 20 NATIONAL WEATHER SERVICE MIAMI FL 5 AM EDT WED JUN 16 1999 DEEP CONVECTION ASSOCIATED WITH ARLENE HAS DECREASED DURING THE NIGHT...AND IS ALSO LESS WIDESPREAD AND VIGOROUS THAN 24 HOURS AGO. THIS COULD MEAN THE START OF A WEAKENING TREND. SATELLITE INTENSITY ESTIMATES FROM TAFB...SAB...AND AFWA REMAIN AT 45 KT. THE INITIAL INTENSITY WILL BE HELD AT 50 KT...ALTHOUGH THE NEW HURRICANE HUNTER PLANE DUE INTO THE STORM AROUND 12Z MAY VERY WELL FIND A WEAKER SYSTEM. ALTHOUGH CIRRUS IS OBSCURING THE LOW LEVEL CENTER...ARLENE APPEARS TO BE MOVING 300/4 KT. THE STORM IS SLOWLY CURVING AROUND THE SUBTROPICAL RIDGE CENTERED OVER THE CENTRAL ATLANTIC. ALL GUIDANCE MODELS AGREE ON A GRADUAL ACCELERATION OVER THE NEXT THREE DAYS IN RESPONSE TO A WESTERLY TROUGH MOVING OFF THE U.S. EAST COAST. THE OFFICIAL FORECAST REFLECTS THIS...ALTHOUGH IT IS A LITTLE SLOWER THAN THE GUIDANCE DUE TO THE MODELS MOVING ARLENE TOO QUICKLY NORTHWARD FOR THE PAST FEW DAYS. THE INTENSITY FORECAST IS PROBLEMATICAL. CONVECTION HAS DECREASED EVEN THOUGH THE ENVIRONMENT APPEARS RELATIVELY FAVORABLE. NORTHERLY SHEAR SHOULD DEVELOP OVER ARLENE IN THE NEXT 24-36 HOURS WHICH SHOULD CAUSE WEAKENING. HOWEVER...MODEL GUIDANCE SUGGESTS THE SHEAR MAY NOT BE STRONG ENOUGH TO CAUSE RAPID WEAKENING. THE OFFICIAL FORECAST CALLS FOR THE SAME WEAKENING AS THE PREVIOUS PACKAGE... EXCEPT TO MAKE ARLENE EXTRATROPICAL BY 72 HOURS. FORECAST WIND RADII HAVE BEEN CHANGED FROM THE PREVIOUS PACKAGE ON THE BASIS OF THE FORECAST MOTION...AND THE EXPECTATION THAT THE NORTHERLY SHEAR WILL SUPPRESS CONVECTION IN THE NORTHWEST QUADRANT. BEVEN/GUINEY FORECAST POSITIONS AND MAX WINDS INITIAL 16/0900Z 30.3N 62.4W 50 KTS 12HR VT 16/1800Z 30.6N 63.0W 50 KTS 24HR VT 17/0600Z 31.6N 64.0W 50 KTS 36HR VT 17/1800Z 33.4N 64.7W 45 KTS 48HR VT 18/0600Z 36.0N 64.5W 40 KTS 72HR VT 19/0600Z 42.0N 62.5W 35 KTS...EXTRATROPICAL NNNN