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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
 
 
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