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HURRICANE OLGA DISCUSSION NUMBER  17
NATIONAL WEATHER SERVICE MIAMI FL
5 PM AST WED NOV 28 2001

THE CLOUD PATTERN ASSOCIATED WITH OLGA CONTINUES TO SHOW SIGNS
OF GRADUAL DECAY...WITH THE CONVECTION DECREASING IN INTENSITY
AND COVERAGE AND THE EYE BECOMING CLOUD-FILLED.  SATELLITE
INTENSITY ESTIMATES REMAIN 77 KT...BUT BASED ON THE SLIGHTLY
LESS IMPRESSIVE APPEARANCE NOW AS COMPARED TO THIS MORNING THE
WINDS ARE DECREASED TO 70 KT.
 
OLGA HAS BEGUN ITS SOUTHWESTWARD TRACK AND THE INITIAL MOTION IS
NOW 225/8.  WATER VAPOR IMAGERY CONTINUES TO INDICATE A BUILDING
MID/UPPER LEVEL RIDGE TO THE WEST AND NORTHWEST OF OLGA...AS WELL
AS A NEWLY-FORMING UPPER LEVEL LOW NEAR 23N60W.  OLGA SHOULD
CONTINUE MOVING GENERALLY SOUTHWESTWARD AROUND THE EASTERN SIDE
OF THE RIDGE AS NOW INDICATE BY ALL GUIDANCE.  THE OFFICIAL FORECAST
TRACK IS SOUTH OF AND FASTER THAN THE PREVIOUS PACKAGE...AND IT
IS A COMPROMISE IN SPEED BETWEEN THE FASTER GFDL AND GLOBAL MODELS
AND THE SLOWER VICBAR...LBAR...AND NHC98.

UPPER LEVEL NORTHERLY FLOW ASSOCIATED WITH THE RIDGE TO THE WEST
OF OLGA IS STARTING TO IMPINGE ON THE HURRICANE...ALTHOUGH SO FAR
THE FASTER MOTION HAS SOMEWHAT DECREASED THE IMPACT.  LARGE-SCALE
MODEL CONTINUE TO FORECAST AN INCREASE IN SHEAR IN THE NEXT 12
TO 24 HR WHICH SHOULD CAUSE OLGA TO WEAKEN FURTHER...AND THIS IS
INDICATED IN THE INTENSITY FORECAST.  THE FORECAST IS STILL TRICKY
BEYOND 36 HR.  CLIMATOLOGY SHOWS THAT MOST ATLANTIC TROPICAL
CYCLONES THAT MOVE SOUTHWESTWARD WEAKEN...OR AT BEST HOLD THEIR OWN.
ON THIS BASIS...AND THE GOOD CHANCE THAT OLGA'S SMALL CORE WILL GET
DISRUPTED BY THE SHEAR...THE CYCLONE SHOULD WEAKEN.  INDEED...THE
AVN AND UKMET BOTH WEAKEN THE SYSTEM TO A TROPICAL WAVE IN 72 HR.
HOWEVER...THE FORECAST TRACK TAKES THE STORM OVER 25C SEA SURFACE
TEMPERATURES BY 72 HR.  SHOULD THE SHEAR DECREASE FOR SOME REASON...
LIKE OLGA FINDING A LIGHT SHEAR AREA IN THE MIDDLE OF AN UPPER LEVEL
TROUGH...THEN WEAKENING WOULD LIKELY BE SLOWER THAT CURRENTLY
FORECAST.  LARGE-SCALE MODELS INDICATE THAT SUCH LIGHT SHEAR REGIONS
MAY FORM NEAR OLGA...BUT THEY DIFFER ON THE DETAILS OF THE SIZE AND
LOCATION OF THESE FEATURES.   BECAUSE OF THE UNCERTAINTY...THE
INTENSITY FORECAST WILL KEEP THE TRENDS FROM THE PREVIOUS FORECAST.
 
FORECASTER BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     28/2100Z 31.1N  57.3W    70 KTS
12HR VT     29/0600Z 30.1N  58.4W    65 KTS
24HR VT     29/1800Z 29.0N  59.9W    60 KTS
36HR VT     30/0600Z 27.9N  62.1W    55 KTS
48HR VT     30/1800Z 27.1N  64.2W    50 KTS
72HR VT     01/1800Z 26.0N  68.0W    40 KTS
 
 
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