ZCZC MIATCDAT2 ALL TTAA00 KNHC DDHHMM 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 NNNN