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| #222565 (Received by flhurricane at: 10:43 PM 16.Oct.2008) TCDAT5 TROPICAL STORM OMAR DISCUSSION NUMBER 15 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL152008 1100 PM EDT THU OCT 16 2008 OMAR HAS A RATHER NON-TROPICAL APPEARANCE THIS EVENING...WITH LITTLE ORGANIZED CONVECTION NEAR THE CENTER. RECENT AMSU MICROWAVE DATA INDICATES THAT OMAR HAS LOST ITS UPPER-LEVEL WARM CORE DUE TO SHEAR...BUT RETAINS A LOW/MID-LEVEL WARM CORE. THAT...COMBINED WITH NO OBVIOUS EVIDENCE OF A LOW-LEVEL/SURFACE FRONTAL SYSTEM NEARBY...MEANS THAT IT REMAINS A TROPICAL CYCLONE DESPITE THE APPEARANCE. A QUICKSCAT OVERPASS NEAR 22Z SHOWED A RELIABLE-LOOKING 65 KT WIND VECTOR AND SEVERAL 60-KT VECTORS. BASED ON THIS...OMAR IS BELIEVED TO HAVE BEEN A HURRICANE UP TO 00Z...BUT HAS NOW LIKELY WEAKENED TO A 60-KT TROPICAL STORM. THE INITIAL MOTION IS SLIGHTLY TO THE RIGHT OF THE PREVIOUS ADVISORY...040/22. OTHER THAN THAT...THERE IS LITTLE CHANGE IN THE FORECAST PHILOSOPHY OR THE FORECAST TRACK. OMAR IS MOVING NORTHEASTWARD BETWEEN A DEEP-LAYER TROUGH TO THE NORTHWEST AND A DEEP-LAYER RIDGE TO THE EAST. THE TROUGH IS FORECAST TO LIFT OUT TO THE NORTHEAST...WHICH SHOULD CAUSE OMAR TO SLOW ITS FORWARD MOTION DURING THE NEXT COUPLE OF DAYS. THE TRACK GUIDANCE IS TIGHTLY CLUSTERED AROUND A NORTH-NORTHEASTWARD MOTION THROUGH 72 HR...AND THE NEW FORECAST TRACK IS JUST TO THE EAST OF THE MODEL CONSENSUS. THERE IS SOME SPREAD IN THE LARGE-SCALE MODELS AFTER 72 HR. THE UKMET AND THE NOGAPS FORECAST OMAR TO TURN EASTWARD OVER THE TOP OF THE ATLANTIC RIDGE...WHILE THE GFS AND ECMWF FORECAST IT TO BE ABSORBED BY A STRONG NON-TROPICAL LOW MOVING EASTWARD ACROSS THE WESTERN ATLANTIC. AT THIS TIME...IT IS UNCLEAR WHICH OF THESE SCENARIOS WILL BE CORRECT. THUS...THE TRACK FORECAST CALLS FOR AN EASTWARD MOTION AT A SPEED BETWEEN THAT OF THE ABOVE MENTIONED GLOBAL MODELS. THE BIGGEST ISSUE ON THE INTENSITY FORECAST IS WHETHER OMAR WILL BE ABLE TO GENERATE ENOUGH CONVECTION TO CONTINUE TO BE CALLED A TROPICAL CYCLONE. ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN SHOW THAT OMAR IS CURRENTLY UNDERGOING ABOUT 40 KT OF SOUTHWESTERLY VERTICAL WIND SHEAR. THE LARGE-SCALE MODELS SUGGEST THAT THE CYCLONE WILL REMAIN IN AT LEAST MODERATE VERTICAL SHEAR...ALTHOUGH THERE IS THE POSSIBILITY THE SHEAR COULD DECREASE SOME IN THE NEXT 24 HR. THE INTENSITY FORECAST FOR THE FIRST 48 HR FOLLOWS THE SCENARIO THAT OMAR WILL DEVELOP ENOUGH CONVECTION TO KEEP IF FROM DEGENERATING INTO A REMNANT LOW...BUT NOT ENOUGH TO KEEP IT FROM WEAKENING. THUS...THIS PART OF THE FORECAST SHOWS GRADUAL WEAKENING. AFTER 48 HR...OMAR WILL CROSS THE 26C SEA SURFACE TEMPERATURE ISOTHERM...ENCOUNTER INCREASING WIND SHEAR... AND BEGIN MERGING WITH A FRONTAL SYSTEM. THE INTENSITY FORECAST CALLS FOR EXTRATROPICAL TRANSITION AT 72-96 HR...AND KEEPS OMAR AT 35 KT DURING THIS TIME. IF THE NOGAPS AND UKMET FORECASTS VERIFY... OMAR COULD RE-INTENSIFY AS AN EXTRATROPICAL LOW OVER THE EASTERN ATLANTIC. HOWEVER...THAT IS TOO UNCERTAIN TO INCLUDE IN THE FORECAST AT THIS TIME. FORECAST POSITIONS AND MAX WINDS INITIAL 17/0300Z 23.8N 58.1W 60 KT 12HR VT 17/1200Z 26.4N 56.2W 55 KT 24HR VT 18/0000Z 29.3N 54.0W 50 KT 36HR VT 18/1200Z 31.3N 51.9W 45 KT 48HR VT 19/0000Z 33.3N 49.6W 40 KT 72HR VT 20/0000Z 37.0N 43.5W 35 KT...BECOMING EXTRATROPICAL 96HR VT 21/0000Z 38.0N 35.5W 35 KT...EXTRATROPICAL 120HR VT 22/0000Z 38.0N 28.0W 35 KT...EXTRATROPICAL $$ FORECASTER BEVEN |
