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| #200760 (Received by flhurricane at: 5:08 AM 13.Jul.2008) TCDAT2 HURRICANE BERTHA DISCUSSION NUMBER 41 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 500 AM EDT SUN JUL 13 2008 THE CENTER OF BERTHA HAS BECOME MORE DIFFICULT TO LOCATE THIS MORNING. INFRARED IMAGERY SUGGESTS THE SYSTEM IS DRIFTING SOUTHWARD. HOWEVER...A COUPLE OF RECENT AMSU MICROWAVE OVERPASSES SUGGEST THE LOW-LEVEL CENTER IS NORTH OF THE CENTER SEEN IN THE IR IMAGERY...POSSIBLY DUE TO THE ONSET OF NORTHERLY VERTICAL WIND SHEAR. THE INITIAL POSITION IS AN AVERAGE OF LOCATIONS FROM THE IR AND MICROWAVE DATA. SATELLITE INTENSITY ESTIMATES ARE 65 KT FROM TAFB AND 55 KT FROM SAB. THE INITIAL INTENSITY REMAINS 65 KT...BUT THIS COULD BE GENEROUS. THE INITIAL MOTION IS STATIONARY. LARGE-SCALE MODEL ANALYSES SHOW THAT BERTHA IS EMBEDDED IN THE SUBTROPICAL RIDGE...WITH WATER VAPOR IMAGERY SHOWING A MID-LEVEL SHORTWAVE RIDGE ABOUT TO PASS TO THE NORTH OF THE CYCLONE. THIS COMBINATION OF FEATURES SHOULD KEEP BERTHA NEARLY STATIONARY DURING THE NEXT 12 HR OR SO...FOLLOWED BY A SLOW MOTION TOWARD THE NORTHWEST AND NORTH AS THE MID-LEVEL RIDGE AMPLIFIES THE SUBTROPICAL RIDGE EAST OF BERTHA AND A MID/UPPER- LEVEL TROUGH MOVES INTO THE EASTERN UNITED STATES. AFTER 72 HR...A DEEP-LAYER LOW FORMING OVER THE NORTH CENTRAL ATLANTIC WILL LIKELY STEER BERTHA EASTWARD OR EAST-SOUTHEASTWARD. THE LARGE-SCALE MODELS GENERALLY AGREE ON THIS SCENARIO...BUT THERE REMAINS SOME SPREAD IN BOTH THE DIRECTION AND SPEED OF MOTION. THE NEW FORECAST TRACK IS SHIFTED TO THE LEFT OF THE PREVIOUS TRACK DURING THE FIRST 72 HR...AND EVENTUALLY WINDS UP AT THE 5 DAY POSITION FROM THE PREVIOUS FORECAST. THE NEW TRACK IS IN BEST AGREEMENT WITH THE ECMWF. BERTHA HAS ESSENTIALLY BEEN STATIONARY FOR 24 HR...AND IT IS LIKELY UPWELLING COLDER WATER UNDERNEATH IT. THIS SHOULD CAUSE A WEAKENING TREND AS LONG AS THE CYCLONE REMAINS STATIONARY. WHEN BERTHA STARTS MOVING...THE FORECAST TRACK TAKES IT OVER COOLER WATER AND INTO INCREASING VERTICAL WIND SHEAR. DESPITE THESE ISSUES...THE GFDL AND HWRF MAINTAIN BERTHA AS A HURRICANE FOR 5 DAYS...WHILE THE SHIPS MODEL FORECASTS WEAKENING TO 35 KT BY 5 DAYS. THE NEW INTENSITY FORECAST COMPROMISES BETWEEN THESE EXTREMES...CALLING FOR BERTHA TO WEAKEN TO 50 KT BY THE END OF THE FORECAST PERIOD. FORECAST POSITIONS AND MAX WINDS INITIAL 13/0900Z 29.8N 62.5W 65 KT 12HR VT 13/1800Z 30.1N 62.8W 60 KT 24HR VT 14/0600Z 30.8N 63.2W 60 KT 36HR VT 14/1800Z 32.0N 63.6W 60 KT 48HR VT 15/0600Z 33.4N 63.5W 55 KT 72HR VT 16/0600Z 35.5N 62.0W 50 KT 96HR VT 17/0600Z 35.5N 59.0W 50 KT 120HR VT 18/0600Z 34.5N 55.0W 50 KT $$ FORECASTER BEVEN |
