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| #199417 (Received by flhurricane at: 5:08 AM 06.Jul.2008) TCDAT2 TROPICAL STORM BERTHA DISCUSSION NUMBER 13 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 500 AM EDT SUN JUL 06 2008 NOT MANY CHANGES TO REPORT WITH BERTHA THIS MORNING. THE CYCLONE CONTINUES TO MOVE QUICKLY WESTWARD ALONG A HEADING OF ABOUT 280/19 TO THE SOUTH OF THE SUBTROPICAL RIDGE. THE OBJECTIVE GUIDANCE IS IN VERY GOOD AGREEMENT THROUGH ABOUT 48 HOURS ON A GENERAL WEST-NORTHWESTWARD MOTION...AND A GRADUAL SLOWING OF THE FORWARD SPEED AS HEIGHTS BUILD TO THE NORTHWEST OF BERTHA OVER THE SUBTROPICAL WESTERN ATLANTIC. THE MODELS STILL DIVERGE SIGNIFICANTLY AT DAYS 3 THROUGH 5...BUT THEY HAVE GENERALLY COME INTO A LITTLE BETTER AGREEMENT ON A TURN TOWARD THE NORTHWEST BY THEN...ALTHOUGH AT A FORWARD SPEED OF ABOUT HALF OF BERTHA'S CURRENT MOTION. THE OUTLIER IS THE HWRF THAT FORECASTS A MUCH SOONER TURN TOWARD THE NORTH ALONG ABOUT 58W. NONE OF THE MODELS...HOWEVER...FORECAST BERTHA TO REACH 30 DEGREES NORTH LATITUDE WITHIN FIVE DAYS. WHILE THE RIDGE OVER THE WESTERN ATLANTIC IS FORECAST TO BE MUCH WEAKER BY THE END OF THE FIVE-DAY FORECAST PERIOD...THE MODELS DO NOT DEPICT A HIGH-AMPLITUDE TROUGH THAT WOULD ACCELERATE BERTHA INTO THE MIDDLE LATITUDES. SO IT IS FAR TOO EARLY TO DETERMINE IF OR WHEN BERTHA WILL RECURVE. THE NEW OFFICIAL TRACK FORECAST IS ADJUSTED SLIGHTLY TO THE RIGHT...TOWARD BUT NOT AS FAR EAST AS THE CONSENSUS. THE WATERS BENEATH BERTHA ARE CURRENTLY ABOUT 26 CELSIUS AND CONTINUE TO GET WARMER AS THE CYCLONE PROCEEDS WESTWARD...AND BY 48 HOURS FROM NOW THEY SHOULD BE APPROACHING 28 CELSIUS. VERTICAL WIND SHEAR SHOULD NOT STRENGTHEN DURING THIS PERIOD...SO GRADUAL STRENGTHENING IS FORECAST IN LINE WITH NEARLY ALL OF THE OBJECTIVE INTENSITY GUIDANCE...AND BERTHA COULD BE A HURRICANE IN A COUPLE OF DAYS. BEYOND THAT TIME...THE WATERS WILL BE PLENTY WARM...SO THE DETERMINING FACTOR ON THE INTENSITY OF BERTHA WILL PROBABLY BE WIND SHEAR. THE UPPER-LEVEL PATTERN FORECAST BY THE MODELS AT DAYS 3 THROUGH 5 IS COMPLEX...AND IT IS HARD TO DETERMINE JUST HOW MUCH SHEAR WILL BE AFFECTING BERTHA DURING THAT PERIOD. DESPITE TAKING INTO ACCOUNT A MODEST INCREASE IN SHEAR BASED ON THE GFS MODEL WIND FIELDS...THE SHIPS AND LGEM FORECAST 70-75 KT AT 72-120 HOURS...SO THE NEW OFFICIAL INTENSITY FORECAST PEAKS AT 70 KT...A LITTLE HIGHER THAN THE PREVIOUS ADVISORY. GIVEN THE UNCERTAINTIES IN THE ATMOSPHERIC PATTERN AT THOSE LONG RANGES...HOWEVER...THIS IS NOT A HIGH CONFIDENCE INTENSITY FORECAST. FORECAST POSITIONS AND MAX WINDS INITIAL 06/0900Z 17.3N 43.2W 45 KT 12HR VT 06/1800Z 17.9N 46.1W 50 KT 24HR VT 07/0600Z 18.7N 49.5W 55 KT 36HR VT 07/1800Z 19.6N 52.5W 60 KT 48HR VT 08/0600Z 20.4N 55.1W 65 KT 72HR VT 09/0600Z 22.5N 60.0W 70 KT 96HR VT 10/0600Z 24.5N 64.0W 70 KT 120HR VT 11/0600Z 27.0N 67.5W 70 KT $$ FORECASTER KNABB |
