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| #200212 (Received by flhurricane at: 4:37 AM 10.Jul.2008) TCDAT2 HURRICANE BERTHA DISCUSSION NUMBER 29 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 500 AM EDT THU JUL 10 2008 DVORAK T-NUMBERS REMAIN AT 5.0 CORRESPONDING TO 90 KT...WHICH IS USED FOR THE ADVISORY INTENSITY. THERE HAS BEEN SOME EROSION OF DEEP CONVECTION OVER THE SOUTHWEST QUADRANT OF THE HURRICANE...PROBABLY DUE TO THE COMBINATION OF INCREASED SOUTH-SOUTHWESTERLY SHEAR AND THE INTRUSION OF DRIER AIR. GIVEN THIS EVOLUTION...I HAVE ELECTED TO BACK OFF SLIGHTLY ON THE INTENSITY PREDICTION. THE CURRENT OFFICIAL WIND SPEED FORECAST IS A BLEND OF THE STATISTICAL/DYNAMICAL GUIDANCE SHIPS AND LGEM ALONG WITH THE PURELY DYNAMICAL MODELS GFDL AND HWRF. SEA SURFACE TEMPERATURES SHOULD REMAIN SUFFICIENTLY WARM FOR STRENGTHENING THROUGH AT LEAST 3-4 DAYS...BUT THE SHIPS MODEL DIAGNOSES 27-31 KT OF VERTICAL SHEAR IN 3-5 DAYS. THEREFORE WEAKENING IS LIKELY TO OCCUR IN THE LATTER PART OF THE FORECAST PERIOD. BERTHA'S FORWARD MOTION HAS SLOWED A BIT AND IS NOW ABOUT 315/8. AN ANTICYCLONE TO THE NORTH OF THE HURRICANE WILL CONTINUE TO SLIDE EASTWARD OVER THE NEXT DAY OR TWO...SO THE TRACK OF THE HURRICANE IS LIKELY TO BEND TOWARD THE NORTH THROUGH 48 HOURS. BEYOND THAT TIME...THE TRACK FORECAST IS COMPLICATED BY THE FACT THAT THE DYNAMICAL MODELS...IN PARTICULAR THE GFS...SHOW A MID-LEVEL ANTICYCLONE DEVELOPING TO THE NORTH OF BERTHA IN AROUND 72 HOURS. THIS DEVELOPMENT WOULD AT LEAST RETARD THE NORTHWARD PROGRESS OF THE HURRICANE AND...IF THE ANTICYCLONE TURNS OUT TO BE STRONG ENOUGH...COULD EVEN FORCE A TURN TO THE LEFT AS INDICATED BY THE GFS. THE GFDL AND HWRF MODELS RESPOND TO THE INCREASED RIDGING MAINLY BY SLOWING THE TROPICAL CYCLONE AS IT CROSSES 30N LATITUDE. IT IS CERTAINLY POSSIBLE FOR BERTHA TO MOVE ERRATICALLY IN ABOUT 3 DAYS...BUT THE DETAILS OF THAT MOTION ARE OF COURSE UNKNOWN. BY THE END OF THE FORECAST PERIOD...THE FLOW AHEAD OF A LARGE- AMPLITUDE 500 MB TROUGH NEARING THE U.S EAST COAST IS EXPECTED TO CAUSE BERTHA TO MOVE NORTH-NORTHEASTWARD WITH SOME INCREASE IN FORWARD SPEED. THE RADII OF 12-FT SEAS HAVE BEEN ADJUSTED BY THE TROPICAL ANALYSIS AND FORECAST BRANCH BASED ON A JASON SATELLITE ALTIMETER PASS OVER BERTHA...WHICH IS QUITE CONSISTENT WITH THE LATEST OUTPUT FROM THE NORTH ATLANTIC HURRICANE WAVE MODEL. FORECAST POSITIONS AND MAX WINDS INITIAL 10/0900Z 26.0N 59.4W 90 KT 12HR VT 10/1800Z 27.0N 60.3W 95 KT 24HR VT 11/0600Z 28.0N 61.1W 95 KT 36HR VT 11/1800Z 29.1N 61.8W 95 KT 48HR VT 12/0600Z 29.9N 62.0W 90 KT 72HR VT 13/0600Z 31.0N 62.0W 80 KT 96HR VT 14/0600Z 32.5N 62.0W 70 KT 120HR VT 15/0600Z 36.0N 61.0W 60 KT $$ FORECASTER PASCH |
