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| #200183 (Received by flhurricane at: 11:08 PM 09.Jul.2008) TCDAT2 HURRICANE BERTHA DISCUSSION NUMBER 28 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 1100 PM EDT WED JUL 09 2008 THERE HAVE BEEN NO APPRECIABLE CHANGES IN THE SATELLITE APPEARANCE OF THE HURRICANE DURING THE PAST FEW HOURS. OBJECTIVE DVORAK ESTIMATES APPROACHED 100 KT A FEW HOURS AGO BUT HAVE SINCE COME BACK DOWN IN LINE WITH THE 00Z SUBJECTIVE ESTIMATES...WHICH SUPPORT MAINTAINING THE ADVISORY INTENSITY AT 90 KT. BERTHA CURRENTLY LIES OVER SEA-SURFACE TEMPERATURES OF ALMOST 28 CELSIUS...AND THE WATERS ALONG THE FORECAST TRACK WILL REMAIN ABOUT THE SAME FOR THE NEXT COUPLE OF DAYS. GLOBAL MODELS DO NOT APPEAR TO FORECAST AN INCREASE IN VERTICAL WIND SHEAR IMPACTING THE TROPICAL CYCLONE DURING THAT PERIOD...SO ALL OF THE OBJECTIVE INTENSITY MODELS FORECAST AT LEAST A LITTLE STRENGTHENING. THE OFFICIAL FORECAST AGAIN CALLS FOR BERTHA TO REGAIN MAJOR HURRICANE STATUS...IN AGREEMENT WITH THE HWRF MODEL. GRADUAL WEAKENING IS FORECAST AT DAYS 3 THROUGH 5 DUE TO COOLING SSTS AND THE POTENTIAL FOR AN INCREASE IN SHEAR...IN LINE WITH THE INTENSITY MODEL CONSENSUS. THE SHIPS AND LGEM...HOWEVER...FORECAST MORE RAPID WEAKENING THAN THE GFDL AND HWRF...SO HOW MUCH WEAKENING OCCURS NEAR THE END OF THE FORECAST IS RATHER UNCERTAIN...AND DEPENDS IN PART ON HOW FAST BERTHA MOVES TOWARD COOLER WATERS BY THAT TIME. THE HURRICANE CONTINUES MOVING NORTHWESTWARD OR ALONG 315 DEGREES AT ABOUT 10 KT...WITH STEERING PROVIDED BY A MID-LEVEL RIDGE TO ITS NORTH. AS THAT RIDGE SLIDES EASTWARD AND WEAKENS DURING THE NEXT TWO TO THREE DAYS...BERTHA IS FORECAST TO GRADUALLY TURN TOWARD THE NORTH AND SLOW DOWN. THIS PORTION OF THE FORECAST IS AGREED UPON WELL BY THE VARIOUS DYNAMICAL MODELS...BUT THEN THEY START TO DIVERGE...ESPECIALLY IN TERMS OF FORWARD SPEED...DUE TO VARYING DEPICTIONS OF WHEN A HIGH-AMPLITUDE TROUGH WILL EMERGE OFF THE EAST COAST OF THE UNITED STATES. THE UKMET...NOGAPS...AND GFDL FORECAST BERTHA TO SPEED UP BEYOND 72 HOURS AHEAD OF A SLIGHTLY MORE PROGRESSIVE TROUGH...WHILE THE 18Z GFS AND THE 12Z ECMWF FORECAST BERTHA TO STALL AT 72-120 HOURS...WAITING FOR THAT TROUGH TO PERHAPS PICK BERTHA UP LATER. THE NEW OFFICIAL TRACK FORECAST IS SLOWED DOWN A LITTLE AT THE END COMPARED TO THE PREVIOUS ADVISORY...AS AN APPROXIMATE COMPROMISE BETWEEN THESE EXTREMES. IT IS IMPORTANT TO NOTE THAT ALL OF THE MODELS SHOW A RELATIVELY SLOW MOTION AT AROUND 72 HOURS...NOT FAR EAST OF BERMUDA...AND BERTHA COULD MOVE ERRATICALLY DURING THAT TIME FRAME. THEREFORE... INTERESTS ON THAT ISLAND SHOULD CLOSELY MONITOR THE PROGRESS OF BERTHA FOR THE NEXT SEVERAL DAYS. CONSIDERING THE COMBINED FORECAST UNCERTAINTIES IN THE TRACK...INTENSITY...AND SIZE OF BERTHA...THERE IS ALREADY A 43% CHANCE THAT BERMUDA WILL EXPERIENCE SUSTAINED WINDS OF TROPICAL STORM FORCE DURING THE NEXT FIVE DAYS...AS CONVEYED IN THE WIND SPEED PROBABILITY PRODUCT ACCOMPANYING THIS ADVISORY PACKAGE. FORECAST POSITIONS AND MAX WINDS INITIAL 10/0300Z 25.5N 58.9W 90 KT 12HR VT 10/1200Z 26.4N 59.9W 100 KT 24HR VT 11/0000Z 27.5N 60.8W 100 KT 36HR VT 11/1200Z 28.7N 61.5W 100 KT 48HR VT 12/0000Z 29.7N 61.8W 95 KT 72HR VT 13/0000Z 31.0N 62.0W 85 KT 96HR VT 14/0000Z 32.5N 61.5W 75 KT 120HR VT 15/0000Z 35.5N 60.5W 65 KT $$ FORECASTER KNABB/BROWN |
