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| #200051 (Received by flhurricane at: 4:40 AM 09.Jul.2008) TCDAT2 HURRICANE BERTHA DISCUSSION NUMBER 25 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 500 AM EDT WED JUL 09 2008 THE CLOUD PATTERN ASSOCIATED WITH BERTHA APPEARS TO HAVE BECOME SLIGHTLY MORE CIRCULAR AND ORGANIZED OVER THE PAST FEW HOURS. A SLIGHT EXPANSION OF THE CIRRUS CANOPY OVER THE SOUTHERN AND WESTERN PORTIONS OF THE CIRCULATION SUGGESTS THAT VERTICAL SHEAR MAY BE DECREASING. THIS IS CONSISTENT WITH WATER VAPOR ANIMATIONS WHICH SHOW AN UPPER-LEVEL CYCLONE NEAR 27N 68W MOVING WESTWARD AND FARTHER AWAY FROM THE TROPICAL CYCLONE. DVORAK SATELLITE CURRENT INTENSITY ESTIMATES ARE ONLY 55 TO 65 KT AND THEREFORE THE CURRENT WIND SPEED IS ADJUSTED TO 65 KT FOR THIS ADVISORY. THE SHIPS MODEL WHICH USES THE GFS UPPER-LEVEL WIND FORECAST DOES NOT SHOW A DECREASE IN SHEAR OR ANY STRENGTHENING. HOWEVER THE GFDL AND HWRF MODELS...BOTH OF WHICH HAVE SOME LIMITED SKILL IN INTENSITY PREDICTION...FORECAST A MODEST INCREASE IN STRENGTH OVER THE NEXT 48 HOURS OR SO. MY WIND SPEED FORECAST ONLY HINTS AT THE POSSIBILITY OF RE-INTENSIFICATION. HOWEVER BERTHA WILL CONTINUE TO TRAVERSE WARM WATERS FOR A COUPLE OF DAYS AND SINCE OTHER ENVIRONMENTAL FACTORS DO NOT APPEAR INHIBITING...IT WOULD NOT BE SURPRISING TO SEE IT BECOME STRONGER AGAIN. ALTHOUGH THERE HAS BEEN A FAIR AMOUNT OF SCATTER IN THE CENTER FIXES OVERNIGHT...MY ESTIMATE OF INITIAL MOTION IS STILL NORTHWESTWARD OR 305/9. THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE TRACK FORECAST REASONING FROM THE PREVIOUS ADVISORY PACKAGE. A MID-TROPOSPHERIC ANTICYCLONE TO THE NORTH OF BERTHA IS LIKELY TO WEAKEN AS A 500 MB SHORTWAVE TROUGH MOVES OFF OF THE U.S. EAST COAST IN 2-3 DAYS. THIS EVOLUTION SHOULD CAUSE THE MOTION OF THE HURRICANE TO SLOW FURTHER AND TO TURN MORE NORTHWARD AROUND THAT TIME. GLOBAL MODELS INDICATE THAT THE TROUGH WILL MISS BERTHA AND LEAVE IT IN WEAK STEERING CURRENTS UNTIL NEAR THE END OF THE FORECAST PERIOD WHEN A NEW AND STRONGER TROUGH APPROACHES THE EAST COAST. A VERY SLOW MOTION IS INDICATED AROUND DAYS 3-4 DUE TO THE WEAK STEERING AND BERTHA COULD ALSO MOVE ERRATICALLY AROUND THAT TIME. BY DAY 5 THE OFFICIAL FORECAST SHOWS A SLIGHT INCREASE IN NORTH-NORTHEASTWARD MOTION IN RESPONSE TO THE SECOND TROUGH. THE OFFICIAL TRACK FORECAST IS QUITE CLOSE TO THE VARIABLE DYNAMICAL MODEL CONSENSUS...TVCN. IT IS STILL TOO EARLY TO KNOW WHETHER BERTHA WILL HAVE AN IMPACT ON BERMUDA. INTERESTS ON THAT ISLAND SHOULD CONTINUE TO MONITOR THE PROGRESS OF THIS HURRICANE OVER THE NEXT COUPLE OF DAYS. FORECAST POSITIONS AND MAX WINDS INITIAL 09/0900Z 23.5N 56.5W 65 KT 12HR VT 09/1800Z 24.2N 57.8W 65 KT 24HR VT 10/0600Z 25.4N 59.2W 70 KT 36HR VT 10/1800Z 26.8N 60.2W 70 KT 48HR VT 11/0600Z 28.0N 61.0W 70 KT 72HR VT 12/0600Z 30.0N 61.5W 70 KT 96HR VT 13/0600Z 31.0N 61.5W 65 KT 120HR VT 14/0600Z 33.0N 61.0W 60 KT $$ FORECASTER PASCH |
