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| #200711 (Received by flhurricane at: 11:08 PM 12.Jul.2008) TCDAT2 HURRICANE BERTHA DISCUSSION NUMBER 40 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 1100 PM EDT SAT JUL 12 2008 BERTHA HAS SEEMINGLY DROPPED ANCHOR A COUPLE HUNDRED MILES SOUTHEAST OF BERMUDA...AND IT HAS BEEN ABOUT 12 HOURS SINCE THE CENTER HAS MOVED ENOUGH TO ASSIGN A MEANINGFUL FORWARD MOTION. THE TROPICAL CYCLONE IS STUCK WITHIN A LARGE BREAK IN THE SUBTROPICAL RIDGE THAT IT SHARES WITH TWO MID- TO UPPER-LEVEL LOWS...ONE NEAR THE BAHAMAS AND THE OTHER ABOUT 1000 N MI TO ITS EAST. THE EASTERN LOW IS FORECAST BY THE MODELS TO LIFT OUT AS A BIT OF RIDGING FORMS JUST NORTHEAST OF BERTHA TOMORROW...AND ONCE THAT FEATURE MATERIALIZES...BERTHA SHOULD BEGIN MOVING SLOWLY NORTHWARD AGAIN. IN A COUPLE OF DAYS...THE NEXT MID-LATITUDE TROUGH SHOULD REACH THE U.S. NORTHEAST COAST AND START PUSHING BERTHA EASTWARD. VERY LATE IN THE FIVE-DAY FORECAST PERIOD...A NEW MID- TO UPPER-LEVEL LOW IS FORECAST BY ALL MODELS TO DEVELOP CLOSE BY TO THE NORTHEAST OF BERTHA...AND THE TROPICAL CYCLONE COULD HAVE A SOUTHWARD COMPONENT OF MOTION BY THEN. BASED ON THE MODEL GUIDANCE IN THIS SCENARIO...THE NEW OFFICIAL TRACK FORECAST HAS BEEN ADJUSTED SLIGHTLY WESTWARD FROM THE PREVIOUS ONE THROUGH ABOUT 72 HOURS...SINCE NEARLY ALL OF DYNAMICAL MODELS HAVE SHIFTED A BIT IN THAT DIRECTION. IT IS VERY IMPORTANT TO EMPHASIZE THAT EVEN SLOW-MOVING TROPICAL CYCLONES SUCH AS THIS ONE CAN POSE CHALLENGING TRACK FORECAST UNCERTAINTIES...AND THAT IT IS DIFFICULT TO DETERMINE EXACTLY HOW CLOSE THE CENTER OF BERTHA WILL COME TO BERMUDA. EVIDENCE OF THAT UNCERTAINTY LIES IN SOME OF THE LATEST 48-HOUR MODEL TRACKS...WITH THE GFDL AND HWRF JUST A HAIR EAST OF THE OFFICIAL...BUT THE GFS IS FARTHER WEST. THE SPREAD AT 4-5 DAYS IS EVEN LARGER BUT THE SENSE IS FOR A GENERAL EASTWARD MOTION AT A SLIGHTLY GREATER FORWARD SPEED. THE INITIAL INTENSITY REMAINS 65 KT...SINCE THE CONVECTIVE PATTERN HAS NOT CHANGED MUCH SINCE WE HAD AIRCRAFT DATA THIS AFTERNOON...AND KEEPING BERTHA AS A HURRICANE IS SUPPORTED BY THE LATEST SUBJECTIVE DVORAK CI NUMBERS. FOLLOWING A BRIEF STRONG BURST AROUND 00Z...THE COLD CLOUD TOPS HAVE BEEN GRADUALLY ON THE DECLINE. A SLOW WEAKENING TREND IS PROBABLY NOT TOO FAR IN THE FUTURE...GIVEN THE EFFECTS OF OCEAN UPWELLING UNDER A STATIONARY HURRICANE...AND SINCE WHEN BERTHA DOES GET MOVING IT SHOULD ENCOUNTER STEADILY DECREASING SEA-SURFACE TEMPERATURES. THE NEW OFFICIAL INTENSITY FORECAST IS NOT MUCH OF A CHANGE FROM THE PREVIOUS ADVISORY AND IS CONSISTENT WITH THE INTENSITY MODEL CONSENSUS. FORECAST POSITIONS AND MAX WINDS INITIAL 13/0300Z 29.9N 62.4W 65 KT 12HR VT 13/1200Z 30.2N 62.5W 65 KT 24HR VT 14/0000Z 30.8N 62.8W 60 KT 36HR VT 14/1200Z 32.0N 63.2W 60 KT 48HR VT 15/0000Z 33.2N 63.2W 60 KT 72HR VT 16/0000Z 35.5N 61.5W 55 KT 96HR VT 17/0000Z 35.5N 58.0W 50 KT 120HR VT 18/0000Z 34.5N 55.0W 50 KT $$ FORECASTER KNABB |
