Hurricane BERTHA

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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
 
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FORECASTER KNABB
 
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