Hurricane BERTHA

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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
 
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