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Hurricane BERTHA


ZCZC MIATCDAT2 ALL
TTAA00 KNHC DDHHMM
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
 
NNNN


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