Hurricane BERTHA

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HURRICANE BERTHA DISCUSSION NUMBER  32
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
1100 PM EDT THU JUL 10 2008
 
A SERIES OF MICROWAVE OVERPASSES THROUGHOUT THE DAY INDICATE THAT
THE INNER EYEWALL HAS BEEN SLOWLY DETERIORATING...WHILE A NEW OUTER
EYEWALL HAS BEEN TAKING SHAPE.  THE MOST RECENT IMAGE...FROM SSMIS
AT 2319Z...DEPICTS THE OUTER EYEWALL AS A COMPLETELY CLOSED RING AT
A RADIUS OF ABOUT 60 N MI...WITH BARELY HALF OF THE INNER EYEWALL
REMAINING.  TRENDS IN GEOSTATIONARY INFRARED IMAGERY REFLECT THE
EVOLVING STRUCTURE...AS THE EYE HAS JUST ABOUT DISAPPEARED WHILE
CLOUD TOPS HAVE BEEN COOLING ABOVE THE OUTER RING SEEN IN THE
MICROWAVE.  IN ADDITION...A QUIKSCAT PASS BACK AT 23Z DEPICTED A
SECONDARY WIND MAXIMUM IN THE OUTER EYEWALL.  DVORAK INTENSITY
ESTIMATES AT 00Z WERE ALL 75 KT...BUT IT IS HARD TO ESTIMATE EVEN
VERY SHORT-TERM INTENSITY CHANGES GIVEN THE ONGOING STRUCTURAL
CHANGES...EVEN THOUGH THE OCEAN AND ATMOSPHERIC ENVIRONMENTS ARE NOT
PROHIBITIVE FOR STRENGTHENING.  IT IS POSSIBLE THAT BERTHA WILL
WEAKEN SOME UPON THE FINAL DEMISE OF THE INNER EYEWALL...AND IT IS
ALSO POSSIBLE THAT THE NEW OUTER EYEWALL WILL EVENTUALLY CONTRACT
AND BE ACCOMPANIED BY SOME STRENGTHENING...BUT I AM NOT GOING TO
TRY TO EXPLICITLY FORECAST SUCH CHANGES.  THE MOST PRUDENT
APPROACH...WHICH IS ALSO SUPPORTED BY THE OBJECTIVE GUIDANCE...IS
TO KEEP THE INTENSITY LEVEL FOR THE NEXT COUPLE OF DAYS...FOLLOWED
BY GRADUAL WEAKENING AS THE WIND SHEAR POTENTIALLY INCREASES AND
BERTHA PASSES OVER GRADUALLY COOLER SEA-SURFACE TEMPERATURES.  THE
NEW INTENSITY FORECAST HAS JUST A SLIGHTLY LOWER PEAK INTENSITY
THAN THE PREVIOUS ADVISORY.

BERTHA APPEARS TO BE GRADUALLY SLOWING DOWN...AND THE INITIAL MOTION
ESTIMATE IS NOW 315/7.  THE FORECAST AND ITS SUPPORTING REASONING
HAVE NOT REALLY CHANGED FOR THE FIRST 48 HOURS OR SO...INCLUDING A
VERY SLOW FORWARD MOTION IN A COUPLE OF DAYS NEAR BERMUDA.  BEYOND
THAT TIME...ESSENTIALLY ALL OF THE MODELS HAVE SHIFTED TO THE EAST
AND FORECAST A FASTER MOTION AT 4-5 DAYS...THIS TIME FORECASTING
BERTHA TO BE PULLED NORTHWARD A LITTLE SOONER BY A TROUGH ALONG THE
U.S. EAST COAST BY THEN...WITH SLIGHTLY LESS RIDGING TO THE NORTH
TO BLOCK BERTHA'S POLEWARD PROGRESS.  THE NEW OFFICIAL TRACK
FORECAST DOES NOT COMPLETELY BUY INTO THIS MUCH OF A SHIFT IN THE
MODELS...BUT IT IS ADJUSTED EASTWARD AND MOST OF THE WAY TOWARD THE
CONSENSUS.  SINCE THIS NEW TRACK STILL DOES NOT FORECAST BERTHA TO
BE GOING ANY FASTER THAN ABOUT 5-7 KT AT THE END OF THE FIVE-DAY
PERIOD...THIS ADJUSTMENT DOES NOT REPRESENT A HUGE CHANGE IN WHERE
WE EXPECT BERTHA TO BE IN FIVE DAYS.

THE LATEST WIND SPEED PROBABILITY PRODUCT ACCOMPANYING THIS ADVISORY
PACKAGE INDICATES THAT...GIVEN UNCERTAINTIES IN THE FORECASTS OF
BERTHA'S TRACK...INTENSITY...AND SIZE...THERE ROUGHLY A 50% CHANCE
THAT WINDS OF TROPICAL STORM FORCE WILL AFFECT BERMUDA DURING THE
NEXT FIVE DAYS.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      11/0300Z 27.7N  61.2W    75 KT
 12HR VT     11/1200Z 28.4N  61.9W    75 KT
 24HR VT     12/0000Z 29.5N  62.6W    75 KT
 36HR VT     12/1200Z 30.4N  62.6W    75 KT
 48HR VT     13/0000Z 31.1N  62.3W    70 KT
 72HR VT     14/0000Z 32.5N  61.5W    65 KT
 96HR VT     15/0000Z 34.5N  60.5W    60 KT
120HR VT     16/0000Z 37.0N  59.0W    55 KT
 
$$
FORECASTER KNABB
 
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