Hurricane BERTHA

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HURRICANE BERTHA DISCUSSION NUMBER  36
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
1100 PM EDT FRI JUL 11 2008
 
THE GEOSTATIONARY SATELLITE APPEARANCE OF BERTHA HAS NOT CHANGED
THAT MUCH SINCE THE RECONNAISSANCE AIRCRAFT DEPARTED SEVERAL HOURS
AGO...AND THE SUBJECTIVE DVORAK ESTIMATES ARE ABOUT THE SAME...SO
THE ADVISORY INTENSITY REMAINS 80 KT.  BOTH INFRARED AND MICROWAVE
IMAGERY SUGGEST THERE IS STILL SOMETHING OF AN INNER EYEWALL THAT
REFUSES TO COMPLETELY GO AWAY...WHILE THE OUTER EYEWALL PERSISTS AS
A CLOSED RING AT A SLIGHTLY SMALLER RADIUS THAN 24 HOURS AGO.  ALL
OF THAT MEANS THAT THE WELL-ADVERTISED EYEWALL REPLACEMENT CYCLE IS
NOT EXACTLY PROCEEDING AT A BLISTERING PACE...AND I AM REALLY NOT
SURE HOW THE INNER CORE STRUCTURE WILL EVOLVE DURING THE NEXT 24
HOURS OR SO.  PERSISTENCE MIGHT BE A GOOD GUIDE...SO A VERY SLOW
CHANGE IN BERTHA'S STRUCTURE WILL BE THE ASSUMPTION DRIVING THE
FLAT INTENSITY FORECAST THROUGH 24 HOURS.  VERY GRADUAL WEAKENING
IS FORECAST THEREAFTER...IN ACCORDANCE WITH A CONSENSUS OF OUR
PRIMARY INTENSITY MODELS...AS BERTHA TAKES ITS TIME MOVING TOWARD
COOLER WATERS.  THE NEW OFFICIAL INTENSITY FORECAST IS ALMOST AN
EXACT COPY OF THE PREVIOUS ONE.

BERTHA IS CAUGHT IN A VERY WEAK STEERING REGIME AS IT CONTINUES
MOVING...BARELY...AT 335/4.  THE DYNAMICAL MODELS DO NOT OFFER MUCH
PROGNOSTIC EVIDENCE THAT A MID-LATITUDE TROUGH WITH SUFFICIENT
AMPLITUDE WILL BE ABLE TO GET BERTHA MOVING MUCH FASTER WITHIN THE
FIVE-DAY FORECAST PERIOD.  THE SHORTWAVE TROUGH CURRENTLY ALONG THE
U.S. NORTHEAST COAST WILL ALMOST CERTAINLY BYPASS BERTHA WELL TO
THE NORTH...AND NONE OF THE MODELS FORECAST BERTHA TO ACCELERATE
AHEAD OF THE NEXT TROUGH WITHIN FIVE DAYS...ALTHOUGH THAT TROUGH
LOOKS TO BE STRONGER AND MIGHT TRULY PICK UP BERTHA BEYOND THAT
TIME.  IN BETWEEN THOSE TWO TROUGHS...THE MODELS DISAGREE ON HOW
MUCH OF A RIDGE MIGHT FORM NORTH OF BERTHA...AND SOME OF THEM
FORECAST BERTHA TO STALL WITHIN THE NEXT DAY OR TWO.  IN
ADDITION...MOST OF THE MODELS SHOW AT LEAST SOME SOUTHWARD
COMPONENT OF MOTION ON DAY 5...IN RESPONSE TO A CUTOFF LOW FORECAST
TO BE NOT FAR NORTHEAST OF BERTHA BY THAT TIME.  THE RESULT OF THE
COMPLICATED SET OF INDIVIDUAL MODEL TRACKS IS A CONSENSUS THAT
LEADS ME TO SLOW DOWN THE OFFICIAL FORECAST JUST SLIGHTLY...ALSO
WITH A NUDGE TO THE WEST...ALONG A TRACK SIMILAR TO THE PREVIOUS
ADVISORY THAT SIMPLY BENDS BERTHA'S MOTION GRADUALLY TO THE
RIGHT... AT A SPEED OF ABOUT 3 KT THROUGHOUT THE FIVE-DAY FORECAST. 
SOME ERRATIC MOTION AT SOME POINT ALONG THE WAY WOULD NOT BE
SURPRISING.

THE PROBABILITY OF WINDS OF TROPICAL STORM FORCE OCCURRING IN
BERMUDA WITHIN THE NEXT FIVE DAYS...AS CONVEYED IN THE WIND SPEED
PROBABILITY PRODUCT INCLUDED IN EACH ADVISORY PACKAGE...HAS BEEN
GRADUALLY RISING DURING THE PAST COUPLE OF DAYS...AND HAS NOW
REACHED 58%.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      12/0300Z 29.4N  62.4W    80 KT
 12HR VT     12/1200Z 30.0N  62.6W    80 KT
 24HR VT     13/0000Z 30.6N  62.6W    80 KT
 36HR VT     13/1200Z 31.2N  62.5W    75 KT
 48HR VT     14/0000Z 31.7N  62.4W    70 KT
 72HR VT     15/0000Z 33.0N  62.0W    65 KT
 96HR VT     16/0000Z 34.0N  61.5W    60 KT
120HR VT     17/0000Z 35.0N  60.5W    60 KT
 
$$
FORECASTER KNABB
 
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