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Hurricane BERTHA


ZCZC MIATCDAT2 ALL
TTAA00 KNHC DDHHMM
HURRICANE BERTHA DISCUSSION NUMBER  37
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
500 AM EDT SAT JUL 12 2008
 
INFRARED SATELLITE IMAGERY SUGGESTS THAT THE INNER EYEWALL REMNANT
OF BERTHA IS FINALLY DISSIPATING...WITH THE CLOUD TOPS COOLING IN
THE LARGE OUTER EYEWALL.  SATELLITE INTENSITY ESTIMATES REMAIN
UNCHANGED FROM 6 HR AGO...AND BASED ON THIS AND PERSISTENCE FROM THE
PREVIOUS ADVISORY THE INITIAL INTENSITY REMAINS 80 KT.  THE CIRRUS
OUTFLOW REMAINS GOOD IN THE SOUTHERN SEMICIRCLE BUT HAS DECREASED
SOMEWHAT IN THE NORTHERN SEMICIRCLE.
 
THE INITIAL MOTION IS 340/4...AND THE 4 KT FORWARD SPEED COULD BE
GENEROUS.  BERTHA IS CURRENTLY EMBEDDED IN THE SUBTROPICAL RIDGE IN
AN AREA OF WEAK STEERING FLOW...WITH A MID/UPPER-LEVEL SHORTWAVE
TROUGH AND ASSOCIATED SURFACE COLD FRONT PASSING TO THE NORTH OF
THE HURRICANE.  THE LARGE-SCALE MODELS FORECAST THE TROUGH TO
BYPASS BERTHA WITH A MID-LEVEL SHORTWAVE RIDGE FORMING NORTH OF THE
HURRICANE.  THE ECMWF...NOGAPS...AND CANADIAN MODEL RESPOND TO THIS
BY JOGGING BERTHA TO THE WEST CLOSER TO BERMUDA....WHILE THE GFS...
HWRF..AND GFDL CALL FOR A NORTHWARD MOTION A LITTLE FARTHER TO THE
EAST.  THIS PORTION OF THE NEW FORECAST TRACK CALLS FOR A NORTHWARD
MOTION FOR ABOUT 72 HR...WITH THE 48-72 HR POSITION A LITTLE TO THE
WEST OF THE PREVIOUS TRACK IN DEFERENCE TO THE MODELS FORECASTING
A WESTWARD JOG.  AFTER 72 HR...THE LARGE-SCALE MODELS FORECAST THAT
A NEW TROUGH OVER THE EASTERN UNITED STATES WILL BE STRONG ENOUGH
TO LIFT BERTHA TO THE NORTHEAST OR EAST AT A FASTER FORWARD SPEED.
HOWEVER...THIS IS A SIGNIFICANT CHANGE FROM THE PREVIOUS MODELS
RUNS...AND THERE IS CONSIDERABLE SPREAD IN BOTH THE DIRECTION AND
SPEED OF MOTION.  THUS...THE NEW FORECAST FOR THOSE TIMES IS ONLY
SLIGHTLY FASTER THAN THE PREVIOUS FORECAST...AND CONSIDERABLY
SLOWER THAN THE CURRENT GUIDANCE.

THERE IS LITTLE CHANGE TO THE INTENSITY FORECAST PHILOSOPHY FROM THE
LAST SEVERAL ADVISORIES.  BERTHA SHOULD MAINTAIN ITS INTENSITY FOR
THE FIRST 24-36 HR WHILE UNDER AN UPPER-LEVEL ANTICYCLONE AND OVER
WARM SURFACE TEMPERATURES.  BY 36 HR...THE HURRICANE SHOULD BEGIN
TO WEAKEN AS IT REACHES COOLER WATERS.  THEN...BY 72 HR THE
VERTICAL WIND SHEAR IS FORECAST TO INCREASE...WHICH SHOULD INCREASE
THE WEAKENING RATE.  THE INTENSITY FORECAST IS IDENTICAL TO THE
LAST ADVISORY THROUGH 48 HR...THEN SHOWS MORE WEAKENING DUE TO THE
EXPECTED SHEAR.
 
A TROPICAL STORM WARNING WILL LIKELY BE ISSUED FOR BERMUDA LATER
THIS MORNING.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      12/0900Z 29.7N  62.5W    80 KT
 12HR VT     12/1800Z 30.1N  62.6W    80 KT
 24HR VT     13/0600Z 30.7N  62.6W    80 KT
 36HR VT     13/1800Z 31.3N  62.6W    75 KT
 48HR VT     14/0600Z 31.9N  62.6W    70 KT
 72HR VT     15/0600Z 33.5N  62.5W    60 KT
 96HR VT     16/0600Z 35.0N  61.5W    50 KT
120HR VT     17/0600Z 36.5N  59.5W    50 KT
 
$$
FORECASTER BEVEN
 
NNNN


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