Tropical Storm BERTHA

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TROPICAL STORM BERTHA DISCUSSION NUMBER  60
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
1100 PM EDT THU JUL 17 2008
 
BERTHA CONTINUES TO BE A RELENTLESS CYCLONE AS A NEW BURST OF DEEP
CONVECTION RECENTLY FORMED OVER THE SOUTHEASTERN SEMICIRCLE THUS
MAINTAINING THE OVERALL SATELLITE APPEARANCE.  BASED ON THIS...THE
INITIAL INTENSITY IS HELD AT 50 KT FOR THIS ADVISORY.  WHILE
VERTICAL SHEAR IS EXPECTED TO REMAIN RELATIVELY LOW FOR THE NEXT
DAY OR SO...BERTHA IS EXPECTED TO MOVE OVER INCREASINGLY COLDER
WATERS.  THIS SHOULD RESULT IN WEAKENING IN THE SHORT-TERM. BEYOND
48 HOURS...BAROCLINIC DYNAMICS AND POSSIBLE EXTRATROPICAL
TRANSITION COULD RESULT IN SOME RESTRENGTHENING.  HOWEVER...THERE
ARE DIFFERENCES AMONG THE VARIOUS DYNAMICAL MODELS WITH SOME MODELS
SHOWING BERTHA BECOMING A LARGE AND POWERFUL EXTRATROPICAL SYSTEM
WHILE OTHER MODELS SHOW A WEAKENING CYCLONE BEING ABSORBED INTO A
LARGER NON-TROPICAL LOW.  GIVEN THIS UNCERTAINTY...THE OFFICIAL
FORECAST SHOWS SLOW WEAKENING EARLY WITH A LITTLE RESTRENGTHENING
AT DAYS 4 AND 5. 

BERTHA CONTINUES TO ROTATE AROUND A MIDDLE TO UPPER LEVEL LOW
RESULTING IN A TURN TOWARDS THE EAST...AND THE INITIAL MOTION IS
NOW 100/15.  THE CYCLONE IS EXPECTED TO TURN NORTHEASTWARD AND
ACCELERATE DURING THE NEXT DAY OR SO AS IT COMPLETES ITS ROTATION
AROUND THE LOW AND THEN ENCOUNTERS STRONG SOUTHWESTERLY FLOW
AHEAD OF A MID-LATITUDE TROUGH MOVING OFF THE CANADIAN MARITIMES. 
AS WOULD BE EXPECTED WITH AN ACCELERATING CYCLONE...TRACK GUIDANCE 
DIVERGES QUITE A BIT DUE TO THEIR DIFFERENT FORWARD SPEEDS.  IN
GENERAL...TRACK GUIDANCE IS FASTER THAN BEFORE BUT THEY HAVE NOT
DEMONSTRATED GOOD RUN TO RUN CONSISTENCY.  THE OFFICIAL FORECAST IS
NUDGED TOWARDS THE CONSENSUS OF THE DYNAMICAL MODELS WHICH RESULTS
IN A SLIGHTLY FASTER TRACK FORECAST THAN BEFORE. 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      18/0300Z 33.9N  55.4W    50 KT
 12HR VT     18/1200Z 34.8N  53.7W    50 KT
 24HR VT     19/0000Z 36.7N  51.2W    45 KT
 36HR VT     19/1200Z 38.8N  48.6W    45 KT
 48HR VT     20/0000Z 41.2N  45.2W    40 KT
 72HR VT     21/0000Z 47.0N  36.5W    45 KT...EXTRATROPICAL
 96HR VT     22/0000Z 56.0N  24.0W    45 KT...EXTRATROPICAL
120HR VT     23/0000Z...ABSORBED
 
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