Hurricane BERTHA

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HURRICANE BERTHA DISCUSSION NUMBER  31
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
500 PM EDT THU JUL 10 2008
 
A RECENT TRMM OVERPASS AND GOES VISIBLE IMAGERY INDICATE THAT THE
OUTER CONVECTIVE BANDING NOTED EARLIER HAS WRAPPED UP INTO AN OUTER
EYEWALL WITH A DIAMETER OF ABOUT 75 N MI.  THIS FEATURE SURROUNDS
THE ORIGINAL INNER EYE WHICH IS ABOUT 10 N MI WIDE.  BASED ON
SATELLITE INTENSITY ESTIMATES AND WARMING OF THE CLOUD TOPS IN THE
INNER EYEWALL...THE INITIAL INTENSITY IS REDUCED TO 75 KT.  THE
CIRRUS OUTFLOW REMAINS GOOD IN THE NORTHWESTERN SEMICIRCLE...AND
HAS INCREASED SINCE THIS MORNING IN THE SOUTHEASTERN SEMICIRCLE.

THE INITIAL MOTION IS 315/9.  WATER VAPOR IMAGERY SHOWS A COMPLEX
STEERING PATTERN...CONSISTING OF A BROAD MID/UPPER-LEVEL TROUGH
OVER THE NORTHEASTERN UNITED STATES AND A MID-LEVEL RIDGE NORTH OF
BERTHA THAT ARE MOVING EASTWARD...AND AN UPPER-LEVEL LOW NEAR
28N69W THAT IS MOVING SLOWLY WESTWARD.  IN THE SHORT TERM...THIS
PATTERN SHOULD CAUSE BERTHA TO TURN NORTHWARD AS SHOWN IN THE NEW
FORECAST TRACK.  DURING THE 36-72 HR PERIOD...A MID-LEVEL RIDGE
DEVELOPING BEHIND THE SHORTWAVE TROUGH IS LIKELY TO BLOCK BERTHA'S
NORTHWARD MOTION...WITH THE POSSIBILITY OF ERRATIC MOTION NEAR OR
TO THE EAST OF BERMUDA.  THEREAFTER...THE LARGE-SCALE MODELS
CURRENTLY AGREE THAT A SECOND TROUGH MOVING THROUGH THE
NORTHWESTERN ATLANTIC WILL BYPASS BERTHA...LEAVING THE STORM
TRAPPED IN THE SUBTROPICAL RIDGE TO THE NORTH OR NORTHEAST OF
BERMUDA.  IN A MAJOR SHIFT FROM PREVIOUS RUNS...NONE OF THE
DYNAMICAL MODELS SHOW BERTHA REACHING 40N IN 5 DAYS.  BASED ON
THIS...THE FORECAST TRACK WILL CALL FOR A SLOWER NORTHWARD MOTION
THAN IN THE PREVIOUS PACKAGE... WITH THE TRACK SMOOTHING THROUGH
SOME OF THE ERRATIC MOTION FORECASTS IN THE MODELS.  THE NEW TRACK
IS CLOSER TO BERMUDA THAN THE PREVIOUS TRACK...AND BERTHA COULD COME
EVEN CLOSER TO BERMUDA IF THE JOGS TO THE LEFT IN THE UKMET AND
CANADIAN MODELS VERIFY.

AN UPPER-LEVEL ANTICYCLONE IS BUILDING OVER BERTHA...AND THE STORM
STILL HAS A DAY OR TWO MORE OVER WARM SEA SURFACE TEMPERATURES. 
THUS...EVEN THOUGH NONE OF THE FORECAST MODELS CALL FOR SIGNIFICANT
STRENGTHENING...THE HURRICANE COULD RE-INTENSIFY AT THE END OF THE
EYEWALL REPLACEMENT CYCLE.  BASED ON THIS...THE NEW INTENSITY
FORECAST CALLS FOR MODEST STRENGTHENING IN 24 HR.  LATER IN THE
FORECAST PERIOD...BERTHA IS EXPECTED TO MOVE OUT FROM UNDER THE
ANTICYCLONE AND ENCOUNTER VERTICAL WIND SHEAR.  THIS SHOULD OCCUR
AS BERTHA REACHES COOLER WATERS AND RESULT IN A WEAKENING TREND.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      10/2100Z 27.2N  60.8W    75 KT
 12HR VT     11/0600Z 28.0N  61.7W    75 KT
 24HR VT     11/1800Z 29.0N  62.2W    80 KT
 36HR VT     12/0600Z 29.8N  62.5W    80 KT
 48HR VT     12/1800Z 30.5N  62.5W    75 KT
 72HR VT     13/1800Z 31.5N  62.5W    70 KT
 96HR VT     14/1800Z 33.0N  62.5W    60 KT
120HR VT     15/1800Z 36.0N  62.0W    50 KT
 
$$
FORECASTER BEVEN
 
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