Hurricane BERTHA

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HURRICANE BERTHA DISCUSSION NUMBER  41
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL022008
500 AM EDT SUN JUL 13 2008
 
THE CENTER OF BERTHA HAS BECOME MORE DIFFICULT TO LOCATE THIS
MORNING.  INFRARED IMAGERY SUGGESTS THE SYSTEM IS DRIFTING
SOUTHWARD.  HOWEVER...A COUPLE OF RECENT AMSU MICROWAVE OVERPASSES
SUGGEST THE LOW-LEVEL CENTER IS NORTH OF THE CENTER SEEN IN THE IR
IMAGERY...POSSIBLY DUE TO THE ONSET OF NORTHERLY VERTICAL WIND
SHEAR.  THE INITIAL POSITION IS AN AVERAGE OF LOCATIONS FROM THE IR
AND MICROWAVE DATA.  SATELLITE INTENSITY ESTIMATES ARE 65 KT FROM
TAFB AND 55 KT FROM SAB.  THE INITIAL INTENSITY REMAINS 65 KT...BUT
THIS COULD BE GENEROUS.

THE INITIAL MOTION IS STATIONARY.  LARGE-SCALE MODEL ANALYSES SHOW
THAT BERTHA IS EMBEDDED IN THE SUBTROPICAL RIDGE...WITH WATER VAPOR
IMAGERY SHOWING A MID-LEVEL SHORTWAVE RIDGE ABOUT TO PASS TO THE
NORTH OF THE CYCLONE.  THIS COMBINATION OF FEATURES SHOULD KEEP
BERTHA NEARLY STATIONARY DURING THE NEXT 12 HR OR SO...FOLLOWED BY
A SLOW MOTION TOWARD THE NORTHWEST AND NORTH AS THE MID-LEVEL RIDGE
AMPLIFIES THE SUBTROPICAL RIDGE EAST OF BERTHA AND A MID/UPPER-
LEVEL TROUGH MOVES INTO THE EASTERN UNITED STATES.  AFTER 72 HR...A
DEEP-LAYER LOW FORMING OVER THE NORTH CENTRAL ATLANTIC WILL LIKELY
STEER BERTHA EASTWARD OR EAST-SOUTHEASTWARD.  THE LARGE-SCALE
MODELS GENERALLY AGREE ON THIS SCENARIO...BUT THERE REMAINS SOME
SPREAD IN BOTH THE DIRECTION AND SPEED OF MOTION.  THE NEW FORECAST
TRACK IS SHIFTED TO THE LEFT OF THE PREVIOUS TRACK DURING THE FIRST
72 HR...AND EVENTUALLY WINDS UP AT THE 5 DAY POSITION FROM THE
PREVIOUS FORECAST.  THE NEW TRACK IS IN BEST AGREEMENT WITH THE
ECMWF.

BERTHA HAS ESSENTIALLY BEEN STATIONARY FOR 24 HR...AND IT IS LIKELY
UPWELLING COLDER WATER UNDERNEATH IT.  THIS SHOULD CAUSE A
WEAKENING TREND AS LONG AS THE CYCLONE REMAINS STATIONARY.  WHEN
BERTHA STARTS MOVING...THE FORECAST TRACK TAKES IT OVER COOLER
WATER AND INTO INCREASING VERTICAL WIND SHEAR.  DESPITE THESE
ISSUES...THE GFDL AND HWRF MAINTAIN BERTHA AS A HURRICANE FOR 5
DAYS...WHILE THE SHIPS MODEL FORECASTS WEAKENING TO 35 KT BY 5
DAYS.  THE NEW INTENSITY FORECAST COMPROMISES BETWEEN THESE
EXTREMES...CALLING FOR BERTHA TO WEAKEN TO 50 KT BY THE END OF THE
FORECAST PERIOD.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      13/0900Z 29.8N  62.5W    65 KT
 12HR VT     13/1800Z 30.1N  62.8W    60 KT
 24HR VT     14/0600Z 30.8N  63.2W    60 KT
 36HR VT     14/1800Z 32.0N  63.6W    60 KT
 48HR VT     15/0600Z 33.4N  63.5W    55 KT
 72HR VT     16/0600Z 35.5N  62.0W    50 KT
 96HR VT     17/0600Z 35.5N  59.0W    50 KT
120HR VT     18/0600Z 34.5N  55.0W    50 KT
 
$$
FORECASTER BEVEN
 
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