Hurricane DARBY

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TROPICAL STORM DARBY DISCUSSION NUMBER  16
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 AM PDT FRI JUL 30 2004

DARBY HAS SIGNIFICANTLY DECREASED IN ORGANIZATION DURING THE PAST
SEVERAL HOURS.  THE LOW-LEVEL CENTER HAS BECOME EXPOSED TO THE
SOUTH OF THE CONVECTION...AND THE CONVECTION HAS DECREASED
SIGNIFICANTLY IN COVERAGE.  SATELLITE INTENSITY ESTIMATES ARE 65 KT
FROM AFWA AND SAB...AND 45 KT FROM TAFB.  BASED ON THIS...DARBY IS
DOWNGRADED TO A 55 KT TROPICAL STORM.
 
THE INITIAL MOTION IS 280/12...AND OVER THE LAST FEW HOURS THE
CYCLONE MAY BE MOVING DUE WEST.  DARBY IS ON THE SOUTH SIDE OF A
STRONG LOW/MID LEVEL RIDGE...WHILE A MID/UPPER LEVEL TROUGH IS
LOCATED NORTH OF THE HAWAIIAN ISLANDS.  THE RIDGE SHOULD KEEP DARBY
ON A BASICALLY WESTWARD COURSE THROUGH ABOUT 72 HR.  AFTER THAT...
THERE IS A CHANCE THE CYCLONE COULD TURN MORE NORTHWARD IF THE
CYCLONE STILL HAS ANY VERTICAL DEPTH.  THE NOGAPS...WHICH KEEPS
DARBY STRONG THROUGH THE FORECAST PERIOD...TURNS IT NORTHWARD IN
RESPONSE TO THE CENTRAL PACIFIC TROUGH.  OTHER LARGE-SCALE
MODELS...WHICH FORECAST A WEAKER SYSTEM...SHOW A MORE WESTERLY
MOTION.  THE OFFICIAL FORECAST CALLS FOR A WESTWARD TRACK FOR 72
HR... FOLLOWED BY A WEST-NORTHWESTWARD MOTION.  THE TRACK IS NUDGED
A LITTLE SOUTH OF THE PREVIOUS PACKAGE AND IS IN BEST AGREEMENT
WITH THE GFDN AND BAMM. 

THE INTENSITY FORECAST IS PROBLEMATIC.  NORMALLY A CYCLONE OF
THIS ORGANIZATION AT THIS LOCATION CONTINUES WEAKENING UNTIL
DISSIPATION...AS INDICATED BY THE GFS...GFDL...AND SHIPS MODELS. 
HOWEVER...SEA SURFACE TEMPERATURES ARE A LITTLE WARMER THAN NORMAL
IN THE CENTRAL PACIFIC...AND THE FORECAST TRACK MOVES THE CYCLONE
OVER INCREASING SSTS AFTER IT CROSSES 140W.  THIS MAY BE REFLECTED
IN THE NOGAPS FORECAST.  THE INTENSITY FORECAST TENDS TO
COMPROMISE...CALLING FOR DARBY TO WEAKEN TO A DEPRESSION IN 36 HR
THEN LINGER THROUGH THE REST OF THE FORECAST PERIOD.  ALTERNATIVE
SCENARIOS INCLUDE DARBY COULD DISSIPATE BEFORE IT REACHES THE
WARMER WATER...OR THAT INCREASING VERTICAL SHEAR ASSOCIATED WITH
THE TROUGH COULD DISSIPATE IT DESPITE THE WARMER WATER.
 
FORECASTER BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      30/1500Z 18.6N 132.2W    55 KT
 12HR VT     31/0000Z 18.9N 134.1W    45 KT
 24HR VT     31/1200Z 19.1N 136.8W    35 KT
 36HR VT     01/0000Z 19.3N 139.7W    30 KT
 48HR VT     01/1200Z 19.4N 142.5W    25 KT
 72HR VT     02/1200Z 19.5N 147.5W    25 KT
 96HR VT     03/1200Z 20.5N 152.0W    25 KT
120HR VT     04/1200Z 22.0N 155.0W    25 KT
 
 
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