Tropical Storm DARBY

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TROPICAL STORM DARBY DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 PM PDT MON JUL 26 2004
 
THE DEPRESSION IS UPGRADED TO A TROPICAL STORM ON THE BASIS OF T2.5
DVORAK CLASSIFICATIONS FROM TAFB AND SAB.  OVER THE PAST SEVERAL
HOURS...THERE HAS BEEN A GRADUAL INCREASE IN DEEP CONVECTION CLOSE
TO AND NORTH OF THE PRESUMED CENTER OF CIRCULATION...AND SOME
BANDING IS STILL EVIDENT IN THE WESTERN SEMICIRCLE.  THERE IS GOOD
OUTFLOW IN THE NORTHWEST QUADRANT BUT NONE IS EVIDENT ELSEWHERE.
THE CIRCULATION OF DARBY APPEARS TO BE IN ITS FORMATIVE
STAGE...STILL ELONGATED AND NOT YET HAVING CLEANLY SEPARATED FROM
THE ITCZ.  THIS SUGGESTS THAT DEVELOPMENT OVER THE NEXT DAY OR SO
SHOULD BE MORE GRADUAL RATHER THAN RAPID.  THE GFDL AND SHIPS
MODELS...ON THE OTHER HAND...TAKE DARBY TO HURRICANE STRENGTH IN
24-36 HOURS.  THE OFFICIAL FORECAST...WHILE MORE AGGRESSIVE THAN
THE PREVIOUS ADVISORY...TAKES 48 HOURS TO REACH THAT THRESHOLD.
 
THE INITIAL MOTION REMAINS DIFFICULT TO DETERMINE...BUT IS ESTIMATED
TO BE 280/12.  I DON'T HAVE A GREAT DEAL OF CONFIDENCE IN THE MODEL
SOLUTIONS TONIGHT.  CURIOUSLY...MOST OF THE DYNAMICAL GUIDANCE
CALLS FOR AN IMMEDIATE NORTHWESTWARD MOTION.  THE MODELS MAY BE
REACTING TO A MID- TO UPPER-LEVEL LOW AHEAD OF DARBY NEAR
21N/122W...OR PERHAPS HAVE A POOR INITIALIZATION OF THE LOW-LEVEL
STEERING FLOW.  THERE ARE ALSO SUGGESTIONS IN THE GLOBAL MODELS
THAT THE UPPER-LOW COULD END UP WEAKENING THE SUBTROPICAL RIDGE
NORTH OF DARBY IN A COUPLE OF DAYS...YET THE MODELS TEND TO BEND
THE TRACK BACK TO THE WEST ABOUT THIS TIME.  IT'S USUALLY A LOSING
PROPOSITION TO TRY TO OUT-INTEGRATE THE GLOBAL GUIDANCE...SO THE
OFFICIAL FORECAST ATTEMPTS TO ACCOMODATE THIS GUIDANCE WITH A
SHORT-TERM BEND TO THE RIGHT FOLLOWED BY A MORE WESTWARD TRACK.  I
WOULD NOT BE SURPRISED HOWEVER...IF IT TURNS OUT THAT DARBY MOVES
MORE WESTWARD INITIALLY AND THEN TURNS NORTHWESTWARD IN A COUPLE OF
DAYS.   
 
FORECASTER FRANKLIN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      27/0300Z 12.5N 116.1W    35 KT
 12HR VT     27/1200Z 13.1N 117.7W    40 KT
 24HR VT     28/0000Z 13.9N 119.6W    45 KT
 36HR VT     28/1200Z 14.5N 121.5W    55 KT
 48HR VT     29/0000Z 15.0N 123.5W    65 KT
 72HR VT     30/0000Z 15.5N 127.5W    65 KT
 96HR VT     31/0000Z 16.0N 131.5W    65 KT
120HR VT     01/0000Z 16.5N 135.5W    60 KT
 
 
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