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Tropical Depression TWELVE-E


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TROPICAL DEPRESSION TWELVE-E DISCUSSION NUMBER   2
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
8 AM PDT WED SEP 08 2004
 
THERE HAS BEEN A MODEST INCREASE IN THE OVERALL AMOUNT OF DEEP
CONVECTION OVER THE PAST SEVERAL HOURS...BUT THE BASIC STRUCTURE
HAS NOT CHANGED.  EASTERLY SHEAR IS KEEPING THE LOW-LEVEL
CIRCULATION CENTER LOCATED NEAR AND JUST UNDERNEATH THE NORTHEAST
EDGE OF THE CONVECTION.  DVORAK INTENSITY ESTIMATES RANGE FROM 25
TO 35 KT FROM THE THREE AGENCIES.  THE QUIKSCAT PASS FROM LATE LAST
NIGHT SUGGESTED PEAK WINDS WERE AT LEAST 30 KT AND SO THE INITIAL
INTENSITY ESTIMATE IS INCREASED TO THAT VALUE FOR THIS ADVISORY.

QUIKSCAT AMBIGUITIES AND MORNING MICROWAVE DATA INDICATE THAT THE
DEPRESSION IS MOVING AT 270/8.  SYNOPTIC STEERING IS BEING PROVIDED
BY A MID- TO UPPER-LEVEL HIGH OVER BAJA CALIFORNIA THAT IS FORECAST
BY GLOBAL MODELS TO PERSIST OVER THE NEXT COUPLE OF DAYS...AND THIS
PATTERN SHOULD RESULT IN A WEST-NORTHWESTERLY TRACK OVER THAT TIME
FRAME.  BY DAY THREE...THE CYCLONE IS EXPECTED TO BE SOUTH OF A
WEAKNESS IN THE SUBTROPICAL RIDGE THAT WOULD PROVIDE AN OPPORTUNITY
FOR A MORE NORTHWARD TRACK.  HOWEVER...NONE OF THE EXPLICIT TRACK
GUIDANCE TAKES THE CYCLONE INTO THIS WEAKNESS...PERHAPS BECAUSE THE
MODELS GENERALLY KEEP THE SYSTEM QUITE WEAK THROUGHOUT THE FORECAST
PERIOD.  ADDITIONAL UNCERTAINTY STEMS FROM A POOR INITIALIZATION OF
THE LOW-LEVEL VORTEX...WHICH APPEARS TO RESULT IN A SPURIOUS
NORTHWEST TRACK INITIALLY IN THE GFS AND GFDL.  I AM INCLINED TO
DISCOUNT THIS NORTHWEST TRACK IN FAVOR OF CONTINUITY WITH THE
CURRENT MOTION ESTIMATE.  THE OFFICIAL FORECAST IS ADJUSTED SOUTH
OF THE PREVIOUS ADVISORY FOR THE FIRST 72 HOURS...AND IS SLIGHTLY
NORTH OF AND SLOWER THAN THE PREVIOUS ADVISORY AFTER THAT. 

CIMSS ANALYSES SHOW 25 KT OF EASTERLY SHEAR OVER THE DEPRESSION. 
GLOBAL MODELS SUGGEST THAT THIS SHEAR WILL PERSIST OVER THE NEXT 36
HOURS OR SO...AND THE OFFICIAL FORECAST SHOWS ONLY SLOW
STRENGTHENING DURING THIS TIME.  SSTS UNDER THE FORECAST TRACK
DECREASE SLIGHTLY OVER THE NEXT FIVE DAYS...SO IF THE SHEAR
GRADUALLY DIMINSHES AS EXPECTED THE CYCLONE WILL LIKELY BE SLOW TO
SPIN DOWN.

FORECASTER FRANKLIN
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      08/1500Z 15.2N 111.3W    30 KT
 12HR VT     09/0000Z 15.4N 112.6W    35 KT
 24HR VT     09/1200Z 15.9N 114.5W    40 KT
 36HR VT     10/0000Z 16.5N 116.5W    45 KT
 48HR VT     10/1200Z 17.0N 119.0W    55 KT
 72HR VT     11/1200Z 17.5N 123.5W    55 KT
 96HR VT     12/1200Z 18.0N 129.0W    45 KT
120HR VT     13/1200Z 18.5N 135.0W    35 KT
 
 
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