510
WTNT42 KNHC 120233
TCDAT2
TROPICAL STORM FRED DISCUSSION NUMBER 18
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL072009
1100 PM AST FRI SEP 11 2009
FRED CONTINUES TO PRODUCE DEEP CONVECTION...ALTHOUGH THE COLDEST
CLOUD TOPS ARE NOW DISPLACED MORE THAN A DEGREE NORTH OF THE
LOW-LEVEL CENTER. THIS DISPLACEMENT IS DUE TO ABOUT 20 KNOTS OF
SOUTHERLY SHEAR AS ANALYZED BY THE SHIPS MODEL AND UW-CIMSS. DVORAK
DATA T-NUMBERS AT 0000 UTC WERE 2.5 FROM SAB AND 3.0 FROM TAFB...
AND A QUIKSCAT PASS FROM AROUND 2100 UTC DID NOT SHOW ANY WINDS
HIGHER THAN ABOUT 45 KNOTS. THE INITIAL INTENSITY OF 50 KNOTS IS
BASED ON A BLEND OF THE DATA-T NUMBERS AND ASSUMES THAT RAIN WAS
ATTENUATING THE WIND SIGNAL SOMEWHAT IN THE QUIKSCAT DATA. WITH THE
WIND SHEAR EXPECTED TO CONTINUE...AND FRED MOVING OVER MARGINAL
SSTS AND INTO A MORE STABLE ENVIRONMENT...GRADUAL WEAKENING IS
EXPECTED THROUGH THE PERIOD. THE OFFICIAL FORECAST HAS BEEN
ADJUSTED DOWNWARD FOR THE FIRST 24 HOURS TO ACCOUNT FOR THE INITIAL
INTENSITY...AND IS SIMILAR TO THE PREVIOUS FORECAST AFTER THAT
TIME...IN BEST AGREEMENT WITH THE LGEM MODEL. IT IS POSSIBLE THAT
FRED COULD DISSIPATE BEFORE THE END OF THE 5-DAY PERIOD...AS
INDICATED BY THE SHIPS MODEL.
A 2016 UTC SSMI PASS...A 1941 UTC SSMIS PASS...AND THE QUIKSCAT DATA
ALL SUGGEST THAT THE LOW-LEVEL CENTER OF FRED IS LOCATED FARTHER
SOUTHEAST THAN PREVIOUSLY ESTIMATED. THE INITIAL MOTION ESTIMATE IS
VERY SLOW...060/02. A TURN TOWARD THE NORTH AND THE NORTHWEST WITH
A GRADUAL INCREASE IN FORWARD SPEED IS EXPECTED OVER THE NEXT
COUPLE OF DAYS AS THE DEEP-LAYER TROUGH OVER THE NORTH ATLANTIC
BEGINS TO LIFT OUT. BY 72 HOURS...LOW- TO MID-LEVEL RIDGING WILL
BUILD IN TO THE NORTH OF WHAT IS LEFT OF FRED...AND SHOULD IMPART A
FASTER MOTION TOWARD THE WEST-NORTHWEST THROUGH THE END OF THE
PERIOD. THE TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT ON THIS
SCENARIO...WITH THE EXCEPTION OF THE UKMET...WHICH IS A FAR
SOUTHERN OUTLIER. THE NEW OFFICIAL FORECAST IS ADJUSTED TO THE
RIGHT OF THE PREVIOUS TRACK DUE TO THE MORE EASTWARD INITIAL
POSITION...AND IS NEAR OR JUST TO THE RIGHT OF THE TVCN MODEL
CONSENSUS.
THE INITIAL 34-KT AND 50-KT WIND RADII WERE ADJUSTED BASED ON THE
QUIKSCAT DATA.
FORECAST POSITIONS AND MAX WINDS
INITIAL 12/0300Z 18.0N 33.6W 50 KT
12HR VT 12/1200Z 18.1N 33.5W 40 KT
24HR VT 13/0000Z 18.4N 33.6W 40 KT
36HR VT 13/1200Z 19.2N 34.4W 35 KT
48HR VT 14/0000Z 20.0N 35.9W 30 KT
72HR VT 15/0000Z 22.0N 40.5W 25 KT...REMNANT LOW
96HR VT 16/0000Z 23.5N 46.0W 20 KT...REMNANT LOW
120HR VT 17/0000Z 25.0N 51.5W 20 KT...REMNANT LOW
FORECASTER BRENNAN
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