[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 241455
TCDAT1

TROPICAL STORM OPHELIA DISCUSSION NUMBER  15
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL162011
1100 AM AST SAT SEP 24 2011

DURING THE PAST 6 HOURS...DEEP CONVECTION HAS REDEVELOPED OVER AND
TO THE NORTHEAST OF THE PREVIOUSLY EXPOSED LOW-LEVEL CIRCULATION
CENTER. SATELLITE CURRENT INTENSITY ESTIMATES ARE T3.0/45 KT FROM
TAFB AND T3.1/47 KT FROM UW-CIMSS ADT. THEREFORE...THE INITIAL
INTENSITY WILL REMAIN AT 45 KT FOR THIS ADVISORY.

THE INITIAL MOTION ESTIMATE IS 310/11 KT. THERE IS NO SIGNIFICANT
CHANGE TO THE PREVIOUS FORECAST TRACK REASONING. A FAIRLY LARGE
SPREAD REMAINS IN THE MODEL GUIDANCE WITH THE GFS AND GFS ENSEMBLE
MODELS SHOWING A MORE WESTWARD MOTION DUE TO A WEAKER CYCLONE
EXPECTED...WHEREAS THE GFDL AND HWRF MODELS SHOW A MORE NORTHERLY
TRACK DUE TO A MORE ROBUST AND STRONGER SYSTEM. THE ECMWF...
UKMET...AND NOGAPS MODELS ARE IN BETWEEN THESE TWO EXTREMES AND ARE
FAIRLY SIMILAR TO EACH OTHER IN BOTH TRACK AND INTENSITY. AS A
RESULT...THE OFFICIAL FORECAST TRACK IS VERY NEAR THE PREVIOUS
ADVISORY TRACK...AND LIES DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE
AND TO THE LEFT OF THE CONSENSUS MODEL..TVCN...AFTER 72 HOURS.

SIMILAR TO THIS TIME YESTERDAY...VERY COLD-TOPPED CONVECTION HAS
REDEVELOPED IN THE FORM OF A CURVED BAND IN THE NORTHEAST QUADRANT.
GIVEN THAT OPHELIA IS EXPECTED TO REMAIN A LARGER THAN AVERAGE
TROPICAL CYCLONE...AND THE FACT THAT IT HAS ALREADY SURVIVED
SOUTHWESTERLY VERTICAL SHEAR OF NEAR 30 KT...I SEE NO REASON WHY
THIS CYCLONE SHOULD DISSIPATE DURING THE FORECAST PERIOD LIKE SOME
OF THE GUIDANCE IS INDICATING. THE CURRENT SHEAR OF 25 KT IS
EXPECTED TO CONTINUE FOR THE NEXT 48 HOURS OR SO...WITH A GRADUAL
INCREASE TO 30 KT AFTER THAT. HOWEVER...AS THE PREVIOUS DISCUSSION
ALLUDED TO...THE GLOBAL MODELS ARE STILL SUGGESTING THAT AN
UPPER-LEVEL TROUGH NORTH OF THE LESSER ANTILLES COULD FRACTURE AND
ALLOW RESTRENGTHENING OF THE TROPICAL CYCLONE AFTER 72 HOURS AS THE
OPHELIA MOVES INTO THIS REGION OF UPPER-LEVEL DIVERGENCE. THIS
SCENARIO IS MAINTAINED IN THE OFFICIAL INTENSITY FORECAST...WHICH
SHOWS MODEST STRENGTHENING AT DAYS 4 AND 5 WHILE THE CYCLONE IS
STILL OVER SSTS OF NEAR 28C. THE INTENSITY FORECAST LEANS TOWARD
THE INTENSITY CONSENSUS MODEL...ICON...THROUGH 72 HOURS...AND IS A
LITTLE LOWER THAN THE CONSENSUS AFTER THAT.

FORECAST POSITIONS AND MAX WINDS

INIT  24/1500Z 17.8N  56.0W   45 KT  50 MPH
 12H  25/0000Z 18.8N  57.4W   40 KT  45 MPH
 24H  25/1200Z 19.9N  59.1W   40 KT  45 MPH
 36H  26/0000Z 20.9N  60.7W   35 KT  40 MPH
 48H  26/1200Z 21.6N  61.8W   35 KT  40 MPH
 72H  27/1200Z 23.3N  63.1W   35 KT  40 MPH
 96H  28/1200Z 25.3N  63.4W   40 KT  45 MPH
120H  29/1200Z 27.3N  63.4W   50 KT  60 MPH

$$
FORECASTER STEWART



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