[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 242040
TCDAT1

TROPICAL STORM OPHELIA DISCUSSION NUMBER  16
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL162011
500 PM AST SAT SEP 24 2011

DURING THE PAST 6 HOURS...CONVECTION HAS WAXED AND WANED...AND THE
INNER CORE WIND FIELD HAS BECOME RATHER AMORPHOUS. BOTH NOAA AND
AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT HAVE HAD GREAT DIFFICULTY
IN TRYING TO IDENTIFY A WELL-DEFINED CIRCULATION CENTER. IN FACT...
THE PREVIOUS ALLEGED CENTER THAT WE HAVE BEEN TRACKING FOR THE PAST
24 HOURS NO LONGER EXISTS BASED ON RECON DATA...AND THE LONE VORTEX
REPORTED APPEARS TO HAVE DEGENERATED AS WELL. THE INITIAL POSITION
IS THE MEAN POSITION OF ALL OF THE PREVIOUS AND NEWLY-FORMED SMALL
SWIRLS NOTED IN SATELLITE IMAGERY AND RECON DATA. THE INITIAL
INTENSITY OF 40 KT IS BASED ON AN EARLIER NOAA DROPSONDE REPORT OF
40 KT IN THE NORTHEAST QUADRANT.

THE INITIAL MOTION ESTIMATE IS 295/10 KT BASED ON A 24-HOUR TREND.
OTHER THAN THE UNCERTAINTY IN THE EXACT LOCATION OF OPHELIA...
THERE IS NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK
REASONING. A LARGE SPREAD REMAINS IN THE NHC MODEL GUIDANCE WITH
THE GFS AND GFS ENSEMBLE MODELS ALONG THE SOUTHERN EDGE OF THE
GUIDANCE ENVELOP INDICATING MORE WESTWARD MOTION...WHEREAS THE GFDL
AND HWRF MODELS ARE ON THE FAR RIGHT SIDE INDICATING A MORE
NORTHERLY TRACK. THE REMAINDER OF THE GUIDANCE IS SPREAD OUT EVENLY
BETWEEN THESE TWO EXTREMES. THE OPHELIA CIRCULATION IS EXPECTED TO
MOVE NORTHWESTWARD AND EVENTUALLY NORTHWARD AROUND THE SOUTHWESTERN
PERIPHERY OF A LARGE SUBTROPICAL RIDGE THAT IS FORECAST TO WEAKEN
AND SHIFT SLOWLY EASTWARD THROUGHOUT THE FORECAST PERIOD. THE
OFFICIAL FORECAST TRACK IS LEFT OF THE PREVIOUS ADVISORY...AND LIES
DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE CLOSE TO THE CONSENSUS
MODEL..TVCN.

ALTHOUGH THE CONVECTIVE PATTERN IS SOMEWHAT RAGGED IN OVERALL
APPEARANCE...NEW CONVECTION HAS BEEN DEVELOPING IN THE INNER CORE
REGION NEAR THE ALLEGED LOW-LEVEL CENTER...WHICH MIGHT PRODUCE SOME
SHORT TERM STRENGTHENING. IN THE LONGER TERM...HOWEVER...VERTICAL
SHEAR IS EXPECTED TO GRADUALLY INCREASE FROM THE SOUTHWEST THROUGH
THE NEXT 72 HOURS...WHICH SHOULD INDUCE GRADUAL WEAKENING. SINCE
OPHELIA HAS BEEN ABLE TO MAINTAIN A FAIRLY LARGE CIRCULATION...ANY
WEAKENING SHOULD BE LESS THAN WHAT WOULD USUALLY OCCUR FOR A MORE
TYPICAL TROPICAL CYCLONE. AFTER THAT TIME...SOME OF THE GLOBAL
MODELS...LIKE THE ECMWF...ARE STILL FORECASTING AN UPPER-LEVEL
TROUGH NORTH OF THE LESSER ANTILLES TO FRACTURE...WHICH ALLOWS A
SMALL ANTICYCLONE TO DEVELOP ABOVE OPHELIA. THIS SCENARIO IS
EXPECTED TO RESULT IN SOME RESTRENGTHENING OF THE CYCLONE AT 96 AND
120 HOURS WHILE THE OPHELIA IS STILL OVER SSTS OF 28C. THE INTENSITY
FORECAST LEANS TOWARD THE INTENSITY CONSENSUS MODEL ICON.

FORECAST POSITIONS AND MAX WINDS

INIT  24/2100Z 17.9N  57.4W   40 KT  45 MPH
 12H  25/0600Z 18.7N  58.8W   40 KT  45 MPH
 24H  25/1800Z 19.6N  60.4W   40 KT  45 MPH
 36H  26/0600Z 20.5N  61.6W   35 KT  40 MPH
 48H  26/1800Z 21.4N  62.7W   35 KT  40 MPH
 72H  27/1800Z 23.0N  64.0W   35 KT  40 MPH
 96H  28/1800Z 24.5N  64.5W   40 KT  45 MPH
120H  29/1800Z 27.0N  64.5W   50 KT  60 MPH

$$
FORECASTER STEWART



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