[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 220240
TCDAT1

TROPICAL STORM OPHELIA DISCUSSION NUMBER   5
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL162011
1100 PM AST WED SEP 21 2011

INFRARED AND MICROWAVE SATELLITE IMAGERY THIS EVENING SHOWS THAT
OPHELIA CONTINUES TO BE AFFECTED BY SOUTHWESTERLY VERTICAL WIND
SHEAR...WITH THE LOW-LEVEL CENTER REMAINING NEAR THE SOUTHWESTERN
EDGE OF THE MAIN CONVECTIVE MASS.  SATELLITE INTENSITY ESTIMATES
FROM TAFB AND SAB ARE 45 KT...AND NOAA BUOY 41041 RECENTLY REPORTED
1-MINUTE MEAN WINDS OF 45 KT.  BASED ON THE BUOY AND THE LIKELIHOOD
THAT IT DID NOT SAMPLE THE MAXIMUM WIND...THE INITIAL INTENSITY
REMAINS 50 KT.

THE INITIAL MOTION IS 280/13.  THERE IS LITTLE CHANGE TO EITHER THE
TRACK FORECAST PHILOSOPHY OR THE FORECAST TRACK SINCE THE LAST
ADVISORY.  A LOW/MID-LEVEL RIDGE TO THE NORTH OF OPHELIA SHOULD
STEER THE CYCLONE WESTWARD TO WEST-NORTHWESTWARD FOR THE NEXT 2-3
DAYS.  AFTER THAT TIME...THE STORM SHOULD APPROACH THE WESTERN
PERIPHERY OF THE RIDGE AS A DEEP-LAYER TROUGH MOVES EASTWARD INTO
THE EASTERN UNITED STATES.  THIS SHOULD ALLOW OPHELIA TO TURN
TOWARD THE NORTHWEST WITH SOME REDUCTION IN FORWARD SPEED.  THE NEW
FORECAST TRACK IS AN UPDATE OF THE PREVIOUS TRACK AND LIES IN THE
CENTER OF THE TRACK GUIDANCE ENVELOPE.

ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN INDICATE THAT
OPHELIA IS EXPERIENCING 20-25 KT OF SHEAR DUE IN PART TO AN
UPPER-LEVEL TROUGH TO THE NORTHWEST OF THE STORM.  ALL OF THE
DYNAMICAL MODELS INDICATE THAT MODERATE TO STRONG SHEAR IS LIKELY
TO CONTINUE FOR THE NEXT 72 HOURS...AND IN RESPONSE MOST GUIDANCE
SHOWS LITTLE CHANGE IN STRENGTH OR WEAKENING DURING THAT TIME.  THE
INTENSITY FORECAST IS MORE UNCERTAIN AFTER 72 HR.  THE UPPER-LEVEL
TROUGH IS FORECAST TO SLOW ITS WESTWARD MOTION AS IT MOVES NORTH OF
HISPANIOLA.  THIS COULD ALLOW OPHELIA TO CATCH UP WITH THE TROUGH
AND ENCOUNTER STRONGER SHEAR...OR TURN NORTHWARD EAST OF THE TROUGH
AND ENCOUNTER LIGHTER SHEAR.  THE GFS TENDS TOWARD THE LATTER
SOLUTION...WHILE THE GFDL MAKE OPHELIA A HURRICANE AFTER 96 HR.  ON
THE OTHER HAND...THE ECMWF SHOWS VERY STRONG UPPER-LEVEL WINDS NEAR
THE STORM FROM 96-120 HR.  DUE TO THE UNCERTAINTY...THERE IS NO
CHANGE TO THE PREVIOUS INTENSITY FORECAST.

FORECAST POSITIONS AND MAX WINDS

INIT  22/0300Z 13.7N  46.2W   50 KT  60 MPH
 12H  22/1200Z 14.0N  48.1W   50 KT  60 MPH
 24H  23/0000Z 14.3N  50.9W   50 KT  60 MPH
 36H  23/1200Z 15.2N  53.4W   50 KT  60 MPH
 48H  24/0000Z 16.3N  55.8W   50 KT  60 MPH
 72H  25/0000Z 18.5N  60.0W   45 KT  50 MPH
 96H  26/0000Z 21.5N  64.0W   45 KT  50 MPH
120H  27/0000Z 23.5N  66.5W   45 KT  50 MPH

$$
FORECASTER BEVEN



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