[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 170832
TCDAT4
HURRICANE IRENE DISCUSSION NUMBER  51
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 AM EDT WED AUG 17 2005

THE LAST COUPLE OF SSMI AND TRMM OVERPASSES CLEARLY INDICATE THAT
IRENE HAS SLOWED ITS FORWARD SPEED...AND THAT MID- TO UPPER-LEVEL
WESTERLY SHEAR HAS ERODED THE EYE FEATURE AND HAS TILTED IT
EASTWARD OF THE LOW-LEVEL CENTER BY AT LEAST 30 NMI. SATELLITE
INTENSITY ESTIMATES HAVE ALSO COME DOWN...SO THE INTENSITY HAS BEEN
DECREASED TO 75 KT FOR THIS ADVISORY.

THE INITIAL MOTION ESTIMATE IS 080/7...EVEN THOUGH THE MOTION OVER
THE PAST 9 HOURS HAS BEEN 090/06.  A VIGOROUS SHORTWAVE TROUGH
MOVING QUICKLY EASTWARD OFF THE MID-ATLANTIC AND NORTHEASTERN U.S.
COASTS IS INDUCING WESTERLY MID- TO UPPER-LEVEL SHEAR ON THE
SYSTEM...WHILE THE SURFACE CIRCULATION IS LAGGING BEHIND IN WEAKER
LOW-LEVEL FLOW AHEAD OF A COLD FRONT AND A BROAD FRONTAL LOW.
HOWEVER...AS BOTH THE SHORTAVE TROUGH AND FRONTAL LOW APPROACH
IRENE FROM THE WEST...THE HURRICANE IS EXPECTED TO QUICKLY
ACCELERATE TO THE NORTHEAST IN RESPONSE TO THE INCREASING
SOUTHWESTERLY STEERING FLOW.  ONCE IRENE MOVES NORTH OF 41N
LATITUDE WHERE SSTS DROP OFF SHARPLY INTO THE 14-16C RANGE...THE
CYCLONE SHOULD QUICKLY DECOUPLE AND BECOME AN EXTRATROPICAL LOW.
THEREAFTER...EXTRATROPICAL IRENE SHOULD HOOK UP WITH THE
MID-LATITUDE WESTERLIES AND RACE OFF TO THE NORTHEAST TOWARD
GREENLAND AND ICELAND. THE OFFICIAL TRACK IS SIMILAR TO THE
PREVIOUS FORECAST AND IS JUST A LITTLE TO THE RIGHT OF CONSENSUS OF
THE NOW TIGHTLY PACKED NHC MODEL GUIDANCE.

ALTHOUGH THE VERTICAL SHEAR IS FORECAST TO GRADUALLY INCREASE...
IRENE WILL STILL BE OVER 27-28C SSTS FOR ANOTHER 24 HOURS OR SO.
THIS MAY ALLOW FOR BRIEF PERIODS OF DEEP CONVECTION TO FORM NEAR
THE CENTER AND KEEP THE SYSTEM GOING AS A HURRICANE. HOWEVER...ONCE
IRENE MOVES NORTH OF 41N...THE MUCH COLDER WATER AND STRONG
SOUTHWESTERLY SHEAR OF 30-40 KT SHOULD BRING ABOUT A QUICK DEMISE
TO THIS SYSTEM AS A TROPICAL CYCLONE.  AFTER 48 HOURS...IRENE IS
EXPECTED TO BECOME ABSORBED BY A LARGER EXTRATROPICAL LOW. THE
OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND
THE SHIPS INTENSITY MODEL.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      17/0900Z 36.6N  60.1W    75 KT
 12HR VT     17/1800Z 37.7N  57.6W    65 KT
 24HR VT     18/0600Z 41.0N  52.8W    60 KT
 36HR VT     18/1800Z 46.7N  46.5W    50 KT...EXTRATROPICAL
 48HR VT     19/0600Z 54.7N  39.2W    50 KT...EXTRATROPICAL
 72HR VT     20/0600Z...ABSORBED BY EXTRATROPICAL LOW

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