[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 161429
TCDAT4
HURRICANE IRENE DISCUSSION NUMBER  48
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT TUE AUG 16 2005

IRENE IS MAINTAINING INTENSITY AT THIS TIME...AS AN EYE HAS BEEN
EVIDENT ON BOTH VISIBLE AND INFRARED IMAGES.  SSM/I IMAGERY ALSO
SHOWS AN EYE...THAT IS CIRCULAR AND 20 N MI IN DIAMETER WITH A
CLOSED WALL...ALBEIT A LITTLE WEAK OVER THE SOUTHERN SEM
THE INITIAL MOTION ESTIMATE IS 090/08 BASED ON THE LAST 6 HOURS OF
MICROWAVE SATELLITE POSITIONS. THE EASTWARD MOTION IS LIKELY DUE TO
A SHORTWAVE CAUGHT UP IN THE FASTER WESTERLIES WELL TO THE NORTH OF
IRENE THAT IS BYPASSING THE CYCLONE. THIS HAS RESULTED IN WEAKER
WESTERLY STEERING FLOW ON THE SOUTH SIDE OF AN EAST-WEST ORIENTED
JETSTREAM. HOWEVER...A MUCH STRONGER AND HIGHER AMPLITUDE SHORTWAVE
TROUGH OVER THE EASTERN GREAT LAKES AND OHIO VALLEY REGION IS
EXPECTED TO MOVE QUICKLY EASTWARD AND GRADUALLY LIFT OUT IRENE TO
THE NORTHEAST BY 36 HOURS...AND THEN RAPIDLY ACCELERATE THE CYCLONE
BY 48 HOURS. BETWEEN 36-48 HOURS...IRENE WILL BE MOVING OVER
SUB-24C SSTS...SO TRANSITION TO A POWERFUL EXTRATROPICAL LOW
PRESSURE SYSTEM IS EXPECTED. DUE TO THE MORE EASTWARD INITIAL
POSITION...THE OFFICIAL FORECAST TRACK IS A LITTLE TO THE RIGHT OF
THE PREVIOUS FORECAST...AND IS CLOSE TO THE NHC MODEL CONSENSUS.

THE INITIAL EASTWARD MOTION WILL KEEP IRENE OVER WARMER WATER A
LITTLE LONGER THAN PREVIOUSLY EXPECTED...BUT A GRADUAL INCREASE IN
THE WESTERLY VERTICAL SHEAR WILL LIKELY OVERCOME ANY POSITIVE
CONTRIBUTION FROM THE 28C SSTS. ONCE IRENE HITS THE ICY WATERS OF
THE NORTH ATLANTIC IN ABOUT 48 HOURS...ANY REMAINING TROPICAL
CHARACTERISTICS SHOULD QUICKLY DISAPPEAR. HOWEVER...WITH SUCH A
STRONG SHORTWAVE TROUGH AND JETSTREAM APPROACHING...THE INCREASING
BAROCLINIC ENERGY SHOULD KEEP IRENE A RATHER POTENT EXTRATROPICAL
LOW PRESSURE SYSTEM OVER THE FAR NORTH ATLANTIC.
 ICIRCLE.  DVORAK CLASSIFICATIONS GIVE A T-NUMBER OF 4.5 WHICH
SUPPORTS A CURRENT INTENSITY OF 75 KT.  EVEN THOUGH THE HURRICANE
LOOKS QUITE HEALTHY AT THE MOMENT...ONLY A SMALL NORTHWARD
COMPONENT OF MOTION WILL BRING THE SYSTEM INTO AN ENVIRONMENT OF
MUCH HIGHER SHEAR...GREATER THAN 30 KNOTS.  THE OFFICIAL INTENSITY
FORECAST...LIKE THE PREVIOUS ONE...SHOWS A WEAKENING TREND
COMMENCING WITHIN 12-24 HOURS AND THEN WEAKENS IRENE AT A SLIGHTLY
FASTER RATE THAN PREDICTED BY THE SHIPS GUIDANCE.  GLOBAL MODELS
INDICATE THAT IRENE WILL BEGIN TO INTERACT WITH A BAROCLINIC ZONE
IN ABOUT 36 HOURS.  AFTER EXTRACURRICULAR TRANSITION...THE SYSTEM
WILL LIKELY MERGE WITH OR BECOME ABSORBED BY ANOTHER EXTRATROPICAL
CYCLONE NEAR SOUTHERN GREENLAND.

INITIAL MOTION IS ESTIMATED TO BE 080/9.  THE HURRICANE HAS MOVED ON
A MORE EASTERLY TRACK THAN ANTICIPATED...IN PART DUE TO A LACK OF
MID-LEVEL RIDGING TO ITS SOUTHEAST...RESULTING IN A MORE ZONAL
STEERING CURRENT THAN EXPECTED.  HOWEVER AS A 500 MB TROUGH MOVES
INTO THE CANADIAN MARITIME DURING THE NEXT DAY OR SO...A GRADUAL
TURN TO THE LEFT...AHEAD OF THIS TROUGH...IS ANTICIPATED.  THE
OFFICIAL TRACK FORECAST IS IN GOOD AGREEMENT WITH THE LATEST
GFS...WHICH APPEARS TO HAVE A GOOD INITIALIZATION OF THE TROPICAL
CYCLONE.

FORECASTER PASCH


FORECAST POSITIONS AND MAX WINDS

INITIAL      16/1500Z 36.7N  62.4W    75 KT
 12HR VT     17/0000Z 37.2N  60.9W    70 KT
 24HR VT     17/1200Z 38.3N  58.8W    65 KT
 36HR VT     18/0000Z 41.0N  54.5W    55 KT...BECOMING EXTRATROPICAL
 48HR VT     18/1200Z 46.0N  48.0W    50 KT...EXTRATROPICAL
 72HR VT     19/1200Z 55.0N  36.0W    45 KT...EXTRATROPICAL
 96HR VT     20/1200Z...ABSORBED


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