[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 250858
TCDAT4

HURRICANE IRENE DISCUSSION NUMBER  20
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL092011
500 AM EDT THU AUG 25 2011

A HURRICANE HUNTER AIRCRAFT INVESTIGATING IRENE A FEW HOURS AGO
FOUND THAT THE CENTRAL PRESSURE HAD DROPPED TO 950 MB.
HOWEVER...FLIGHT-LEVEL AND SFMR-OBSERVED SURFACE WIND DATA
SUPPORTED A CURRENT INTENSITY OF NO MORE THAN 100 KT...AND THIS IS
PROBABLY GENEROUS.  IT IS PRESUMED THAT THE INTENSIFICATION PROCESS
WAS HALTED BY AN EYEWALL REPLACEMENT EVENT AS SUGGESTED BY
MICROWAVE IMAGERY AND AIRCRAFT OBSERVATIONS.  SINCE THE ENVIRONMENT
APPEARS TO BE CONDUCIVE WITH WEAK SHEAR...WARM WATERS...AND AN
UPPER-LEVEL OUTFLOW ANTICYCLONE OVER THE HURRICANE...THE OFFICIAL
FORECAST SHOWS RE-STRENGTHENING WITHIN A DAY OR SO.  AFTER A COUPLE
OF DAYS...THE SHIPS GUIDANCE SHOWS A SIGNIFICANT INCREASE IN
VERTICAL SHEAR...SO A STEADY DECREASE IN INTENSITY IS LIKELY.
HOWEVER...SINCE IRENE HAS SUCH A LARGE AND INTENSE CIRCULATION...IT
WILL PROBABLY BE RATHER SLOW TO WEAKEN.  GIVEN THE LIMITATIONS IN
OUR ABILITY TO PREDICT INTENSITY CHANGE...THERE IS SIGNIFICANT
UNCERTAINTY AS TO JUST HOW STRONG IRENE WILL BE WHEN IT NEARS THE
EASTERN AND NORTHEASTERN U.S. COAST.

THE INITIAL MOTION CONTINUES TO BE ABOUT 315/10.  THERE HAS BEEN NO
CHANGE TO THE TRACK FORECAST REASONING.  IRENE SHOULD TURN
NORTHWARD AS IT MOVES THROUGH A BREAK IN THE SUBTROPICAL RIDGE
DURING THE NEXT 48 HOURS.  THEREAFTER...A SHORTWAVE TROUGH MOVING
THROUGH THE GREAT LAKES IS LIKELY TO TURN THE HURRICANE TOWARD THE
NORTH-NORTHEAST.  IN THE LATTER PART OF THE FORECAST
PERIOD...GLOBAL MODELS SHOW THE SHORTWAVE BYPASSING IRENE AND THE
HURRICANE BECOMING EMBEDDED WITHIN A BROAD AND SLOW-MOVING
MID-TROPOSPHERIC TROUGH OVER THE EASTERN UNITED STATES.  THIS
STEERING PATTERN SHOULD PREVENT IRENE FROM MOVING WELL OFFSHORE OF
THE NORTHEASTERN STATES.  THE OFFICIAL TRACK FORECAST HAS BEEN
SHIFTED VERY SLIGHTLY TO THE WEST OF THE PREVIOUS ONE AND IS
ROUGHLY IN THE MIDDLE OF THE GUIDANCE ENVELOPE...WHICH IS BRACKETED
BY THE U.K. MET. OFFICE MODEL ON THE EAST AND THE GFDL MODEL ON THE
WEST.

HURRICANE AND TROPICAL STORM WATCHES ARE NOW NECESSARY FOR A PORTION
OF THE SOUTHEAST AND CENTRAL U.S. EAST COAST.

FORECAST POSITIONS AND MAX WINDS

INIT  25/0900Z 24.6N  76.2W  100 KT 115 MPH
 12H  25/1800Z 26.1N  77.1W  110 KT 125 MPH
 24H  26/0600Z 28.2N  77.7W  115 KT 135 MPH
 36H  26/1800Z 30.4N  77.5W  110 KT 125 MPH
 48H  27/0600Z 32.3N  77.1W  100 KT 115 MPH
 72H  28/0600Z 36.3N  75.4W   90 KT 105 MPH
 96H  29/0600Z 42.5N  72.5W   60 KT  70 MPH...INLAND
120H  30/0600Z 51.0N  67.0W   45 KT  50 MPH...POST-TROP/EXTRATROP

$$
FORECASTER PASCH

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