[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 080854
TCDAT4
TROPICAL STORM IRENE DISCUSSION NUMBER  15
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 AM EDT MON AUG 08 2005

A NEW BURST OF CONVECTION HAS DEVELOPED JUST EAST OF THE CENTER OF
IRENE DURING THE PAST 6 HR...ALTHOUGH THE BURST IS LESS IMPRESSIVE
THAT THE ONE AT THIS TIME LAST NIGHT.  SATELLITE INTENSITY ESTIMATES
ARE 30 KT FROM ALL AGENCIES...AND BASED ON THIS IRENE MAY HAVE
WEAKENED TO A DEPRESSION.  HOWEVER...ANY DOWNGRADE CAN WAIT UNTIL
AFTER DATA FROM THE NEXT QUIKSCAT OVERPASS ARRIVES.  THE INITIAL
INTENSITY REMAINS 35 KT FOR THIS PACKAGE.

THE 12-HR INITIAL MOTION IS 295/12...AND THE SHORT-TERM MOTION MAY
BE A LITTLE LEFT OF THAT AND FASTER.  IRENE REMAINS SOUTH OF A
LOW/MID-LEVEL RIDGE...WITH LARGE-MODELS FORECASTING THIS TO PERSIST
FOR 48-72 HR.  THIS SHOULD KEEP IRENE ON A GENERAL WEST-NORTHWEST
HEADING FOR THAT TIME.  BEYOND 72 HR...MANY MODELS FORECAST IRENE
TO TURN MORE NORTHWARD INTO A WEAKNESS IN THE RIDGE CAUSED BY A
FRONTAL SYSTEM MOVING OFF THE U. S. NORTHEAST COAST.  THE DYNAMICAL
MODELS AGREE ON THIS SCENARIO...WITH THE UKMET AND NOGAPS TURNING
IRENE NORTHEASTWARD AFTER 96 HR...THE NOGAPS DOING SO IN SPITE OF A
SIGNIFICANT WESTWARD SHIFT SINCE ITS LAST RUN.  ON THE OTHER
HAND...THE BAM MODELS DO NOT SHOW A NORTHWARD TURN...CONTINUING
IRENE WEST-NORTHWESTWARD THROUGH 120 HR.  THE OFFICIAL FORECAST
TRACK CONTINUES TO SHOW A NORTHERLY TURN AFTER 72 HR...ALTHOUGH THE
ENTIRE TRACK HAS BEEN SHIFTED SOMEWHAT WESTWARD FROM 6 HR AGO.
HOWEVER...THE TRACK IS NOT AS FAR WEST AS THE NOGAPS AND UKMET ARE
THROUGH 96 HR...AND IT MAY HAVE TO BE SHIFTED FARTHER WESTWARD
AFTER THE NEXT ROUND OF MODEL RUNS.

IRENE IS IN AN ENVIRONMENT OF MODERATE NORTHWESTERLY VERTICAL
SHEAR...AND THIS WILL LIKELY CONTINUE FOR 36-48 HR.  THIS SHOULD
LIMIT STRENGTHENING EVEN THOUGH THE STORM WILL BE MOVING OVER
WARMER SEA SURFACE TEMPERATURES.  AFTER THAT TIME...THE LARGE-SCALE
MODELS ARE IN POOR AGREEMENT ON THE EVOLUTION OF THE UPPER-LEVEL
WINDS NEAR IRENE.  THE NOGAPS BUILDS A LARGE ANTICYCLONE OVER THE
STORM...WHILE THE GFS...UKMET...AND CANADIAN BUILD A MORE MODEST
RIDGE.  HOW MUCH OF THIS DEVELOPMENT IS DUE TO THE MODEL OUTFLOW
FROM IRENE IS UNKNOWN...BUT IT IS POSSIBLE THAT THE MODELS MAY BE
OVERDOING THIS MORE FAVORABLE ENVIRONMENT.  THE INTENSITY FORECAST
IS BASED ON A GRADUAL DECREASE IN SHEAR THAT WILL ALLOW SOME
STRENGTHENING THROUGH 72 HR.  AFTER THAT...THE FORECAST TRACK TAKES
IRENE OVER COOLER SSTS...WHICH SHOULD LIMIT STRENGTHENING.  THE
INTENSITY FORECAST IS BASED ON THE SHIPS MODEL.  IT SHOULD BE NOTED
THAT THE GFDL IS AGAIN DISSIPATING IRENE IN LESS THAN 48 HR.

FORECASTER BEVEN


FORECAST POSITIONS AND MAX WINDS

INITIAL      08/0900Z 21.9N  48.9W    35 KT
 12HR VT     08/1800Z 22.6N  50.5W    35 KT
 24HR VT     09/0600Z 23.4N  52.4W    35 KT
 36HR VT     09/1800Z 24.2N  54.1W    40 KT
 48HR VT     10/0600Z 25.1N  55.9W    45 KT
 72HR VT     11/0600Z 27.0N  58.5W    50 KT
 96HR VT     12/0600Z 30.0N  60.5W    50 KT
120HR VT     13/0600Z 33.0N  60.0W    50 KT


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