Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  39
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 PM PDT SAT OCT 15 2011
 
THE LOW-LEVEL CENTER OF IRWIN BECAME EXPOSED TO THE NORTHEAST OF THE
DEEP CONVECTION AROUND THE RELEASE OF THE PREVIOUS ADVISORY...AND
IT HAS BECOME INCREASINGLY SEPARATED SINCE THAT TIME. THIS
ASYMMETRIC CLOUD PATTERN IS DUE TO ABOUT 20 KT OF NORTHEASTERLY
VERTICAL WIND SHEAR. THE INITIAL WIND SPEED IS HELD AT 35 KT...IN
AGREEMENT WITH THE SATELLITE CLASSIFICATION FROM TAFB...BUT THIS
COULD BE GENEROUS. IRWIN LIES OVER THE COOL WATER WAKE OF
JOVA...AND SATELLITE DATA INDICATE THAT THE CENTER OF IRWIN IS
CURRENTLY OVER WATERS OF ABOUT 23C. THESE COOL WATERS...STABLE
AIR...AND AN INCREASE IN EASTERLY WIND SHEAR SHOULD CAUSE IRWIN TO
WEAKEN SOME AND EVENTUALLY DEGENERATE TO A REMNANT LOW IN TWO TO
THREE DAYS. THE OFFICIAL INTENSITY FORECAST IS UNCHANGED FROM
EARLIER AND IS ON THE LOW SIDE OF THE GUIDANCE ENVELOPE...IN
AGREEMENT WITH THE GFDL AND HWRF MODELS.
 
IRWIN IS MOVING SOUTH-SOUTHWESTWARD AT ABOUT 8 KT. A TURN TO THE
WEST IS FORECAST DURING THE NEXT 24 TO 36 HOURS AS A LOW- TO
MID-LEVEL RIDGE BECOMES ORIENTED EAST-TO-WEST TO THE NORTH OF THE
TROPICAL CYCLONE. IN 48 TO 72 HOURS...THE MODELS SUGGEST THAT
IRWIN...OR ITS REMNANTS...COULD COME TO A HALT...PERHAPS REVERSING
EASTWARD OR SOUTHEASTWARD AS IT BECOMES SHALLOW IN NATURE AND IS
STEERED BY THE LOW-LEVEL FLOW. THE OFFICIAL FORECAST HAS BEEN
ADJUSTED SOUTHWARD AND IS A LITTLE SLOWER THAN THE PREVIOUS ONE TO
COME INTO BETTER AGREEMENT WITH THE LATEST GUIDANCE.

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  15/2100Z 15.6N 107.1W   35 KT  40 MPH
 12H  16/0600Z 14.8N 107.7W   30 KT  35 MPH
 24H  16/1800Z 14.3N 108.4W   30 KT  35 MPH
 36H  17/0600Z 14.2N 109.4W   30 KT  35 MPH
 48H  17/1800Z 14.1N 109.9W   30 KT  35 MPH
 72H  18/1800Z 14.0N 110.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  19/1800Z 14.0N 110.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  20/1800Z...DISSIPATED
 
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FORECASTER CANGIALOSI
 
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