Tropical Depression IRWIN

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TROPICAL DEPRESSION IRWIN DISCUSSION NUMBER  41
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 AM PDT SUN OCT 16 2011
 
A SMALL BURST OF COLD-TOPPED CONVECTION HAS REDEVELOPED OVER AND
SOUTH OF THE CIRCULATION CENTER DURING THE PAST 6 HOURS....WHICH IS
ENOUGH TO PRODUCE A DATA-T NUMBER OF T2.5/35 KT FROM TAFB USING THE
SHEAR PATTERN TECHNIQUE. A PARTIAL ASCAT OVERPASS AT 16/0416Z
CAUGHT THE WESTERN SEMICIRCLE OF IRWIN AND INDICATED A LARGE PATCH
OF 25-KT WINDS EXISTED AT THAT TIME. BASED ON THE ASCAT DATA AND
THE RECENT INCREASE IN DEEP CONVECTION...THE INITIAL INTENSITY IS
BEING HELD AT 30 KT FOR THIS ADVISORY.

THE INITIAL MOTION ESTIMATE IS 210/04 KT. IRWIN IS EXPECTED TO MOVE
SLOWLY WEST-SOUTHWESTWARD AND THEN WESTWARD OVER THE NEXT 48 HOURS
AROUND THE SOUTHERN PERIPHERY OF A LARGE SUBTROPICAL RIDGE THAT
EXTENDS EAST-WEST ACROSS NORTHERN MEXICO AND BAJA CALIFORNIA. AFTER
THAT TIME...A WEAKENED AND MORE VERTICALLY SHALLOW IRWIN IS
FORECAST TO BECOME NEARLY STATIONARY OR JUST LOOP AROUND IN THE
SAME GENERAL AREA AS THE LOW-LEVEL STEERING FLOW COLLAPSES. THE
OFFICIAL FORECAST TRACK IS JUST AN EXTENSION OF THE PREVIOUS
ADVISORY TRACK AND LIES CLOSE TO THE CONSENSUS MODEL...TVCN.
 
ALTHOUGH IRWIN IS NOW MOVING OVER WARMER WATER AND AWAY FROM THE
COLD WAKE LEFT BEHIND BY FORMER HURRICANE JOVA...WATER VAPOR
IMAGERY INDICATES STRONG NORTH TO NORTHEASTERLY UPPER-LEVEL WINDS
AND VERY DRY AIR READY TO AFFECT THE SYSTEM FAIRLY SOON. IN FACT...
THE SHIPS INTENSITY MODEL INDICATES THE VERTICAL SHEAR WILL
INCREASE TO MORE THAN 35 KT BY 36 HOURS...WHICH SHOULD INDUCE RAPID
WEAKENING DESPITE THE 29C SSTS THAT THE CYCLONE WILL BE TRAVERSING.
THE OFFICIAL INTENSITY FORECAST SHOWS IRWIN WEAKENING AND BECOMING
A NON-CONVECTIVE REMNANT LOW PRESSURE SYSTEM WITHIN 36-48 HOURS.
HOWEVER...THIS WEAKENING COULD OCCUR SOONER BASED ON THE ABUNDANCE
OF VERY DRY AIR THAT LURKS JUST TO THE NORTH OF THE CYCLONE...WHICH
SHOULD INHIBIT THE DEVELOPMENT OF PERSISTENT DEEP CONVECTION NEAR
THE CENTER.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  16/0900Z 14.3N 107.5W   30 KT  35 MPH
 12H  16/1800Z 14.0N 108.2W   25 KT  30 MPH
 24H  17/0600Z 13.9N 108.9W   25 KT  30 MPH
 36H  17/1800Z 14.0N 109.4W   25 KT  30 MPH
 48H  18/0600Z 13.8N 109.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  19/0600Z 13.5N 109.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  20/0600Z 13.0N 108.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  21/0600Z...DISSIPATED
 
$$
FORECASTER STEWART
 
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