Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  35
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 PM PDT FRI OCT 14 2011
 
THE CONVECTIVE ORGANIZATION OF IRWIN INCREASED AGAIN OVER THE PAST
FEW HOURS...WITH TOPS COOLING NEAR THE CENTER AND THE CONVECTIVE
PATTERN TAKING ON A MORE BANDED STRUCTURE. BASED ON A SHIP
OBSERVATION OF 37-KT WINDS AND THE 18Z TAFB DVORAK ESTIMATE...THE
INTENSITY IS RAISED TO 40 KT. IRWIN WILL BE MOVING THROUGH A
MARGINAL ENVIRONMENT FOR THE NEXT SEVERAL DAYS...CHARACTERIZED BY
DRY MID-LEVEL AIR...THE COOL WAKE LEFT FROM HURRICANE JOVA...AND
MODERATE TO STRONG VERTICAL SHEAR. DESPITE THIS...MOST OF THE
INTENSITY GUIDANCE CONTINUES TO INSIST ON GRADUAL STRENGTHENING
THROUGH THE PERIOD. GIVEN THE LESS THAN IDEAL ENVIRONMENT AND THE
SPORADIC NATURE OF THE CONVECTIVE ORGANIZATION DURING THE PAST
COUPLE OF DAYS...THE OFFICIAL FORECAST SHOWS NO CHANGE FOR THE
FIRST 72 HOURS. GRADUAL DECAY TO A REMNANT LOW BY DAY 5 IS EXPECTED
AS THE SHEAR INCREASES FURTHER AND THE GLOBAL MODELS SHOW THE
VORTEX WEAKENING OR DISSIPATING BY THAT TIME.
 
THE INITIAL MOTION ESTIMATE IS 180/4...AS IRWIN IS LOCATED TO THE
SOUTH OF A MID-LEVEL RIDGE BUILDING SOUTHEASTWARD OVER BAJA
CALIFORNIA AND WESTERN MEXICO. THE CYCLONE WILL GRADUALLY TURN
TOWARD THE WEST OVER THE NEXT COUPLE OF DAYS AS THE RIDGE TO THE
NORTH BUILDS WESTWARD AND TAKES ON A MORE EAST/WEST
ORIENTATION...AND THE TRACK GUIDANCE IS IN GOOD AGREEMENT ON THIS
SCENARIO. AFTER THAT TIME THE RIDGE BREAKS DOWN...WHICH SHOULD
RESULT IN A SLOW AND/OR ERRATIC MOTION...AS INDICATED BY THE LARGE
SPREAD IN THE GUIDANCE AT THAT TIME. THE NEW NHC TRACK FORECAST IS
TRENDED TOWARD THE NEW MULTI-MODEL CONSENSUS...AND IS A LITTLE
FASTER AND A BIT TO THE RIGHT OF THE PREVIOUS ONE THROUGH 36 HOURS.
AFTER THAT TIME THE NHC TRACK IS ESSENTIALLY AN UPDATE OF THE
PREVIOUS ONE...SHOWING LITTLE OR NO MOTION AT DAYS 3 THROUGH 5 DUE
TO THE LARGE UNCERTAINTY AT THIS TIME RANGE.
 

FORECAST POSITIONS AND MAX WINDS
 
INIT  14/2100Z 18.0N 106.9W   40 KT  45 MPH
 12H  15/0600Z 17.2N 107.1W   40 KT  45 MPH
 24H  15/1800Z 16.0N 107.4W   40 KT  45 MPH
 36H  16/0600Z 15.1N 107.9W   40 KT  45 MPH
 48H  16/1800Z 14.7N 108.9W   40 KT  45 MPH
 72H  17/1800Z 15.0N 109.8W   35 KT  40 MPH
 96H  18/1800Z 15.0N 110.0W   30 KT  35 MPH
120H  19/1800Z 15.0N 110.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER BRENNAN
 
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