000
WTPZ41 KNHC 131438
TCDEP1
 
TROPICAL STORM IRWIN DISCUSSION NUMBER  30
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 AM PDT THU OCT 13 2011

DEEP CONVECTION HAS MADE A COMEBACK NEAR THE CENTER OF IRWIN OVER
THE PAST FEW HOURS...WITH A CONVECTIVE BAND ALSO FORMING NORTHWEST
OF THE CENTER. BASED ON THE 1200 UTC TAFB DVORAK ESTIMATE...THE
SYSTEM IS UPGRADED TO A TROPICAL STORM WITH AN INTENSITY OF 35 KT.
THE SHIPS MODEL ANALYSES SHOW THAT THE VERTICAL SHEAR HAS GRADUALLY
DECREASED OVER IRWIN IN THE LAST 12 HOURS...WHICH MAY HAVE ALLOWED
THE RECENT INCREASE IN CONVECTION. THE INTENSITY GUIDANCE SHOWS
LITTLE CHANGE IN THE FIRST 36 HOURS AS THE CYCLONE MOVES OVER WARM
WATERS BUT REMAINS IN A RELATIVELY DRY MID-LEVEL ENVIRONMENT...AND
THE NHC FORECAST FOLLOWS THAT THINKING. THE GFDL...LGEM...AND SHIPS
MODELS THEN SHOW IRWIN STRENGTHENING TO STRONG TROPICAL STORM
STATUS AS IT REMAINS OVER WARM WATERS IN LIGHT TO MODERATE EASTERLY
TO SOUTHEASTERLY SHEAR. HOWEVER...MANY OF THE GLOBAL MODELS SHOW
THE CIRCULATION WEAKENING OR DISSIPATING BY THE END OF THE PERIOD.
GIVEN THIS UNCERTAINTY...THE NHC FORECAST SHOWS ONLY A SMALL
INCREASE IN INTENSITY AT 48 HOURS...WITH NO CHANGE AFTER THAT TIME.

IRWIN HAS SLOWED AND TURNED A LITTLE TO THE RIGHT...WITH AN
ESTIMATED INITIAL MOTION OF 060/8. ALL OF THE GUIDANCE SHOWS IRWIN
MAKING AN ABRUPT ANTICYCLONIC LOOP IN THE NEXT 72 HOURS...AS A
MID-LEVEL RIDGE BUILDS NORTH OF THE CYCLONE. THE GUIDANCE SUITE HAS
TRENDED TOWARD A SHARPER LOOP DURING THE FIRST 72 HOURS...AND
TOWARD A MORE SOUTHERLY TRACK AT DAYS 4 AND 5. THE OFFICIAL
FORECAST HAS BEEN ADJUSTED TOWARD THE NEW MULTI-MODEL CONSENSUS...
AND LIES TO THE RIGHT OF THE PREVIOUS PACKAGE THROUGH 48 HOURS AND
TO THE LEFT OF IT AT DAYS 4 AND 5.

GIVEN THE SMALL AREA OF TROPICAL STORM FORCE WINDS AND THE EXPECTED
SHARP TURN TOWARD THE SOUTH OVER THE NEXT COUPLE OF DAYS...TROPICAL
STORM WATCHES OR WARNINGS ARE NOT REQUIRED FOR THE COAST OF MEXICO
AT THIS TIME. 


FORECAST POSITIONS AND MAX WINDS
 
INIT  13/1500Z 18.7N 107.2W   35 KT  40 MPH
 12H  14/0000Z 18.7N 106.3W   35 KT  40 MPH
 24H  14/1200Z 18.2N 105.7W   35 KT  40 MPH
 36H  15/0000Z 17.2N 105.3W   35 KT  40 MPH
 48H  15/1200Z 16.2N 105.6W   40 KT  45 MPH
 72H  16/1200Z 15.0N 107.0W   40 KT  45 MPH
 96H  17/1200Z 14.5N 108.5W   40 KT  45 MPH
120H  18/1200Z 15.0N 110.0W   40 KT  45 MPH
 
$$
FORECASTER BRENNAN