Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  34
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 AM PDT FRI OCT 14 2011

CONVENTIONAL SATELLITE IMAGERY INDICATES THAT IRWIN IS CHARACTERIZED
BY A SHAPELESS BLOB OF CONVECTION. CLOUDS TOPS ASSOCIATED WITH
EARLIER BURSTS OF DEEP CONVECTION JUST TO THE SOUTHWEST OF THE
CENTER HAVE WARMED AND RECENT AMSU IMAGES SHOWED JUST A FEW
FRAGMENTED REMAINING BANDS. DVORAK SATELLITE INTENSITY ESTIMATES
ARE 35 KT FROM TAFB AND SAB...AND THE UW-CIMSS SATCON ESTIMATE IS
33 KT. BASED ON THESE...THE INITIAL WIND SPEED IS REDUCED TO 35 KT.
THE DYNAMICAL AND STATISTICAL INTENSITY MODELS SUGGEST LITTLE
CHANGE THROUGH THE FIRST 48 HOURS...BUT THE OFFICIAL FORECAST WILL
SHOW THE POSSIBILITY OF SOME STRENGTHENING IN THE VERY SHORT TERM.
AFTER DAY 2...THE MODELS DIVERGE...EITHER SHOWING A WEAKENING TREND
OR SOME STRENGTHENING. THE FORMER IS MORE REALISTIC BASED ON THE
EXPECTED INCREASE IN NORTHEASTERLY SHEAR...AFFECTS FROM THE COOLER
SST WAKE OF JOVA...AND THE POTENTIAL FOR DRY AND STABLE MID- TO
UPPER-LEVEL AIR TO POUR INTO THE REGION FROM NORTHERN MEXICO.
THEREFORE...THE OFFICIAL FORECAST CALLS FOR SLOW WEAKENING AFTER 48
HOURS WITH IRWIN BECOMING A REMNANT LOW AT DAY 5.
 
THE INITIAL MOTION ESTIMATE IS 180/3...WITHIN THE LOW- TO MID-LEVEL
FLOW PRODUCED BY A RIDGE OVER NORTHWESTERN MEXICO AND BAJA
CALIFORNIA. THE GENERALLY SOUTHWARD MOTION OF THE CYCLONE IS
FORECAST TO CONTINUE DURING THE NEXT 36-48 HR...WITH AN INCREASE IN
FORWARD SPEED...AS THE MID-LEVEL RIDGE TEMPORARILY BUILDS TO THE
NORTH. THROUGH THE REMAINDER OF THE PERIOD...IRWIN IS EXPECTED TO
BECOME A RELATIVELY WEAK AND SHALLOW SYSTEM AND DRIFT GENERALLY
WESTWARD WITHIN THE LOW-LEVEL FLOW AS IT BECOMES MORE INVOLVED WITH
THE ITCZ. THE NHC FORECAST HAS CHANGED LITTLE FROM THE PREVIOUS
ADVISORY AND IS BASED ON THE HFIP MODEL CONSENSUS... WHICH INCLUDES
THE GLOBAL MODELS. 

 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/1500Z 18.6N 106.8W   35 KT  40 MPH
 12H  15/0000Z 17.9N 106.8W   40 KT  45 MPH
 24H  15/1200Z 16.6N 107.0W   35 KT  40 MPH
 36H  16/0000Z 15.3N 107.4W   35 KT  40 MPH
 48H  16/1200Z 14.8N 108.0W   35 KT  40 MPH
 72H  17/1200Z 15.1N 109.2W   30 KT  35 MPH
 96H  18/1200Z 15.5N 110.0W   30 KT  35 MPH
120H  19/1200Z 15.5N 110.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER ROBERTS/BRENNAN
 
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