ZCZC MIATCDEP1 ALL TTAA00 KNHC DDHHMM TROPICAL STORM IRWIN DISCUSSION NUMBER 12 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 800 PM PDT SAT OCT 08 2011 THERE HAS BEEN NO SIGNIFICANT CONVECTION NEAR THE CENTER OF IRWIN FOR THE PAST 12 HOURS AS EASTERLY VERTICAL WIND SHEAR CONTINUES TO AFFECT THE CYCLONE. THE RATHER UNCERTAIN INITIAL INTENSITY OF 50 KT IS BASED ON A BLEND OF SATELLITE INTENSITY ESTIMATES FROM SAB AND TAFB...AND IT IS POSSIBLE THIS COULD BE GENEROUS. THE INITIAL MOTION IS NOW 050/3. WATER VAPOR IMAGERY SHOWS A LARGE MID/UPPER-LEVEL TROUGH OVER THE WESTERN UNITED STATES AND NORTHWESTERN MEXICO. THE DYNAMICAL MODELS FORECAST THIS TROUGH TO MOVE NORTHEASTWARD DURING THE NEXT 72 HOURS AS A SECOND TROUGH MOVES SOUTHEASTWARD INTO NORTHWESTERN MEXICO. THIS TROUGH IS FORECAST TO BECOME A CUT-OFF LOW OVER THE PACIFIC WEST OF MEXICO BY 120 HOURS. THIS EVOLUTION IS FORECAST TO STEER IRWIN EAST- NORTHEASTWARD TO EASTWARD DURING THE NEXT 72 HOURS. AFTER THAT TIME...THERE IS SIGNIFICANT SPREAD IN THE MODEL GUIDANCE. THE GFS SHOW IRWIN RESPONDING STRONGLY TO THE NEW TROUGH AND TURNING NORTHWARD SOUTH OF BAJA CALIFORNIA. THE GFDL SHOWS A CONTINUED EASTWARD MOTION...WITH THE REMAINDER OF THE DYNAMICAL MODELS SPREAD BETWEEN THESE EXTREMES. OVERALL...THE GUIDANCE IS A LITTLE FASTER AND A LITTLE FARTHER TO THE EAST THAN ON THE PREVIOUS ADVISORY. THE NEW FORECAST TRACK CALLS FOR A GRADUAL TURN TOWARD THE NORTH-NORTHEAST AFTER 72 HOURS IN BEST AGREEMENT WITH THE CANADIAN MODEL. THE NEW TRACK LIES TO THE LEFT OF THE CONSENSUS MODEL. THE INTENSITY FORECAST IS BASED ON THE PREMISE THAT CONVECTION RE-DEVELOPS NEAR THE CENTER OF IRWIN AND THAT THE CYCLONE DOES NOT DECAY TO A REMNANT LOW. THE LATEST DYNAMICAL MODEL RUNS SUGGEST THAT MODERATE EASTERLY SHEAR MAY CONTINUE FOR AS LONG AS 72 HOURS BEFORE CONDITIONS BECOME MORE FAVORABLE FOR STRENGTHENING. INDEED...THE GFDL AND HWRF MODELS SHOW LITTLE CHANGE IN STRENGTH FOR 72 HOURS OR SO...FOLLOWED BY INTENSIFICATION. THE INTENSITY FORECAST IS SIGNIFICANTLY CHANGED FROM THAT OF THE PREVIOUS ADVISORY...CALLING FOR IRWIN TO WEAKEN A LITTLE MORE BEFORE MAINTAINING AN INTENSITY NEAR 45 KT THROUGH 72 HOURS. AFTER THAT TIME...WARMER SEA SURFACE TEMPERATURES AND DECREASED SHEAR MAY ALLOW SOME INTENSIFICATION. THE NEW INTENSITY FORECAST SHOWS THIS...BUT IS NOT AS STRONG AS THE GFDL AND HWRF MODELS. FORECAST POSITIONS AND MAX WINDS INIT 09/0300Z 14.7N 120.3W 50 KT 60 MPH 12H 09/1200Z 15.0N 119.7W 45 KT 50 MPH 24H 10/0000Z 15.3N 118.5W 45 KT 50 MPH 36H 10/1200Z 15.5N 117.3W 45 KT 50 MPH 48H 11/0000Z 15.6N 115.8W 45 KT 50 MPH 72H 12/0000Z 15.5N 113.0W 50 KT 60 MPH 96H 13/0000Z 17.0N 109.0W 55 KT 65 MPH 120H 14/0000Z 20.0N 107.5W 60 KT 70 MPH $$ FORECASTER BEVEN NNNN
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