000 WTPZ41 KNHC 101446 TCDEP1 TROPICAL STORM IRWIN DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 800 AM PDT MON OCT 10 2011 SIMILAR TO WHAT OCCURRED YESTERDAY...IRWIN HAS LOST MOST OF ITS DEEP CONVECTION. ALL THAT REMAINS IS A PATCH OF MEAGER CONVECTION THAT HAS RECENTLY FORMED SOUTH OF THE CIRCULATION CENTER. DVORAK T-NUMBERS HAVE DECREASED ACCORDINGLY...AND A BLEND OF THE LATEST T- AND CI-NUMBERS ARE USED TO SET THE INITIAL INTENSITY ESTIMATE AT 35 KT. IRWIN APPEARS TO BE PULLING IN RELATIVELY COOLER AND MORE STABLE AIR FROM THE WEST...AS EVIDENCED BY THE SOUTHWARD DISPLACEMENT OF THE NEARBY STRATOCUMULUS FIELD. THIS COULD TEMPORARILY BE LIMITING ANY RE-DEVELOPMENT OF SIGNIFICANT DEEP CONVECTION AND HENCE INTENSIFICATION. EASTERLY SHEAR IS ANOTHER LIMITING FACTOR AND IS FORECAST BY THE DYNAMICAL MODELS TO PERSIST AT ROUGHLY THE SAME MAGNITUDE FOR THE NEXT 3-4 DAYS AND POSSIBLY EVEN INCREASE AT THE END OF THE FORECAST PERIOD. WHILE THE DYNAMICAL GUIDANCE SHOWS SOME STRENGTHENING...THE OBJECTIVE STATISTICAL OUTPUT CONTINUES TO SHOW A SLOW DECAY. THE OFFICIAL INTENSITY FORECAST IS REDUCED A BIT FROM THE PREVIOUS ONE TO ACCOUNT FOR THE RECENT WEAKENING. IT ALSO LEANS TOWARD A BLEND OF SHIPS/LGEM AND THE INTENSITY CONSENSUS AIDS...GIVEN HOW MUCH THE SYSTEM IS CURRENTLY STRUGGLING. RECENT CENTER FIXES REMAIN UNCERTAIN...AND THE INITIAL MOTION ESTIMATE APPEARS TO BE 085/06. IRWIN IS EXPECTED TO BE CARRIED ON AN ATYPICAL EASTWARD AND THE NORTHEASTWARD TRACK FOR THE NEXT SEVERAL DAYS AS IT BENDS CYCLONICALLY AROUND A CUT-OFF LOW DEVELOPING SOUTHWEST OF BAJA CALIFORNIA. BY ABOUT 96 HOURS... IRWIN...PERHAPS AS A SHALLOW CYCLONE...IS LIKELY TO BE PULLED BACK TOWARD THE EAST AND EVEN SOUTHEAST INTO A BROAD LOW- TO MID-LEVEL CYCLONIC GYRE OVER MEXICO. THE MODEL GUIDANCE IS IN GOOD AGREEMENT ON THIS SCENARIO...AND THE CURRENT FORECAST IS ESSENTIALLY AN UPDATE OF THE PREVIOUS ONE. FORECAST POSITIONS AND MAX WINDS INIT 10/1500Z 15.0N 117.7W 35 KT 40 MPH 12H 11/0000Z 15.1N 116.4W 35 KT 40 MPH 24H 11/1200Z 15.0N 114.8W 40 KT 45 MPH 36H 12/0000Z 15.1N 112.9W 40 KT 45 MPH 48H 12/1200Z 15.6N 110.9W 40 KT 45 MPH 72H 13/1200Z 16.9N 107.4W 40 KT 45 MPH 96H 14/1200Z 17.1N 105.1W 40 KT 45 MPH 120H 15/1200Z 16.4N 103.9W 35 KT 40 MPH $$ FORECASTER KIMBERLAIN