000 WTPZ41 KNHC 092034 TCDEP1 TROPICAL STORM IRWIN DISCUSSION NUMBER 15 NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011 200 PM PDT SUN OCT 09 2011 IRWIN APPEARS TO BE SLIGHTLY BETTER ORGANIZED. THE CYCLONE HAS BEEN MAINTAINING DEEP CONVECTION IN A BAND WRAPPING AROUND THE SOUTHERN SEMICIRCLE FOR ALMOST 12 HOURS IN AN EASTERLY SHEAR ENVIRONMENT. DVORAK T-NUMBERS HAVE INCREASED SOME...AND THE INITIAL INTENSITY IS RAISED TO 45 KT IN ACCORDANCE WITH THE DVORAK ESTIMATE FROM TAFB. SHROUDED BY ITS CONVECTIVE CLOUD CANOPY...THE CENTER OF IRWIN CONTINUES TO BE DIFFICULT TO LOCATE IN CONVENTIONAL SATELLITE IMAGERY. MICROWAVE FIXES SUGGEST THAT IRWIN IS MOVING 060/04...A TAD TO THE RIGHT OF THE PREVIOUS ESTIMATE. EMBEDDED IN LOW- TO MID-LEVEL WESTERLY FLOW...IRWIN IS EXPECTED TO MOVE GENERALLY EAST-NORTHEASTWARD FOR THE NEXT DAY OR SO. THE INTERACTION OF IRWIN WITH A MID-TROPOSPHERIC SHORTWAVE TROUGH CUTTING OFF INTO A LOW SOUTHWEST OF BAJA CALIFORNIA SHOULD CAUSE THE TRACK TO BEND EAST-SOUTHEAST AND THEN NORTHEAST AROUND THE LOW IN 36-72 HOURS. THE NORTHWARD MIGRATION OF THE PACIFIC ITCZ ALONG WITH A CONTINUED WESTERLY MID-LEVEL STEERING SHOULD GUIDE IRWIN ON AN EASTWARD OR EVEN EAST-SOUTHEASTWARD COURSE LATER ON. THE TRACK GUIDANCE HAS COME INTO MUCH BETTER AGREEMENT ON THIS SCENARIO THIS CYCLE. THE OVERALL EASTWARD SHIFT IN THE GUIDANCE REQUIRES A SIZABLE ADJUSTMENT TO THE PREVIOUS OFFICIAL FORECAST AT DAYS 4-5 BUT IS STILL TO THE LEFT OF THE MULTI-MODEL CONSENSUS. INTENSITY-WISE...IRWIN MAY HAVE SOME STRUGGLES AHEAD. THE STORM COULD BE TAPPING A MORE STABLE AIR MASS TO ITS NORTHWEST WHICH...IN COMBINATION WITH PERSISTENT EASTERLY SHEAR...MAY DIMINISH ITS CHANCE FOR INTENSIFICATION IN THE SHORT TERM. A PERSISTENCE OF EASTERLY SHEAR AND THE POTENTIAL ADVERSE EFFECTS OF THE SST WAKE ASSOCIATED WITH JOVA COULD BE NEGATIVE FACTORS FOR INTENSIFICATION LATER IN THE FORECAST PERIOD. THE STATISTICAL-DYNAMICAL GUIDANCE IS BETTER AGREEMENT THAN BEFORE...BOTH SUGGESTING LITTLE CHANGE IN INTENSITY FOLLOWED BY A SLOW DECAY. THE OFFICIAL FORECAST IS SIMILAR TO THE PREVIOUS ONE AS IN GENERAL AGREEMENT WITH THE INTENSITY CONSENSUS AIDS...ALBEIT SLIGHTLY LOWER AT DAYS 4-5. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 15.2N 119.3W 45 KT 50 MPH 12H 10/0600Z 15.7N 118.4W 45 KT 50 MPH 24H 10/1800Z 15.8N 117.0W 40 KT 45 MPH 36H 11/0600Z 15.6N 115.5W 40 KT 45 MPH 48H 11/1800Z 15.4N 113.8W 40 KT 45 MPH 72H 12/1800Z 16.0N 110.0W 40 KT 45 MPH 96H 13/1800Z 17.5N 106.5W 40 KT 45 MPH 120H 14/1800Z 18.0N 105.0W 45 KT 50 MPH $$ FORECASTER KIMBERLAIN