MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (TAPAH) WARNING NR 005// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 18W (TAPAH), LOCATED APPROXIMATELY 218 NM SOUTH OF KADENA AB, OKINAWA, JAPAN HAS TRACKED NORTHWESTWARD AT 06 KNOTS (KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS DEEP CONVECTIVE BANDING WRAPPING INTO A BROAD, PARTIALLY EXPOSED LOW LEVEL CIRCULATION (LLC). THE SYSTEM HAS EXTENSIVE DEEP CONVECTION IN THE SOUTH AND SOUTHEAST THAT AT TIMES APPEARS TO WRAP INTO THE EASTERN AND NORTHERN QUADRANTS BUT FAILS TO PERSIST. OVER THE LAST SIX HOURS, THE SYSTEM HAS SHOWN MODEST IMPROVEMENTS IN BOTH CONVECTION AND STRUCTURE. THE INITIAL POSITION IS PLACED WITH FAIR CONFIDENCE BASED ON A 192229Z 37GHZ SSMIS MICROWAVE IMAGE THAT SHOWS CONVECTIVE BANDING WRAPPING INTO A BROAD (90NM) CIRCULATION CENTER. THE INITIAL INTENSITY OF 45 KNOTS IS BASED ON EARLIER ASCAT IMAGES AND ADJUSTED FOR THE IMPROVEMENTS IN STRUCTURE. THE INTENSITY IS ABOVE THE PGTW AND RJTD DVORAK FIX ESTIMATES OF T2.0 (30KTS) DUE TO THE BROAD NATURE OF THE SYSTEM CAUSING ERRONEOUSLY LOW DVORAK ESTIMATES. UPPER LEVEL ANALYSIS INDICATES THAT THE WESTERN SIDE OF THE SYSTEM IS IN AN AREA OF LOW TO MODERATE (10-20 KTS) VERTICAL WIND SHEAR WHILE THE EASTERN SIDE OF THE SYSTEM IS IN AN AREA OF MODERATE TO HIGH (15-30 KTS). THE GENERALLY UNFAVORABLE VERTICAL WIND SHEAR IS OFFSET BY STRONG EQUATORWARD OUTFLOW AND HIGH (29-30 CELSIUS) SEA SURFACE TEMPERATURES (SST) IN THE REGION. TS 18W IS TRACKING WEST-NORTHWEST BETWEEN A SUBTROPICAL RIDGE (STR) TO THE NORTHWEST AND A STR TO THE NORTHEAST. THE STR TO THE NORTHEAST HAS BEGUN TO BUILD AND IS TAKING OVER AS THE PRIMARY STEERING MECHANISM FOR THE SYSTEM. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 18W IS FORECAST TO TURN MORE NORTHWESTWARD AS IT BEGINS TO RECURVE AROUND THE STR TO THE NORTHEAST. THE SYSTEM WILL STEADILY INTENSIFY AS IT RECURVES; REACHING 60 KNOTS AS IT ROUNDS THE RIDGE AXIS BY TAU 36. THE PEAK INTENSITY WILL OCCUR SHORTLY AFTER AS AN APPROACHING MID-LATITUDE TROUGH BRIEFLY ENHANCES POLEWARD OUTFLOW, ALLOWING ANOTHER BRIEF PERIOD OF INTENSIFICATION TO A PEAK OF 65 KNOTS. THEREAFTER, INCREASING VERTICAL WIND SHEAR FROM THE TROUGH WILL BEGIN TO WEAKEN THE SYSTEM AND CAUSE THE SYSTEM TO BEGIN EXTRATROPICAL TRANSITION. BY TAU 48, 18W WILL BE TRACKING TO THE NORTHEAST, PASSING BETWEEN CHEJU ISLAND AND SASEBO, JAPAN BEFORE TAU 72. AS TAPAH TRACKS THROUGH THE KOREAN STRAIT, LAND INTERACTION WILL CONTINUE TO WEAKEN THE SYSTEM. THE WEAKENING TREND WILL PERSIST FOR THE DURATION OF THE FORECAST. NUMERICAL GUIDANCE IS IN STRONG AGREEMENT THROUGH THIS PORTION OF THE FORECAST LENDING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK. C. AFTER TAU 72, TS 18W BEGINS TO ENCOUNTER AN INCREASINGLY UNFAVORABLE ENVIRONMENT AS VERTICAL WIND SHEAR REMAINS STRONG AND SSTS CONTINUE TO DECREASE. THE SYSTEM WILL CONTINUE TO WEAKEN AS IT TRACKS THROUGH THIS UNFAVORABLE ENVIRONMENT IN THE SEA OF JAPAN AND BECOMES EMBEDDED IN THE PASSING MID-LATITUDE TROUGH. TS 18W WILL MAKE LANDFALL AS AN EXTRATROPICAL CYCLONE OVER SOUTHERN HOKKAIDO AFTER TAU 84 BEFORE EXITING BACK INTO THE PACIFIC OCEAN AROUND TAU 96. MODEL GUIDANCE BEGINS TO SPREAD AFTER TAU 72; HOWEVER, IT REMAINS IN GENERALLY GOOD AGREEMENT THROUGH TAU 96. DUE TO THE GOOD AGREEMENT IN MODEL GUIDANCE THERE IS OVERALL HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN