MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 31W (YUTU) WARNING NR 38// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 31W (YUTU), LOCATED APPROXIMATELY 430 NM SOUTHEAST OF HONG KONG, HAS TRACKED WESTWARD AT 05 KNOTS OVER THE PAST SIX HOURS. TS 31W HAS CONTINUED TO TRACK AWAY FROM THE PHILIPPINES WITH DECREASING FRICTIONAL EFFECTS. CONSEQUENTLY, ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS A RECONSOLIDATING SYSTEM WITH IMPROVED DEEP CONVECTIVE BANDING, ESPECIALLY OVER THE NORTHERN SEMICIRCLE, WRAPPING INTO A BROAD LOW- LEVEL CIRCULATION CENTER. A 301904Z SSMI 37GHZ IMAGE DEPICTS TIGHTLY CURVED SHALLOW BANDING WRAPPING INTO A WELL-DEFINED MICROWAVE EYE FEATURE, WHICH SUPPORTS THE CURRENT POSITION WITH GOOD CONFIDENCE (WITHIN 30NM). A RECENT ASCAT AMBIGUITY IMAGE AS WELL AS THE RECENT SSMI IMAGE SUGGEST A SOMEWHAT ERRATIC TRACK OVER THE PAST 12 HOURS WITH A DIP TO THE WEST-SOUTHWEST FOLLOWED BY THE RECENT SHARP TURN TO THE NORTHWEST. OVERALL, ENVIRONMENTAL CONDITIONS ARE FAVORABLE WITH LOW (10-15 KT) VERTICAL WIND SHEAR AND ENHANCED POLEWARD OUTFLOW. TS 31W IS TRACKING ALONG THE SOUTHWEST PERIPHERY OF THE SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. THE 34-KNOT WIND RADII OVER THE NORTHWEST (ANALYSIS) AND NORTHEAST QUADRANTS (FORECAST) HAVE BEEN MODIFIED TO INCORPORATE NORTHEAST SURGE WINDS ADJACENT TO THE SYSTEM. B. TS 31W IS FORECAST TO TRACK NORTH-NORTHWESTWARD TO NORTHWARD THROUGH THE FORECAST PERIOD AS IT TRACKS ALONG THE WESTERN PERIPHERY OF THE STR. THE SYSTEM IS FORECAST TO REINTENSIFY TO A PEAK OF 65 KNOTS BY TAU 24 DUE TO THE FAVORABLE ENVIRONMENTAL CONDITIONS. AFTER TAU 24, INCREASING VERTICAL WIND SHEAR (VWS) AND SLIGHTLY COOLER SST VALUES (26C) WILL SERVE TO GRADUALLY WEAKEN THE SYSTEM. AFTER TAU 48, TS 31W WILL RAPIDLY DISSIPATE OVER WATER DUE TO DRY AIR ENTRAINMENT, STRONG VWS AND UPPER-LEVEL CONVERGENCE. THERE IS HIGH UNCERTAINTY IN THE JTWC FORECAST TRACK WITH A LARGE SPREAD OF 530NM AT TAU 72. MOST OF THE MODELS RAPIDLY DISSIPATE THE SYSTEM, EITHER SLOWING THE FORWARD MOTION WHILE LOSING THE CIRCULATION OR EJECTING THE REMNANT CIRCULATION NORTHEASTWARD INTO THE BAROCLINIC ZONE AND STRONG UPPER-LEVEL WESTERLIES. THE JTWC OFFICIAL FORECAST FAVORS A DISSIPATION SCENARIO AND SLOWER TRACK SPEEDS OVER THE NORTHERN SOUTH CHINA SEA DUE TO THE VERY UNFAVORABLE ENVIRONMENTAL CONDITIONS.// NNNN NNNN