MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (KROVANH) WARNING NR 10// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 20W (KROVANH), LOCATED APPROXIMATELY 290 NM SOUTHEAST OF IWO TO, JAPAN HAS TRACKED NORTHWESTWARD AT 11 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A SYMMETRIC CORE STRUCTURE WITH DEEP SPIRAL BANDING WRAPPING INTO A 20NM CLOUD FILLED EYE WHICH SUPPORTS THE INITIAL POSITION WITH HIGH CONFIDENCE. A 162105Z WINDSAT MICROWAVE IMAGE CONTINUES TO SHOW AN IMPROVED CONVECTIVE STRUCTURE WITH A CONSTRICTING MICROWAVE EYE. THE INTENSITY HAS BEEN INCREASED TO 80 KNOTS BASED ON AN ASSESSMENT OF DVORAK INTENSITY ESTIMATES AND REFLECTS THE IMPROVED STRUCTURE OF THE TYPHOON. ENVIRONMENTAL ANALYSIS INDICATES A VERY FAVORABLE ENVIRONMENT WITH NEAR RADIAL OUTFLOW, LOW (5-10 KNOT) VERTICAL WIND SHEAR (VWS), AND SEA SURFACE TEMPERATURES (SSTS) NEAR 30 CELSIUS. TY 20W IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYERED SUB-TROPICAL RIDGE (STR) TO THE NORTHEAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 20W WILL TRACK ON A NORTHWESTWARD TRAJECTORY ALONG THE SOUTHWESTERN PERIPHERY OF THE STEERING STR FOR THE NEXT 24 HOURS BEFORE TURNING POLEWARD AND ROUNDING THE STR AXIS. FAVORABLE ENVIRONMENTAL CONDITIONS WILL SUPPORT RAPID INTENSIFICATION FOR THE NEXT 24 HOURS. BEYOND TAU 24, VWS WILL INCREASE TO GREATER THAN 20 KNOTS DUE TO AN APPROACHING MID-LATITUDE TROUGH FROM THE WEST. INITIALLY THE AFOREMENTIONED TROUGH WILL PROVIDE AN ENHANCED POLEWARD OUTFLOW CHANNEL, ALLOWING TY KROVANH TO PEAK AT 115 KNOTS BY TAU 36. AFTERWARDS THE INCREASED VWS WILL BEGIN TO WEAKEN THE SYSTEM. C. IN THE EXTENDED TAUS, TY 20W WILL TRACK TO THE NORTHEAST FOR THE REMAINDER OF THE FORECAST PERIOD. INCREASING VWS AND COOLER SSTS WILL CONTINUE TO WEAKEN THE SYSTEM AS IT ACCELERATES INTO THE WEAK MID-LATITUDE BAROCLINIC ZONE. THE SYSTEM IS FORECAST TO BEGIN EXTRA- TROPICAL TRANSITION (ETT) AT TAU 96 AND COMPLETE ETT BY THE END OF THE FORECAST PERIOD. DYNAMIC MODEL GUIDANCE CONTINUES TO BE IN OVERALL GOOD AGREEMENT, LENDING TO HIGH CONFIDENCE IN THE JTWC TRACK FORECAST.// NNNN NNNN