MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (KROVANH) WARNING NR 14// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 20W (KROVANH), LOCATED APPROXIMATELY 93 NM SOUTHEAST OF IWO TO, JAPAN HAS TRACKED NORTH-NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A 17NM CLOUD FILLED EYE SURROUNDED BY AN ELONGATED CONVECTIVE CORE THAT HAS BEEN FLARING OVER THE PAST SIX HOURS. WATER VAPOR IMAGERY SHOWS SIGNIFICANT DRY AIR ALONG THE WESTERN PERIPHERY ASSOCIATED WITH A DEEP MID-LATITUDE TROUGH LOCATED TO THE WEST. HOWEVER, THE TROUGH IS ALSO PROVIDING AN ENHANCED POLEWARD OUTFLOW CHANNEL THAT IS SUSTAINING THE CURRENT CONVECTION. A 172228Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS A WELL DEFINED 20NM MICROWAVE EYE WITH THE BULK OF CONVECTION ALONG THE NORTHERN PERIPHERY DUE TO PERSISTENT 20 KNOT SOUTHWESTERLY VERTICAL WIND SHEAR (VWS). THE CURRENT POSITION IS BASED ON THE EYE FEATURE SEEN IN THE ABOVE IMAGERY WITH HIGH CONFIDENCE. THE INITIAL INTENSITY HAS BEEN KEPT AT 105 KNOTS BASED ON AN ASSESSMENT OF DVORAK INTENSITY ESTIMATES FROM ALL AGENCIES AND THE UNCHANGED STRUCTURE OF THE TYPHOON. ENVIRONMENTAL ANALYSIS INDICATES A DETERIORATING ENVIRONMENT AS VWS HAS INCREASED AND PRESSURE ALONG THE WESTERN PERIPHERY HAS BEGUN TO ELONGATE AND TILT THE SYSTEM. TY 20W IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYERED SUB-TROPICAL RIDGE (STR). 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 20W WILL BEGIN TO TURN POLEWARD OVER THE NEXT SIX HOURS AND ROUND THE STR AXIS BEFORE TURNING TO A NORTHEASTERN TRAJECTORY. MARGINAL CONDITIONS WILL PERSIST FOR THE NEXT 12 HOURS ALLOWING TY KROVANH TO MAINTAIN ITS CURRENT INTENSITY. BEYOND TAU 12, CONTINUED DRY AIR ENTRAINMENT AND INCREASING VWS WILL BEGIN TO WEAKEN THE TYPHOON. SSTS WILL DROP BELOW 26 CELSIUS JUST AFTER TAU 48 WHICH WILL INCREASE THE RATE OF WEAKENING. DYNAMIC MODEL GUIDANCE IS IN VERY TIGHT AGREEMENT THROUGH THIS PORTION OF THE FORECAST PERIOD. C. IN THE EXTENDED TAUS, TY KROVANH WILL CONTINUE TO TRACK TO THE NORTHEAST AROUND THE STR. INCREASING VWS AND COOLER SSTS WILL FURTHER WEAKEN THE SYSTEM AS IT ACCELERATES INTO THE WEAK MID-LATITUDE BAROCLINIC ZONE. THE SYSTEM IS FORECAST TO BEGIN EXTRA-TROPICAL TRANSITION AT TAU 96 AND CONTINUE THROUGH THE END OF THE FORECAST PERIOD. DYNAMIC MODEL GUIDANCE DIVERGES IN THIS PORTION OF THE FORECAST DUE TO A COMPLEX STEERING ENVIRONMENT WHICH COULD CAUSE THE SYSTEM TO BECOME QUASI-STATIONARY. ALTHOUGH THE MODEL GUIDANCE CONTINUES TO STRUGGLE IN THE EXTENDED FORECAST, THE TIGHT AGREEMENT IN THE FIRST 72 HOURS, LENDS TO HIGH CONFIDENCE IN THE JTWC TRACK FORECAST.// NNNN NNNN