WDPN32 PGTW 251500 MSGID/GENADMIN/MSG/NAVPACMETOCCEN PEARL HARBOR HI/JTWC// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 18W WARNING NR 21// RMKS/ 1. FOR METEOROLOGISTS. 2. PROGNOSTIC REASONING FOR 251200Z TO 261200Z SEP 2005. A. TYPHOON (TY) 18W (SAOLA) IS LOCATED APPROXIMATELY 245 NM EAST OF TOKYO, JAPAN AND HAS TRACKED NORTHEASTWARD AT 20 KNOTS OVER THE PAST SIX HOURS. ANIMATED WATER VAPOR IMAGERY REVEALS A SIGNIFICANT DECREASE IN THE EXTENT AND ORGANIZATION OF CONVECTION AS DRIER AIR PENETRATES THE SOUTHWESTERN QUADRANT OF THE STORM. B. TS 18W IS THOROUGHLY EMBEDDED IN THE MIDLATITUDE WESTERLIES AND IS ACCELERATING TO THE NORTHEAST AHEAD OF THE MIDLATITUDE TROUGH NOW EXITING JAPAN. THE SYSTEM WILL CONTINUE TO RAPIDLY TRACK TOWARD THE NORTHEAST ALONG THE NORTHWESTERN PERIPHERY OF THE MID LEVEL STEERING RIDGE CENTERED WELL EAST OF JAPAN. THE DYNAMIC AIDS CON- SISTING OF NOGAPS, NCEP GFS, GFDN, JTYM, JGSM, WBAR, EGRR, AFWA MM5, COAMPS, AND TCLAPS ARE IN GOOD AGREEMENT WITH THIS SCENARIO. THIS FORECAST IS BASED ON A CONSENSUS OF ALL AVAILABLE DYNAMIC AIDS THROUGH TAU 24. C. TS 18W IS CURRENTLY UNDERGOING EXTRATROPICAL TRANSITION (XT) AND IS EXPECTED TO COMPLETE XT BY TAU 24. THE SYSTEM CONTINUES TO WEAKEN IN AN ENVIRONMENT OF HIGH VERTICAL WIND SHEAR. FURTHER WEAKENING IS EXPECTED AS THE STORM TRAVERSES LOWER SEA SURFACE TEMPERATURES OVER THE NEXT 24 HOURS. D. CURRENT WIND RADII ARE BASED ON A 250814Z QUIKSCAT PASS AND A 251105Z SSM/I PASS. FORECAST WIND RADII ARE BASED ON CLIMATOLOGY FOR AN AVERAGE-SIZED SYSTEM UNDERGOING XT. 3. FORECAST TEAM: BRAVO// NNNN