MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (ATSANI) WARNING NR 31// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON (TY) 17W (ATSANI), LOCATED APPROXIMATELY 237 NM EAST- NORTHEAST OF IWO TO, HAS TRACKED NORTH-NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY SHOWS A WEAKENING CONVECTIVE STRUCTURE WITH DRY AIR PENETRATING THE NORTHWESTERN EYEWALL. THE SYSTEM HAS MAINTAINED A 12 NM EYE, WHICH SUPPORTS THE CURRENT POSITION WITH HIGH CONFIDENCE. MOST OF THE REMAINING CONVECTION IS CONFINED TO THE SOUTHERN PERIPHERY WHICH IS BEING AIDED BY A ROBUST EQUATORWARD OUTFLOW. A 211738Z SSMI 85GHZ MICROWAVE IMAGE SHOWS A MUCH LARGER MICROWAVE EYE FEATURE WITH AN ERODING EYEWALL. THE CURRENT INTENSITY HAS BEEN LOWERED TO 90 KNOTS BASED ON DECREASING DVORAK INTENSITY ESTIMATES FROM ALL AGENCIES AND REFLECTS THE WEAKENING CONVECTIVE STRUCTURE. ENVIRONMENTAL CONDITIONS ARE MARGINAL DUE TO THE STRONG SUBSIDENCE AND DRY AIR ALONG THE NORTHERN AND WESTERN PERIPHERIES; HOWEVER, SEA SURFACE TEMPERATURES REMAIN FAVORABLE. TY 17W IS ROUNDING THE SOUTHWESTERN PERIPHERY OF THE DEEP-LAYERED SUB-TROPICAL RIDGE (STR) LOCATED TO THE EAST. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 17W WILL CONTINUE TO ROUND THE WESTERN PERIPHERY OF THE DEEP LAYERED STR AND TURN POLEWARD. BY TAU 36, TY ATSANI WILL BEGIN TO ACCELERATE NORTHEASTWARD IN FRONT OF AN APPROACHING MID-LATITUDE TROUGH. ENVIRONMENTAL CONDITIONS WILL REMAIN MOSTLY UNCHANGED THROUGH TAU 72, CREATING A SLOW WEAKENING TREND. BY TAU 72, SEA SURFACE TEMPERATURES WILL DROP BELOW 26 CELSIUS AND THE SYSTEM WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT). C. IN THE EXTENDED FORECAST, TY 17W WILL CONTINUE TO ACCELERATE NORTHEASTWARD AND EMBED FURTHER INTO THE WEAK BAROCLINIC ZONE. BY THE END OF THE FORECAST PERIOD, TY ATSANI WILL SLOW AS IT GETS ENVELOPED BY THE MID-LATITUDE TROUGH AND IS FORECAST TO COMPLETE ETT AS A STRONG COLD-CORE LOW. DYNAMIC MODEL GUIDANCE REMAINS IN TIGHT AGREEMENT LENDING HIGH CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN NNNN