WDPN31 PGTW 232100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR SUBTROPICAL STORM 02W (SURIGAE) WARNING NR 042// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUBTROPICAL STORM (SS) 02W (SURIGAE), LOCATED APPROXIMATELY 247 NM SOUTHEAST OF KADENA AB, HAS TRACKED EASTWARD AT 06 KNOTS OVER THE PAST SIX HOURS. INFRARED SATELLITE IMAGERY SHOWS THAT DEEP CONVECTION HAS VANISHED OVER THE CIRCULATION AS IT PASSES OVER A COLD OCEANIC EDDY WITH COLD SEA SURFACE TEMPERATURES OF 23-24 DEGREES CELSIUS. WATER VAPOR SATELLITE IMAGERY INDICATES THAT AN UPPER-LEVEL SHORTWAVE HAS PASSED OVER THE RYUKYU ISLANDS AND IS NOW SUPERIMPOSING WITH SURIGAE'S CIRCULATION. THIS IS BRINGING ABOUT A REDUCTION IN BAROCLINIC FORCING THAT IS ALLOWING THE CYCLONE TO CONTINUE DECAYING. A 231257Z ASCAT-B PASS INDICATED WINDS IN EXCESS OF 65 KTS IN THE NORTHWEST QUADRANT OF THE STILL WELL-DEFINED CIRCULATION. THE WIND FIELD CONSISTS OF A SYMMETRIC CORE WITH A GROWING ASYMMETRIC EXTENSION TO THE NORTHEAST IN RESPONSE TO WARM ADVECTION AND BAROCLINIC INTERACTION. THE CURRENT INTENSITY IS SET AT 60 KTS, ASSUMING THE WEAKENING TREND HAS CONTINUED SINCE THAT TIME. THE ENVIRONMENT REMAINS HOSTILE WITH VERTICAL WIND SHEAR OF 40- 50 KTS, ABUNDANT DRY AIR IN THE SOUTHERN SEMICIRCLE, AND THE AFOREMENTIONED COLD SEA SURFACE TEMPERATURES. 3. FORECAST REASONING. A. 02W HAS NOW BEEN CLASSIFIED AS A SUBTROPICAL CYCLONE BASED ON ITS LACK OF DEEP CONVECTION, ENTANGLEMENT WITH A COLD-CORE UPPER- LEVEL TROUGH, AND INCREASINGLY ASYMMETRIC STRUCTURE. EXTRATROPICAL TRANSITION IS STILL EXPECTED BY TAU 48. THERE IS NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. SS 02W (SURIGAE) WILL CONTINUE STEADILY WEAKENING OVER THE NEXT 24 HOURS AS IT LOSES BAROCLINIC SUPPORT FROM THE UPPER-LEVEL SHORTWAVE CURRENTLY SUPERIMPOSING WITH THE CIRCULATION, BUT IS EXPECTED TO RETAIN GALE-FORCE WINDS. AROUND 36 HOURS, A SECOND SHORTWAVE TROUGH CURRENTLY ENTERING THE YELLOW SEA WILL APPROACH SURIGAE, REINTRODUCING BAROCLINIC FORCING. THIS MAY STRENGTHEN AND INTENSIFY THE WIND FIELD SLIGHTLY BY 48 HOURS, AND WILL ALSO BRING ABOUT THE CONCLUSION OF EXTRATROPICAL TRANSITION. THE FORECAST TRACK IS SIMILAR TO THE PREVIOUS ONE, PASSING NEAR IWO TO IN AROUND 48 HOURS. THE MODEL GUIDANCE ENVELOPE IS TIGHTLY CLUSTERED, WITH SOME SLIGHT VARIATION IN FORWARD SPEED. OVERALL, THIS FORECAST IS OF HIGH CONFIDENCE.// NNNN