WDPN31 PGTW 240300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (TEMBIN) WARNING NR 21// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TYPHOON 15W (TEMBIN), LOCATED APPROXIMATELY 180 NM SOUTH- SOUTHWEST OF TAIPEI, TAIWAN, HAS TRACKED WEST-SOUTHWESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY AND A 232355Z SSMIS 91 GHZ MICROWAVE IMAGE SHOW THAT THE EYEWALL STRUCTURE HAS ONLY SLIGHTLY DEGRADED AS TY 15W SKIRTED THE SOUTHERN PENINSULA OF TAIWAN. THE CURRENT POSITION IS BASED ON THE CLUSTER OF JAPAN AND TAIWAN RADAR FIXES ALONG WITH THE PGTW AND RJTD CENTER FIXES WITH HIGH CONFIDENCE. THE CURRENT INTENSITY WAS LOWERED BY ONLY TEN KNOTS FROM THE PREVIOUS WARNING, WHEN THE TY WAS MAKING LANDFALL OVER EASTERN TAIWAN, DUE TO MERELY SKIRTING THE SOUTHERN TIP AND LACK OF NOTICEABLE DEGRADATION IN EYEWALL STRUCTURE. UPPER- LEVEL ANALYSIS INDICATES HEALTHY EQUATORWARD EXHAUST ACCOMPANIED BY MODEST RADIAL OUTFLOW. A MORE SOUTHWARD COMPONENT TO THE TRACK HAS DEVELOPED, AS TY 15W TRACKS ALONG THE SOUTHWESTERN EXTENSION OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) CENTERED OVER THE EAST CHINA SEA. WITH TY 16W (BOLAVEN) STILL OVER 700 NM AWAY, WHICH IS BEYOND THE TYPICAL THRESHOLD OF DIRECT CYCLONE INTERACTION, ANY EFFECT TO THAT REGARD SHOULD BE INSIGNIFICANT AT THIS TIME. 3. FORECAST REASONING. A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TY 15W WILL CONTINUE TO STEER WEST-SOUTHWESTWARD OVER THE TAIWAN STRAIT DURING THE NEXT 36 HOURS AND SHOULD BE ABLE TO MAINTAIN CURRENT INTENSITY. AS THE MID-LATITUDE TROUGH OVER MAINLAND CHINA INTENSIFIES, THE STR TO THE NORTH OF TY 15W WILL WEAKEN, THEREBY RESULTING IN QUASI-STATIONARY MOTION. BY TAU 72, THE NEAR EQUATORIAL RIDGE (NER) IS EXPECTED TO INTENSIFY TO THE SOUTHEAST OF TY 15W. THE RESULTANT SOUTHWESTERLY DEEP LAYER STEERING WILL INDUCE A COUNTER-CLOCKWISE LOOP IN THE FORECAST TRACK OF TY 15W. C. AFTER TAU 72, TYPHOON 15W IS EXPECTED TO ACCELERATE EAST- NORTHEASTWARD AS THE NER AMPLIFIES. AS TY 16W (BOLAVEN) PASSES TO THE NORTHEAST, SOME BINARY INTERACTION IS POSSIBLE. UPPER-LEVEL OUTFLOW BEYOND TAU 72 WOULD DEPEND ON THIS INTERACTION AND THE INTENSITY OF THE STR THAT IS FORECAST TO REBUILD WESTWARD BEHIND TY 16W (BOLAVEN). DYNAMIC MODEL GUIDANCE REMAINS IN FAIR AGREEMENT (ALL INDICATING A LOOPING TRACK WITH THE EXCEPTION OF GFDN) WITHIN 72 HOURS. HOWEVER, THERE IS LARGE VARIATION IN THE FORECAST GUIDANCE BEYOND TAU 72 DUE TO THE POSSIBILITY OF INTERACTION WITH TY 16W AND THE VARIATION IN STR ORIENTATION. AS SUCH, THERE REMAINS LOW CONFIDENCE IN THE EXTENDED FORECAST TRACK.// NNNN