WDPN32 PGTW 271500 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TYPHOON 18W (NORU) WARNING NR 023// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.8N 110.3E INITIAL INTENSITY: 105 KTS GEOGRAPHIC REFERENCE: 116 NM EAST OF DA NANG, VIETNAM MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 09 KTS SIGNIFICANT WAVE HEIGHT: 45 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES THAT TY 18W HAS FINALLY SUCCUMBED TO THE PERSISTENT EASTERLY SHEAR THAT HAS BEEN PLAGUING IT FOR SOME TIME. THE SATELLITE DEPICTION HAS ERODED QUITE DRAMATICALLY OVER THE PAST SIX HOURS, HAVING GONE FROM HAVING A SMALL PINHOLE EYE TO HAVING NO DISTINCTLY EVIDENT CENTER IN THE EIR. DEEP CONVECTION CONTINUES TO FIRE HAPHAZARDLY AT VARIOUS POINTS AROUND THE ASSESSED CENTER POSITION BUT IS NOT ORGANIZED INTO CLEAR FEEDER BANDS, AND THUS THE INITIAL POSITION HAS BEEN DIFFICULT TO NAIL DOWN. A 271105Z SSMIS 91GHZ MICROWAVE IMAGE SHOWED AN INTERESTING STRUCTURE, WITH A SOLID BAND OF DEEP CONVECTION WRAPPING UP THE EAST SIDE AND A WEAKER BAND ON THE WEST SIDE BUT NO EYEWALL OR DEEP CONVECTIVE BAND IN THE NORTHERN HEMISPHERE. THE 37GHZ IMAGE SHOWED A MICROWAVE EYE FEATURE AND AN EXTRAPOLATION OF THIS FEATURE, ALONG WITH THE FIRST HINTS OF THE EYE IN VIETNAMESE RADAR DATA, PROVIDED MEDIUM CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY IS ALSO CHALLENGING TO PIN DOWN, AS ALL AGENCY AND OBJECTIVE VALUES HAVE FALLEN OFF THE CLIFF AFTER THE EYE DISAPPEARED, BUT AGENCY CURRENT INTENSITIES (CIS) REMAIN ANYWHERE FROM T5.5 TO T6.5. ADT REMAINS NEAR 100 KNOTS AND THE PGTW ASSESSMENT IS HEDGED SLIGHTLY HIGHER THAN THE ADT, IN KEEPING WITH THE LOWER END OF THE AGENCY CIS, WITH MEDIUM CONFIDENCE. WHILE OUTFLOW REMAINS STRONG AND SSTS ARE CERTAINLY WARM ENOUGH TO SUPPORT THE SYSTEM, EASTERLY SHEAR OVER 30 KNOTS (PER THE CIMSS ANALYSIS) MEANS THE ENVIRONMENT HAS DEGRADED TO THE POINT OF BEING OVERALL UNFAVORABLE. INITIAL WIND RADII BASIS: PERSISTENCE CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) OVER SOUTHERN CHINA. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T6.5 - 127 KTS RJTD: T5.5 - 102 KTS CIMSS ADT: 97 KTS AT 271140Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY UNFAVORABLE VWS: 25-30 KTS SST: 29-30 CELSIUS OUTFLOW: STRONG EQUATORWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TY 18W IS RAPIDLY WEAKENING AT THIS POINT, HAVING DROPPED FROM 130 KNOTS TO 105 KNOTS IN 12 HOURS. THE SYSTEM CONTINUES TO TRACK WESTWARD TOWARDS A RENDEZVOUS WITH THE VIETNAMESE COASTLINE, ALONG THE SOUTHERN SIDE OF THE STRONG STEERING RIDGE ENTRENCHED TO THE NORTH. LANDFALL IN THE VICINITY OF DA NANG, VIETNAM, IS EXPECTED WITHIN THE NEXT 12 HOURS, AROUND 280000Z, THOUGH UNCERTAINTY IN THE INITIAL POSITION MEANS THAT THE TRACK SPEEDS COULD BE SLIGHTLY FASTER THAN CURRENTLY ASSESSED, EQUATING TO A SLIGHTLY EARLIER LANDFALL. THE SYSTEM IS EXPECTED TO CONTINUE TO WEAKEN IN THE FACE OF PERSISTENT, STRONG, EASTERLY VERTICAL WIND SHEAR AND INTENSITY AT LANDFALL IS CURRENTLY EXPECTED TO BE AROUND 90 KNOTS. ONCE ASHORE, TERRAIN INDUCED FRICTIONAL EFFECTS COMBINED WITH THE CONTINUING SHEAR WILL LEAD TO EXTREMELY FAST WEAKENING, AND THE SYSTEM IS EXPECTED TO DISSIPATE OVER NORTHEAST THAILAND NO LATER THAN TAU 48, AND POTENTIALLY AS EARLY AS TAU 36. MODEL DISCUSSION: TRACK GUIDANCE IS IN VERY TIGHT AGREEMENT THROUGH THE FORECAST PERIOD, WITH THE MAXIMUM SPREAD OF ONLY 75NM OCCURRING AT TAU 48, THROUGH AS MENTIONED, THE UNCERTAINTY IN THE INITIAL POSITION COULD LEAD TO SOME TRACK SPEED DEVIATIONS. OVERALL CONFIDENCE IN THE TRACK REMAINS HIGH HOWEVER. INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT THE SYSTEM WILL RAPIDLY WEAKEN THROUGH THE FORECAST PERIOD. IN FACT, THE JTWC FORECAST LIES ABOUT 15 KNOTS ABOVE THE CONSENSUS MEMBERS AT TAU 12, THEN REGAINS THE PACK THEREAFTER AS IT IS UNREALISTIC TO SEE 40 KNOTS OF WEAKENING IN 12 HOURS WITH A SYSTEM STILL OVER WATER. DUE TO UNCERTAINTY IN THE INITIAL INTENSITY, WHICH IS ON THE HIGHER END OF THE AVAILABLE ESTIMATES AND THE IMPACT OF THAT ON THE FORECAST TREND, MEANS THERE IS ONLY MEDIUM CONFIDENCE IN THE JTWC FORECAST INTENSITY. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH INTENSITY 0 - 72 HR: MEDIUM// NNNN