MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (GAEMI) WARNING NR 02// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (GAEMI), LOCATED APPROXIMATELY 435 NM EAST OF HUE, VIETNAM, HAS TRACKED QUASI-STATIONARY OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY DEPICTS A SLOWLY- CONSOLIDATING, LOW-LEVEL CIRCULATION CENTER (LLCC) WITH FRAGMENTED DEEP CONVECTIVE BANDING OVER THE SOUTHERN SEMI-CIRCLE. ANIMATED WATER VAPOR IMAGERY INDICATES THAT THE LACK OF ORGANIZED DEEP CONVECTION OVER THE NORTHERN SEMI-CIRCLE IS LIKELY A RESULT OF INCREASING PRESSURE DUE TO NORTHERLY FLOW ASSOCIATED WITH A DEEP MIDLATITUDE SHORTWAVE TROUGH DIGGING INTO EASTERN CHINA. A 011728Z AMSU IMAGE AS WELL AS A 011324Z ASCAT BULLS-EYE IMAGE AND A 011153Z SSMIS IMAGE PROVIDE EVIDENCE THAT THE SYSTEM HAS BOTH INTENSIFIED AND BECOME QUASI-STATIONARY OVER THE PAST SIX HOURS. THE ASCAT IMAGE DEPICTED 30-35 KNOT WINDS WHILE THE MICROWAVE IMAGERY SHOWS IMPROVED CONVECTIVE BANDING. THE INITIAL POSITION IS BASED ON THIS IMAGERY WITH FAIR CONFIDENCE AND REFLECTS A 30-NM MINOR RELOCATION AT 01/12Z. THE INITIAL INTENSITY IS ASSESSED AT 35 KNOTS BASED ON THE ASCAT DATA AND DVORAK ESTIMATES OF 35 KNOTS. THE 01/12Z 500MB ANALYSIS DEPICTS A LARGE BREAK IN THE SUBTROPICAL RIDGE NORTH OF TS 21W CAUSED BY THE DEEP, BROAD MIDLATITUDE SHORTWAVE TROUGH AND STRONG WESTERLIES EXTENDING AS FAR SOUTH AS 25N. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE; HOWEVER, THE TRACK WAS ADJUSTED DUE TO THE RECENT QUASI-STATIONARY MOTION AND MINOR RELOCATION. B. TS 21W IS FORECAST TO REMAIN EMBEDDED WITHIN A WEAK STEERING ENVIRONMENT THROUGH THE NEXT 36 TO 48 HOURS AND WILL LIKELY DRIFT SOUTHWARD TO SOUTHEASTWARD UNDER THE STEERING INFLUENCE OF THE NEAR- EQUATORIAL RIDGE POSITIONED TO THE SOUTH. AFTER TAU 36, THE SUBTROPICAL RIDGE (STR) IS EXPECTED TO BUILD OVER SOUTHEAST CHINA PROVIDING A STRONG WESTWARD STEERING INFLUENCE. IN GENERAL, DYNAMIC MODEL GUIDANCE IS IN FAIR AGREEMENT WITH ALL MODELS INDICATING THE SOUTHWARD DRIFT THEN ACCELERATION WESTWARD. TS 21W IS FORECAST TO INTENSIFY SLOWLY TO A PEAK INTENSITY OF 55 KNOTS BY TAU 72. DUE TO THE ERRATIC MOTION AND WEAK STEERING INFLUENCE, THERE IS LOW CONFIDENCE IN THE FORECAST TRACK, WHICH IS POSITIONED CLOSE TO THE MULTI-MODEL CONSENSUS. C. IN THE EXTENDED TAUS, TS 21W IS FORECAST TO ACCELERATE WESTWARD AND SHOULD MAKE LANDFALL NEAR TAU 120 ALONG THE CENTRAL COAST OF VIETNAM. DYNAMIC MODEL GUIDANCE IS IN FAIR AGREEMENT WITH SOME MINOR DIFFERENCES IN THE TIMING OF THE WESTWARD TURN AND THE EXACT WESTWARD TRACK. THERE REMAINS LOW CONFIDENCE IN THE FORECAST TRACK THROUGH TAU 120.// NNNN NNNN