MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (GAEMI) WARNING NR 06// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (GAEMI), LOCATED APPROXIMATELY 550 NM EAST OF HUE, VIETNAM, HAS TRACKED SOUTHEASTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY DEPICTS A CONSOLIDATING SYSTEM WITH A CENTRAL DENSE OVERCAST CONCEALING THE LOW-LEVEL CIRCULATION CENTER (LLCC). A 021718Z AMSU IMAGE SHOWS IMPROVED CONVECTIVE BANDING WRAPPING INTO THE CENTER; HOWEVER, THE EXACT CENTER IS DIFFICULT TO ASCERTAIN DUE TO THE LOW RESOLUTION OF THE SENSOR. THEREFORE, THERE IS FAIR CONFIDENCE IN THE INITIAL POSITION AND RECENT MOTION. ANIMATED WATER VAPOR IMAGERY INDICATES IMPROVING POLEWARD OUTFLOW AS THE DEEP MIDLATITUDE SHORTWAVE TROUGH TRACKS EASTWARD INTO WESTERN JAPAN. THE 02/12Z 500MB ANALYSIS SHOWS THAT THIS DEEP TROUGH IS MOVING EASTWARD SLOWLY AND ALSO PROVIDES EVIDENCE THAT THE SUBTROPICAL RIDGE (STR) IS BUILDING EASTWARD INTO SOUTHEAST CHINA. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. PEAK INTENSITY HAS BEEN INCREASED TO 65 KNOTS AT TAU 48 AND 72. B. TS 21W IS FORECAST TO SLOW DOWN AND BECOME QUASI-STATIONARY OVER THE NEXT 12 TO 24 HOURS AS THE SYSTEM BEGINS TO TRANSITION TO THE STEERING INFLUENCE OF THE STR BUILDING TO THE NORTH. AS MENTIONED ABOVE, THE STR IS BUILDING INTO SOUTHEAST CHINA BUT RELATIVELY SLOWLY DUE TO THE DEEP, BROAD TROUGH, WHICH HAS DOMINATED EAST CHINA AND THE EAST CHINA SEA. AFTER TAU 24, THE STR IS FORECAST TO BUILD EASTWARD AND WILL PROVIDE A STRONGER WESTWARD STEERING INFLUENCE, WHICH WILL RESULT IN A SHARP WESTWARD TURN AND A MARKED ACCELERATION. DESPITE THE COMPLEX SCENARIO, DYNAMIC MODEL GUIDANCE HAS BEEN CONSISTENT OVER THE PAST 2 DAYS AND IS IN GOOD AGREEMENT. THERE IS SOME UNCERTAINTY, HOWEVER, IN THE TIMING OF THIS WESTWARD TURN AND THE EXACT TRACK MOTION THROUGH TAU 36, THEREFORE, THERE IS LOW CONFIDENCE IN THE FORECAST TRACK THROUGH TAU 48. AFTER TAU 48, THERE IS INCREASING CONFIDENCE DUE TO THE TIGHT GROUPING OF DYNAMIC MODELS, WHICH TRACK THE SYSTEM WESTWARD. TS 21W IS FORECAST TO INTENSIFY THROUGH TAU 72 UNDER GENERALLY FAVORABLE CONDITIONS. C. IN THE EXTENDED TAUS, TS 21W IS FORECAST TO ACCELERATE WESTWARD AND MAKE LANDFALL ALONG THE EAST COAST OF VIETNAM AFTER TAU 72. THE SYSTEM WILL DISSIPATE BY TAU 120 AS IT INTERACTS WITH THE RUGGED TERRAIN OF THIS REGION. DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT, THEREFORE, THERE IS HIGH CONFIDENCE IN THIS PORTION OF THE TRACK FORECAST.// NNNN NNNN