WDPN31 PGTW 030300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (GAEMI) WARNING NR 07// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM 21W (GAEMI), LOCATED APPROXIMATELY 600 NM EAST OF HUE, VIETNAM, HAS TRACKED EAST-SOUTHEASTWARD AT 05 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED ON A RECENT SATELLITE FIX FROM PGTW AND A 022148Z TRMM PASS. THE CURRENT INTENSITY OF 50 KNOTS IS CONSISTENT WITH DVORAK INTENSITY ESTIMATES FROM 45 TO 55 KNOTS AND A RECENT CIMSS SATCON ESTIMATE OF 53 KNOTS. RECENT ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS DEEP CONVECTION PERSISTING OVER THE SOUTHERN AND WESTERN PERIPHERIES OF A PARTIALLY- EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) EVIDENT IN THE AFOREMENTIONED TRMM PASS. TS 21W LIES JUST TO THE SOUTHWEST OF AN UPPER-LEVEL ANTICYCLONE CENTER, WITH LOW TO MODERATE NORTHEASTERLY VERTICAL WIND SHEAR AND FAVORABLE WESTWARD OUTFLOW ALOFT. TS 21W IS TURNING EQUATORWARD AS THE PRIMARY STEERING INFLUENCE TRANSITIONS FROM A NEAR-EQUATORIAL RIDGE TO THE SOUTH TO A BUILDING SUBTROPICAL RIDGE TO THE NORTH. 3. FORECAST REASONING. A. THERE IS NO MAJOR CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. THE TAU 12 THROUGH TAU 24 FORECAST POSITION HAVE BEEN ADJUSTED SLIGHTLY WESTWARD FROM THE PREVIOUS WARNING BECAUSE THE SYSTEM HAS TRACKED A BIT FARTHER TO THE WEST IN THE PAST SIX HOURS THAN PREVIOUSLY EXPECTED. B. TS 21W IS FORECAST TO SLOW DOWN AND BECOME QUASI-STATIONARY OVER THE NEXT 12 TO 24 HOURS AS THE SYSTEM CONTINUES TRANSITIONING TO THE STEERING INFLUENCE OF THE SUBTROPICAL RIDGE BUILDING TO THE NORTH. THE SUBTROPICAL RIDGE IS BUILDING RELATIVELY SLOWLY AS A DEEP, BROAD TROUGH, WHICH HAS DOMINATED EAST CHINA AND THE EAST CHINA SEA, MOVES OUT. AFTER TAU 24, THE SUBTROPICAL RIDGE WILL BUILD MORE RAPIDLY EASTWARD AND PROVIDE A STRONGER WESTWARD STEERING INFLUENCE, RESULTING 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 36. AFTER TAU 36, 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