WDPN31 PGTW 312100 SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING NR 009// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 04W (CHOI-WAN), LOCATED APPROXIMATELY 517 NM EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST- NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS INTENSE (-82C TOPS) DEEP CONVECTION SPREADING OVER AND OBSCURING THE LOW-LEVEL CIRCULATION CENTER (LLCC), WHICH LIKELY INDICATES A CENTRAL COLD COVER PATTERN WITH ARRESTED DEVELOPMENT. A 311711Z AMSR2 36GHZ IMAGE SUPPORTS THE INITIAL POSITION WITH GOOD CONFIDENCE AND SHOWS CONVECTIVE BANDING WRAPPING INTO A DEFINED LLCC. THE PGTW DVORAK INTENSITY ESTIMATE IS BASED ON THE SBC METHOD AND YIELDS A REASONABLE ESTIMATE OF T2.5/3.0 (35/45 KNOTS), WHICH IS CONSISTENT WITH RECENT ASCAT DATA SHOWING 40- 45 KNOT MAXIMUM WINDS OVER THE SOUTH QUADRANT. THE SYSTEM IS LOCATED WITHIN A MARGINALLY-FAVORABLE ENVIRONMENT WITH NORTHEASTERLY VERTICAL WIND SHEAR (VWS) OFFSET BY GOOD EQUATORWARD OUTFLOW AND WARM SST VALUES (29-30C). TS 04W IS TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH AND NORTHEAST. 3. FORECAST REASONING. A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. B. TS 04W IS FORECAST TO TRACK ALONG THE EASTERN COAST OF THE PHILIPPINES WHILE SLOWLY INTENSIFYING TO A PEAK OF 60 KNOTS BY TAU 48. WITH THE EXCEPTION OF GFS AND HWRF, WHICH TRACK THE SYSTEM INTO THE SOUTH CHINA SEA, THE BULK OF THE GUIDANCE SUPPORTS THE JTWC FORECAST WITH MODERATE CONFIDENCE WITH A 115NM SPREAD IN SOLUTIONS AT TAU 72. DUE TO THE PROXIMITY TO LAND, THE INTENSITY FORECAST IS UNCERTAIN. THE 31/12Z ECMWF ENSEMBLE INDICATES UNCERTAINTY IN THE EXACT TRACK WITH POTENTIAL FOR A TRACK FURTHER WEST, WHICH WOULD LEAD TO MORE SIGNIFICANT WEAKENING THROUGH TAU 72. C. IN THE EXTENDED PERIOD, THE WESTERN PORTIONS OF THE STR WILL CONTINUE TO ERODE ALLOWING THE SYSTEM TO RECURVE NORTHEASTWARD. TS 04W WILL WEAKEN AS IT ENCOUNTERS INCREASING VWS ASSOCIATED WITH A MIDLATITUDE SHORTWAVE TROUGH EXPECTED TO DIG OVER THE EAST CHINA SEA. TS 04W IS EXPECTED TO BEGIN EXTRA-TROPICAL TRANSITION (ETT) NEAR TAU 120 BUT MAY DISSIPATE PRIOR TO COMPLETING ETT. MODEL GUIDANCE DIVERGES WITH A LARGE SPREAD IN SOLUTIONS AT TAU 120. THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS FORECAST AND IS POSITIONED CLOSE TO THE MULTI-MODEL CONSENSUS. BASED ON THE UNCERTAINTY IN THE TIMING AND DEGREE OF RECURVATURE, THERE IS MODERATE CONFIDENCE IN THE JTWC FORECAST TRACK.// NNNN