WDPN31 PGTW 211500 MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHANTHU) WARNING NR 14// RMKS/ 1. FOR METEOROLOGISTS. 2. 12 HOUR SUMMARY AND ANALYSIS. A. TROPICAL STORM (TS) 04W (CHANTHU), LOCATED APPROXIMATELY 210 NM SOUTH-SOUTHWEST OF HONG KONG, HAS TRACKED NORTHWESTWARD AT 05 KNOTS OVER THE PAST SIX HOURS. AT 0600Z THE STORM WEAKENED TO 45 KNOTS (CONFIRMED BY A 21/0228Z ASCAT AND 3.0/3.5 PGTW DVORAK FIX) DUE TO THE EFFECTS OF AN UPPER LEVEL TROUGH. NOW THAT THE TROUGH HAS BEGUN TO PUSH DOWNSTREAM AND WEAKEN THE STORM HAS REBOUNDED BACK TO 55 KNOTS. PGTW IS CURRENTLY ASSESSING 3.5/3.5 OR 55 KNOTS. RECENT ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS SPIRAL BANDING TIGHTENING AROUND A SMALL BUT PERSISTENT AREA OF CENTRAL CONVECTION. STRONG EQUATORWARD DIFFLUENCE HAS ALSO FACILITATED RE- INTENSIFICATION. HOWEVER, OUTFLOW REMAINS LIMITED ON THE NORTH SIDE OF THE CIRCULATION AS A RESULT OF CONTINUOUS, UPPER LEVEL SUBTROPICAL RIDGING TO THE NORTH. 3. FORECAST REASONING A. THE FORECAST PHILOSOPHY HAS NOT CHANGED SINCE THE PREVIOUS PROGNOSTIC REASONING MESSAGE. ONLY MINOR CHANGES HAD BEEN APPLIED TO THE INTENSITY FORECAST TO ACCOUNT FOR FLUCTUATIONS IN THE INITIAL FORECAST INTENSITIES DESCRIBED IN THE PREVIOUS PARA. B. CHANTHU WILL CONTINUE TO TRACK TOWARDS THE NORTHWEST ALONG THE WESTERN PERIPHERY OF THE DEEP LAYER STEERING RIDGE TO THE NORTHEAST. THE MODEL CONSENSUS HAS STEADIED ON PASSAGE JUST TO THE EAST OF HAINAN WITH LANDFALL OCCURRING NEAR LUICHOW PENINSULA WITHIN 24 HOURS. LOW VERTICAL WIND SHEAR COUPLED WITH ADEQUATE UPPER LEVEL VENTING AND WARM WATERS SHOULD SUPPORT MODEST INTENSIFICATION PRIOR TO LANDFALL. LAND INTERACTION WILL LIKELY MODERATE INTENSIFICATION BEYOND TAU 12.// NNNN