WDPN31 PGTW 260300 MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W WARNING NR 07// RMKS/ 1. FOR METEOROLOGISTS. 2. 12 HOUR SUMMARY AND ANALYSIS. A. TROPICAL STORM (TS) 09W HAS STEADILY INTENSIFIED OVER THE PAST 12 HOURS WHILE TRACKING GENERALLY WEST-SOUTHWESTWARD ALONG A BUILDING SUBTROPICAL STEERING RIDGE EXTENSION ON THE POLEWARD SIDE OF THE STORM. THE LOW LEVEL CIRCULATION CENTER (LLCC) HAS CONSOLIDATED, AND SEVERAL RECENT MICROWAVE SATELLITE PASSES HINT AT THE DEVELOPMENT OF A LOW LEVEL EYE FEATURE. B. THE CURRENT POSITION IS BASED ON RECENT SATELLITE FIXES FROM PGTW AND RJTD AND A 252146Z CORIOLIS PASS. THE CURRENT INTENSITY ESTIMATE OF 55 KNOTS IS CONSISTENT WITH DVORAK T-NUMBERS OF 3.5 FROM BOTH PGTW AND RJTD. STRONG CONVERGENT BANDING ON THE SOUTHERN AND SOUTHEASTERN PERIPHERIES OF THE TROPICAL STORM HAVE DRIVEN EXTENSIVE DEEP CONVECTIVE DEVELOPMENT ALONG THE SOUTHERN AND WESTERN PERIPHERIES OF THE STORM. HOWEVER, PERSISTENT NORTHEASTERLY VERTICAL WIND SHEAR ASSOCIATED WITH THE PERIPHERAL FLOW AROUND AN UPPER LEVEL ANTICYCLONE TO THE NORTH HAS LIMITED CONVECTIVE DEVELOPMENT ALONG THE POLEWARD SEMI-CIRCLE OF THE STORM. A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL TO THE EAST CONTINUES TO PROVIDE A STRONG EASTWARD CONVECTIVE VENTING MECHANISM, WHILE AN EMERGING UPPER LEVEL LOW OVER SOUTHEASTERN CHINA IS BEGINNING TO PROVIDE A PATHWAY FOR SIMILAR, THOUGH WEAKER, WESTWARD OUTFLOW. POLEWARD OUTFLOW TOWARD AN A MIDLATITUDE TROUGH OVER THE YELLOW SEA IS DECREASING AS THE WESTARD EXTENSION OF THE SUBTROPICAL RIDGE TO THE NORTH BUILDS. TS 09W LIES OVER AN AREA OF RELATIVELY LOW OCEAN HEAT CONTENT (OHC), BUT IS APPROACHING A PATCH OF HIGHER OHC TO THE WEST. 3. FORECAST REASONING A. FORECAST INTENSITIES HAVE BEEN ADJUSTED UPWARD SINCE THE PREVIOUS ISSUANCE OF THE PROGNOSITIC REASONING MESSAGE IN ANTICI- PATION OF IMPROVING UPPER LEVEL SUPPORT AND PASSAGE OVER AN AREA OF INCREASING OCEAN HEAT CONTENT. B. THE WESTWARD EXTENSION OF THE CURRENT SUBTROPICAL STEERING RIDGE TO THE NORTH OF THE TROPICAL STORM WILL MERGE WITH A SHORTWAVE RIDGE TO THE NORTHWEST OVER THE NEXT DAY OR TWO, PROVIDING A CONTIN- UOUS AND GENERALLY WESTWARD STEERING INFLUENCE THROUGH THE NEXT 24 TO 36 HOURS. DURING THIS PERIOD, TS 09W WILL EXPERIENCE ENHANCED EQUATORWARD OUTFLOW DRIVEN BY THE TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL TO THE EAST AND WESTWARD OUTFLOW ENHANCED BY THE WEAK UPPER LEVEL TROUGH OVER SOUTHEASTERN CHINA. ADDITIONALLY, THE TROPICAL STORM WILL ENCOUNTER THE PREVIOUSLY-MENTIONED PATCH OF ELEVATED OCEAN HEAT CONTENT AFTER TAU 12. HOWEVER, MODERATE NORTHERLY TO NORTHEASTERLY VERTICAL WIND SHEAR IS EXPECTED TO PERSIST DURING THIS PERIOD AS WELL. THUS, THE FORECAST CALLS FOR STEADY INTENSIFI- CATION OVER THE NEXT 48 HOURS. HOWEVER, GIVEN THE OTHERWISE FAVORABLE ENVIRONMENT, EVEN A TEMPORARY RELAXATION IN VERTICAL WIND SHEAR COULD ALLOW TS 09W TO INTENSIFY EVEN MORE VIGOURSLY THAN THE CURRENT FORECAST REFLECTS. AFTER TAU 36, TS 09W SHOULD TAKE A MORE POLEWARD TRACK AS IT REACHES THE SOUTHWESTERN PERIPHERY OF THE STEERING RIDGE. IF THIS TRACK VERIFIES, THE STORM WILL MAKE LANDFALL OVER CENTRAL TAIWAN AND EXPERIENCE A BRIEF PERIOD OF ERRATIC MOTION AS IT INTERACTS WITH HIGH TERRAIN OF THE ISLAND. FOLLOWING THIS INTERACTION, TS 09W WILL REEMERGE IN THE TAIWAN STRAIT AS A WEAKENED SYSTEM AND APPROACH THE SOUTHEASTERN COAST OF CHINA. C. TS 09W IS EXPECTED TO MAKE LANDFALL IN SOUTHEASTERN CHINA AROUND TAU 72 AND WEAKEN THEREAFTER AS IT MOVES INLAND, DISSIPATING BELOW THE WARNING INTENSITY THRESHOLD OF 25 KNOTS BY TAU 120. THE SYSTEM WILL TAKE A MORE POLEWARD TRACK AROUND THE WESTERN PERIPHERY OF THE STEERING RIDGE AS IT COMPLETES THIS DISSIPATION. THE NUMERICAL MODELS ARE IN GOOD GENERAL AGREEMENT WITH BOTH THE NEAR AND EXTENDED TERM FORECASTS, AND THE MODEL CONSENSUS HAS STAYED VERY CONSISTENT OVER THE PAST SEVERAL RUNS. FORECAST TEAM: ALPHA// NNNN