MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (DUJUAN) WARNING NR 09// RMKS// 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. TROPICAL STORM (TS) 21W (DUJUAN), LOCATED APPROXIMATELY 577 NM SOUTHEAST OF KADENA AB, JAPAN HAS TRACKED WEST-NORTHWESTWARD AT 09 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY CONTINUES TO DEPICT A PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH DEEP CONVECTION DISPLACED OVER THE SOUTHWEST QUADRANT DUE TO PERSISTENT NORTHEASTERLY VERTICAL WIND SHEAR (VWS). A 231904Z SSMIS 37GHZ IMAGE INDICATES THAT THE SYSTEM HAS CONSOLIDATED SLIGHTLY WITH DEEP CONVECTION BUILDING OVER THE LLCC WITH CONVECTIVE BANDING OVER THE SOUTHERN SEMI-CIRCLE WRAPPING INTO A MORE DEFINED LLCC. OVERALL, THERE IS GOOD CONFIDENCE IN THE INITIAL POSITION BASED ON THE SSMIS IMAGE. THE INITIAL INTENSITY IS ASSESSED AT 45 KNOTS BASED ON THE 231150Z ASCAT IMAGE. UPPER LEVEL ANALYSIS INDICATES A MARGINAL ENVIRONMENT WITH MODERATE (20 KNOTS) NORTHEASTERLY VWS OFFSET BY DIFFLUENT OUTFLOW. TS DUJUAN IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP LAYERED SUB-TROPICAL RIDGE (STR). 3. FORECAST REASONING. A. BASED ON IMPROVED, MORE STABLE MODEL GUIDANCE AND A DETAILED EXAMINATION OF RECENT 500MB ANALYSES, THE FORECAST PHILOSOPHY NOW SHOWS A STAIR-STEP TRACK VICE A RE-CURVE SCENARIO. B. THE RECENT 500MB ANALYSES DEPICT A SHORTWAVE TROUGH OVER THE KOREAN PENINSULA EXTENDING SOUTHWARD INTO THE EAST CHINA SEA. DUE TO THIS TROUGH, TS 21W IS FORECAST TO TURN NORTHWESTWARD AND TRACK SLOWLY TOWARD THE WEAKNESS IN THE SUBTROPICAL RIDGE (STR) THROUGH TAU 36. THIS TROUGH IS FORECAST TO WEAKEN AND QUICKLY PROPAGATE EASTWARD, THEREFORE, THE STR SHOULD GRADUALLY RE-BUILD POLEWARD OF TS 21W AFTER TAU 36. DESPITE A PERSISTENT BREAK IN THE STR (BETWEEN THE WESTERN AND EASTERN STR), MODEL GUIDANCE GENERALLY INDICATES ZONAL WESTERLY FLOW WITH NO DYNAMIC MIDLATITUDE SHORTWAVE TROUGHS CAPABLE OF INDUCING A RE-CURVE SCENARIO. DYNAMIC GUIDANCE REMAINS UNCERTAIN, HOWEVER, THE MAJORITY OF THE GUIDANCE IS IN GOOD AGREEMENT THROUG TAU 72. TS 21W IS FORECAST TO SLOWLY CONSOLIDATE DUE TO THE BROAD NATURE OF THE LLCC AND MODERATE VWS THROUGH TAU 36, HOWEVER, 12W SHOULD INTENSIFY AT A GREATER RATE AFTER TAU 36 AS VWS DECREASES AND POLEWARD OUTFLOW IMPROVES. C. IN THE EXTENDED TAUS, TS 21W WILL TURN WESTWARD AS IT BEGINS TO TRACK UNDER THE STEERING INFLUENCE OF THE WESTERN STR. DYNAMIC GUIDANCE HAS IMPROVED WITH THE MAJORITY OF MODELS NOW SHOWING A WESTWARD TRACK NEAR TAIWAN. TS 21W SHOULD CONTINUE TO INTENSIFY UNTIL IT APPROACHES TAIWAN AT WHICH POINT IT WILL WEAKEN DUE TO LAND INTERACTION. OVERALL, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST TRACK DUE TO MODEL UNCERTAINTY IN THE EARLY PHASE OF THE FORECAST.// NNNN NNNN