WDPN31 PGTW 040900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (PEIPAH) WARNING NR 004// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 31.0N 131.5E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 486 NM WEST-SOUTHWEST OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 15 KTS SIGNIFICANT WAVE HEIGHT: 18 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH BUILDING CONVECTION ALONG THE NORTHERN PERIPHERY. THE CIRCULATION HAS TRACKED NORTHWARD, JUST OFF THE COAST OF SOUTHEASTERN KYUSHU. AN 89GHZ GMI MICROWAVE IMAGE REVEALS WEAK LOW-LEVEL WRAPPING WITH DEEP CONVECTION CONFINED TO THE NORTHERN SEMI-CIRCLE. THE ENVIRONMENT IS ASSESSED TO BE MARGINALLY FAVORABLE, CHARACTERIZED BY LOW TO MODERATE (15-20 KTS) VERTICAL WIND SHEAR (VWS), VERY WARM (30-31 C) SEA SURFACE TEMPERATURES (SST), AND STRONG POLEWARD OUTFLOW, OFFSET BY TERRAIN INTERACTION OVER KYUSHU. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE AFOREMENTIONED ANIMATED MSI AND 89GHZ GMI MICROWAVE IMAGE. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE LISTED AGENCY DVORAK FIXES AND OBJECTIVE INTENSITY ESTIMATES BELOW. INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE TO THE EAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.0 - 30 KTS RCTP: T2.5 - 35 KTS KNES: T2.5 - 35 KTS CIMSS ADT: 35 KTS AT 040540Z CIMSS AIDT: 39 KTS AT 040540Z CIMSS D-MINT: 36 KTS AT 040603Z CIMSS D-PRINT: 28 KTS AT 040540Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 15-20 KTS SST: 30-31 CELSIUS OUTFLOW: STRONG POLEWARD OTHER FACTORS: TERRAIN INTERACTION ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TROPICAL STORM (TS) 21W IS FORECAST TO TRACK NORTHEASTWARD ALONG THE NORTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE AND SKIRT THE SOUTHERN COAST OF JAPAN THROUGH TAU 24. AFTERWARD, AN UPPER-LEVEL TROUGH IS EXPECTED TO DEEPEN OVER CENTRAL HONSHU AND INTERACT WITH THE WIND FIELD OF 21W, PULLING IT EASTWARD. THE SYSTEM IS FORECAST TO QUICKLY BECOME ABSORBED INTO THE FORMING FRONTAL BOUNDARY ASSOCIATED WITH THE DEEP LAYER TROUGH. AS A RESULT, EXTRATROPICAL TRANSITION IS FORECAST TO BEGIN NEAR TAU 24 AND COMPLETE NO LATER THAN TAU 48. 21W IS EXPECTED TO INTENSIFY SLIGHTLY AS IT SKIRTS THE COAST DUE TO THE ROBUST POLEWARD OUTFLOW AND VERY WARM SST. THE STRONGEST WINDS ARE FORECAST TO REMAIN OFF THE COAST OF JAPAN UNTIL AFTER 21W TRACKS COMPLETELY OFFSHORE BETWEEN TAUS 24 AND 36. AS 21W TRACKS AWAY FROM MAINLAND JAPAN, THE WIND FIELD IS EXPECTED TO RAPIDLY EXPAND IN ASSOCIATION WITH THE DEVELOPING MID-LATITUDE CYCLONE. SOME GLOBAL MODELS DO SHOW A POSSIBILITY FOR 21W TO SEPARATE FROM THE FRONTAL BOUNDARY AND REMAIN QUASI-STATIONARY NEAR THE 150E LONGITUDE BEFORE DISSIPATING, ADDING SOME UNCERTAINTY TO THE END OF THE FORECAST. MODEL DISCUSSION: DETERMINISTIC MODELS ARE IN GOOD AGREEMENT THROUGH TAU 24, BUT QUICKLY DIVERGE AS INTERACTION WITH THE DEEP LAYER TROUGH COMMENCES. THE NORTHERNMOST MODELS (AI MODELS, HAFS, HWRF, AND EGRR) TRACK THE CIRCULATION ASSOCIATED WITH THE MID-LATITUDE CYCLONE, DRIVING THE VORTEX TO ACCELERATE TO THE NORTHEAST. THE SOUTHERNMOST MODELS (GFS, JGSM, AND ECMWF) TRACK THE REMNANT VORTICITY ASSOCIATED WITH 21W AFTER IT GETS ABSORBED INTO THE FRONTAL BOUNDARY. THE JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE THROUGH TAU 24 AND LOW CONFIDENCE THROUGH TAU 48. INTENSITY GUIDANCE IS IN POOR AGREEMENT FOR THE FORECAST PERIOD. HAFS-A AND HWRF TRACK THE MID-LATITUDE CYCLONE AND THEREFORE RAPIDLY INTENSITY AFTER TAU 24. ON THE OTHER HAND, THE GFS AND ECMWF WEAKEN 21W AS IT BECOMES ABSORBED INTO THE FRONTAL BOUNDARY. THE SPLIT IN INTENSITY GUIDANCE CAUSES THERE TO BE LOW CONFIDENCE IN THE JTWC INTENSITY FORECAST AFTER TAU 24. FORECAST CONFIDENCE: TRACK 00-72 HR: LOW INTENSITY 00-72 HR: LOW// NNNN