WDPN31 PGTW 212100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) WARNING NR 001// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 25.4N 138.9E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 595 NM SOUTH OF YOKOSUKA, JAPAN MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 13 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A LARGE CLUSTER OF DEEP PERSISTENT CONVECTION OVER THE NORTHERN SEMICIRCLE OBSCURING A LOW LEVEL CIRCULATION CENTER (LLCC). AN EARLIER 211632Z GMI 89GHZ MICROWAVE IMAGE REVEALS SHALLOW BANDING WRAPPING INTO A LOW LEVEL CIRCULATION CENTER POSITIONED ALONG THE SOUTHWESTERN EDGE OF A CURVED CONVECTIVE BAND. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON EIR IMAGERY AND GMI MICROWAVE IMAGERY. THE INITIAL INTENSITY OF 25 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE IMPROVING CONVECTIVE STRUCTURE AS WELL AS A COMBINATION OF AGENCY DVORAK INTENSITY ESTIMATES, PGTW T1.0, RJTD T1.0, AND KNES T2.0. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE NORTHEAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.0 - 25 KTS RJTD: T1.0 - 25 KTS KNES: T2.0 - 30 KTS FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 0-5 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY. FORECAST DISCUSSION: TD 17W IS FORECAST TO TRACK POLEWARD ALONG THE WESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) BEFORE MAKING LANDFALL INTO HONSHU. ENVIRONMENTAL CONDITIONS OVER THE NEXT 36 HOURS ARE CHARACTERIZED BY WARM SEA SURFACE TEMPERATURES (SSTS) (28- 29C), LOW VERTICAL WIND SHEAR (VWS) (0-5 KNOTS) AND MODERATE POLEWARD OUTFLOW INDUCED BY AN UPPER LEVEL TROUGH SITUATED TO THE SOUTHWEST OF 17W INCREASING THE POLEWARD OUTFLOW, OFFSET BY DRY AIR ENTRAINMENT. LEADING TO AN OVERALL MARGINALLY FAVORABLE ENVIRONMENT FOR DEVELOPMENT WHICH WILL ALLOW 17W TO REACH A POTENTIAL MAXIMUM INTENSITY OF 45 KNOTS BY TAU 36. BETWEEN TAU 36 AND TAU 48 THE SYSTEM IS FORECAST TO TURN AND TRACK TO THE NORTHEAST AS 17W ROUNDS THE SUBTROPICAL RIDGE AXIS. ADDITIONALLY, A SHORTWAVE TROUGH WITH MODERATE TEMPERATURE ADVECTION AND POTENTIAL VORTICITY ADVECTION (PVA) TO THE NORTHWEST OF THE SYSTEM WILL BEGIN TO BREAK DOWN THE STR TO THE WEST WHILE INCREASING VWS. BY TAU 48, 17W WILL CONTINUE TO WEAKEN AS DECAY SHIPS (DSHA) GUIDANCE INDICATES THE SYSTEM WILL MOVE OVER COOLER SSTS BEFORE MAKING LANDFALL INTO HONSHU. BETWEEN TAU 48 AND TAU 72, LAND INTERACTION WILL SIGNIFICANTLY IMPACT INTENSITY AS THE SYSTEM CONTINUES TO TRACK TO THE NORTHEAST BEFORE DISSIPATING. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN TIGHT AGREEMENT WITH THE EXCEPTION OF THE UKMET TRACKER (EGRI), WHICH IS THE WESTERN OUTLIER, THROUGH TAU 48 WITH ONLY A 55NM SPREAD IN THE SOLUTIONS. BETWEEN TAU 48 AND TAU 72 THE GUIDANCE BEGINS TO SPREAD AS UNCERTAINTY INCREASES IN BOTH CROSS TRACK AND ALONG TRACK VARIABILITY. THIS IS POSSIBLY DUE TO HOW MODELS ARE HANDLING THE LAND INTERACTION AND SHORTWAVE TROUGH INTERACTION DIFFERENTLY. THEREFORE, THE JTWC OFFICIAL TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE AND REMAINS CLOSE TO CONSENSUS MODEL TRACK GUIDANCE THROUGH TAU 72. RELIABLE INTENSITY GUIDANCE SHOWS MODERATE SPREAD WITH HWRF SHOWING A HIGH END SOLUTION UP TO 55 KNOTS BY TAU 36, WHILE DECAY SHIPS (DSHA) GUIDANCE INDICATES A MORE CONSERVATIVE 35 KNOT MAXIMUM INTENSITY BY TAU 36. AFTER TAU 48, LAND INTERACTION AND EVENTUAL LANDFALL OCCURS ALL GUIDANCE SHOWS A SHARP DECREASE IN INTENSITY. THUS THE JTWC INTENSITY FORECAST IS PLACED WITH LOW CONFIDENCE GIVEN THE HIGH UNCERTAINTY DUE TO THE BULK OF THE GUIDANCE DISAGREEING ON THE EXTENT OF THE PEAK INTENSITY. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM INTENSITY 0 - 72 HR: LOW// NNNN