WDPN31 PGTW 120300 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (MERBOK) WARNING NR 006// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 21.4N 161.4E INITIAL INTENSITY: 35 KTS GEOGRAPHIC REFERENCE: 318 NM WEST-NORTHWEST OF WAKE ISLAND MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 05 KTS SIGNIFICANT WAVE HEIGHT: 15 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: UPPER-LEVEL CONDITIONS ARE COMPLEX: AN ANTICYCLONE IS CENTERED NEAR 18N 159E WITH A NARROW TUTT POSITIONED ALONG THE NORTHERN PERIPHERY OF THE SYSTEM WHICH IS PRODUCING VIGOROUS WEST-SOUTHWESTERLY FLOW OVER THE NORTHERN HALF OF THE SYSTEM. STRONG DIFFLUENCE OVER THE SOUTHERN HALF OF THE SYSTEM IS MAINTAINING PERSISTENT CURVED CONVECTIVE BANDING BUT LIMITING OVERALL DEVELOPMENT. ADDITIONALLY, ROBUST POLEWARD OUTFLOW INTO THE TUTT CELL TO THE NORTH IS IMPROVING QUICKLY. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS FRAGMENTED BANDING WITH A CLUSTER OF FLARING CONVECTION OVER THE EASTERN QUADRANT. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON AN 112329Z ASCAT-C BULLSEYE IMAGE SHOWING AN ELONGATED CENTER WITH A SWATH OF 30-35 KNOT WINDS OVER THE NORTHERN SEMICIRCLE. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE PGTW DVORAK ESTIMATE AND RECENT ASCAT DATA. INITIAL WIND RADII BASIS: SCATTEROMETER DATA CURRENT STEERING MECHANISM: STEERING IS CURRENTLY TRANSITIONING FROM THE NEAR-EQUATORIAL RIDGE TO THE SOUTHEAST TO THE NORTH-SOUTH ORIENTED STR POSITIONED TO THE EAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T2.0 - 30 KTS KNES: T1.5 - 25 KTS CIMSS SATCON: 42 KTS AT 112234Z CIMSS ADT: 32 KTS AT 112330Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE POLEWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: HIGH INITIAL WIND RADII: LOW 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: AS FORECAST WELL BY THE NUMERICAL MODELS, THE STR HAS RE-ALIGNED IN A MORE NORTH-SOUTH CONFIGURATION WITH A STRONG TUTT CELL POSITIONED TO THE NORTH NEAR 31N 164E. CONSEQUENTLY, TS 15W IS BEGINNING TO TURN MORE POLEWARD AND IS FORECAST TO TRACK NORTHWARD THROUGH TAU 60 THEN NORTH-NORTHEASTWARD THROUGH TAU 72 AS IT BEGINS TO APPROACH THE STR AXIS. TS 15W IS EXPECTED TO INTENSIFY SLOWLY THROUGH TAU 24 UNDER MARGINALLY-FAVORABLE CONDITIONS THEN AT A FASTER RATE OF INTENSIFICATION AFTER TAU 24 AS VERTICAL WIND SHEAR DECREASES TO LOW LEVELS AIDED BY STRONG POLEWARD OUTFLOW. THE PEAK INTENSITY OF 85 KNOTS IS EXPECTED TO OCCUR NEAR TAU 72. NEAR TAU 72, SST WILL GRADUALLY COOL WITH EXTRA-TROPICAL TRANSITION (ETT) EXPECTED TO COMMENCE AS THE SYSTEM BEGINS TO INTERACT WITH THE MIDLATITUDE WESTERLIES AND BAROCLINIC ZONE. TS 15W WILL COMPLETE ETT BY TAU 96 AS IT GAINS FRONTAL CHARACTERISTICS AND ACCELERATES NORTHEASTWARD WITHIN THE STRONG MIDLATITUDE SOUTHWESTERLY FLOW. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 72 WITH A 120NM SPREAD IN SOLUTIONS AT TAU 72. THE 111800Z ECMWF ENSEMBLE (EPS) AND GFS ENSEMBLE (GEFS) ARE ALSO IN TIGHT AGREEMENT LENDING HIGH CONFIDENCE TO THE JTWC FORECAST TRACK. AFTER TAU 72, THE DETERMINISTIC GUIDANCE DIVERGES, MAINLY ALONG-TRACK, WITH A SIGNIFICANT DIFFERENCE IN TRACK SPEEDS THUS LOW CONFIDENCE. RELIABLE INTENSITY GUIDANCE IS IN FAIR AGREEMENT AND INDICATES RAPID INTENSIFICATION (RI): COAMPS-TC (GFS VERSION) INDICATES RI WITH A PEAK INTENSITY OF 98 KNOTS AT TAU 36 WHILE HWRF INDICATES RI AND A PEAK INTENSITY OF 87 KNOTS AT TAU 48. WHILE RI IS FORECASTED AFTER TAU 12, IT IS NOT LIKELY TO BEGIN NOW DUE TO THE MARGINALLY-FAVORABLE UPPER-LEVEL CONDITIONS THUS THE JTWC FORECAST IS SET AT A LOWER PEAK OF 85 KNOTS AT TAU 48. THE COAMPS-TC ENSEMBLE INDICATES A 80 TO 100 PERCENT PROBABILITY OF RI IN THE TAU 12 TO TAU 30 TIMEFRAME. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN