WDPN31 PGTW 122100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 15.1N 152.0E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 359 NM EAST OF SAIPAN MOVEMENT PAST 6 HOURS: EASTWARD AT 07 KTS SIGNIFICANT WAVE HEIGHT: 10 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A COMPACT SYSTEM THAT IS ESSENTIALLY AN IRREGULAR BALL OF CONVECTION INUNDATED BY STRONG NORTHWESTERLY VERTICAL WIND SHEAR, AS INDICATED BY TRANSVERSE BANDING AROUND THE EDGES. THE EIR LOOP ALSO SHOWS A TUTT CELL TO THE NORTH THAT IS SUPPRESSING CONVECTIVE ACTIVITY ALONG THE NORTHERN PERIPHERY OF THE SYSTEM. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON A PARTIAL LOW LEVEL CIRCULATION FEATURE IN THE 121602Z AMSR-2 89GHZ IMAGE. THE INITIAL INTENSITY OF 25KTS IS BASED ON AGENCY DVORAK ESTIMATES OF T1.5 AND IS CONSISTENT WITH THE SUSTAINED 6-HR EIR CONVECTIVE SIGNATURE. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WEAK LOW-LAYER NEAR EQUATORIAL RIDGE TO THE SOUTH. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.5 - 25 KTS KNES: T1.5 - 25 KTS CIMSS SATCON: 29 KTS AT 121532Z CIMSS ADT: 27 KTS AT 121740Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 20-25 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE EASTWARD ANALYSIS CONFIDENCE: INITIAL POSITION: MEDIUM INITIAL INTENSITY: MEDIUM INITIAL WIND RADII: NOT APPLICABLE 3. FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TD 21W WILL BEGIN TO TRACK POLEWARD AS A SUBTROPICAL RIDGE (STR) BUILDS TO THE EAST AND STEERS IT NORTHEASTWARD THEN NORTHWARD. AFTER TAU 48, IT WILL CREST THE STR AXIS AND ACCELERATE NORTHEASTWARD AS THE STR RECEDES, WEAKENED BY A MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST. THE MARGINALLY FAVORABLE ENVIRONMENT WILL FUEL A SLOW INTENSIFICATION TO A PEAK OF 45KTS BY TAU 36. AFTERWARD, INCREASING VWS AND COOLING SST WILL TEMPER ANY FURTHER INTENSIFICATION. BY TAU 72, THE CYCLONE WILL ENTER THE BAROCLINIC ZONE AND BEGIN EXTRA-TROPICAL TRANSITION AND BY TAU 96, WILL TRANSFORM INTO A GALE-FORCE COLD CORE LOW. MODEL DISCUSSION: NUMERICAL MODELS ARE IN OVERALL GOOD AGREEMENT WITH A GRADUAL SPREAD TO APPROXIMATELY 145NM BY TAU 48, THEN SIGNIFICANTLY DIVERGE ALONG- AND ACROSS-TRACK AFTERWARD. IN VIEW OF THIS, THERE IS MODERATE CONFIDENCE IN THE JTWC TRACK FORECAST UP TO TAU 72, THEN LOW AFTERWARD. FORECAST CONFIDENCE: TRACK 0 - 72 HR: MEDIUM TRACK 72-120 HR: LOW INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: LOW// NNNN