WDPN31 PGTW 080900 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) WARNING NR 009// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 23.9N 146.9E INITIAL INTENSITY: 30 KTS GEOGRAPHIC REFERENCE: 311 NM EAST OF IWO TO MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 01 KT SIGNIFICANT WAVE HEIGHT: 14 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL DEPRESSION 21W WITH A COMPLETELY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) AND CONVECTION FLARING ALONG THE EASTERN PERIPHERY DISPLACED ABOUT 150 NM FROM THE LLCC. THE SYSTEM HAS REMAINED QUASI-STATIONARY OVER THE PAST 6 HOURS WITH LITTLE MOVEMENT. ENVIRONMENTAL ANALYSIS INDICATES THAT 21W IS IN A MARGINALLY FAVORABLE ENVIRONMENT FOR INTENSIFICATION CHARACTERIZED BY MODERATE EQUATORWARD OUTFLOW ALOFT, MODERATE TO HIGH (20-25 KTS) VERTICAL WIND SHEAR, DRY AIR ENTRAINMENT, AND WARM (28-29 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC SEEN IN THE ANIMATED MSI. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT WITH A BUILDING LOW TO MID-LEVEL RIDGE TO THE EAST. AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T2.5 - 35 KTS RJTD: T1.5 - 25 KTS CIMSS SATCON: 39 KTS AT 080600Z CIMSS ADT: 32 KTS AT 080600Z CIMSS AIDT: 35 KTS AT 080600Z CIMSS DPRINT: 25 KTS AT080600Z CIMSS DMINT: 29 KTS AT 080606Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 20-25 KTS SST: 28-29 CELSIUS OUTFLOW: MODERATE EQUATORWARD OTHER FACTORS: EXTENSIVE DRY AIR TO THE WEST. ANALYSIS CONFIDENCE: INITIAL POSITION: HIGH 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: 21W IS FORECAST TO BEGIN TRACKING NORTH-NORTHEASTWARD ALONG THE WESTERN PERIPHERY OF A LOW TO MID-LEVEL RIDGE TO THE EAST THROUGH TAU 72. NEAR TAU 72, THE SYSTEM WILL BEGIN INTERACTING WITH THE BAROCLINIC ZONE WITH EXTRATROPCAL TRANSITION FORECASTED TO COMPLETE BETWEEN TAU 84 TO 96. THE CURRENT QUASI-STATIONARY MOVEMENT AND DRIFTING OF THE VORTEX MAY CAUSE THE FORECAST TRACK TO BE PULLED SLIGHTLY TO THE EAST OR WEST IF IT CONTINUES TO PERSIST. THE TIMING OF THE RIDGE BUILDING AND CREATING A MORE DEFINED STEERING ENVIRONMENT WILL DICTATE THE REMAINDER OF THE FORECAST. REGARDING INTENSITY, 21W IS FORECAST TO GRADUALLY INTENSIFY TO 45 KNOTS AROUND TAU 48 DUE TO THE WIND FIELD BECOMING MORE SYMMETRIC AND THE SEA SURFACE TEMEPERATURES REMAINING CONDUCIVE THROUGH THAT TIME. AFTER TAU 48, THE TRANSITION INTO THE BAROCLINIC ZONE WILL ALLOW THE SYSTEM TO CONTINUE TO INTENSIFY TO AROUND 60 KTS AS POLEWARD OUTFLOW VASTLY IMPROVES. HOWEVER, NEAR TAU 96, VERTICAL WIND SHEAR WILL INCREASE SHARPLY AS THE VORTEX INTERACTS WITH THE JET AND SEA SURFACE TEMPERATURES WILL RAPIDLY DROP. THIS WILL CAUSE THE SYSTEM TO WEAKEN AS IT TRANSITIONS INTO AN EXTRATROPICAL LOW WITH AN EXPANSIVE WIND FIELD. MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT REGARDING THE TRACK OF 21W WITH A 115 NM CROSS-TRACK SPREAD AT TAU 72. MODELS BEGIN TO DIVERGE AFTER TAU 72 AS EXTRATROPICAL TRANSITION BEGINS AND TRACK SPEEDS DIFFER. THE JTWC TRACK FORECAST IS PLACED CLOSE TO CONSENSUS WITH HIGH CONFIDENCE THROUGH TAU 72 AND MEDIUM THEREAFTER. INTENSITY GUIDANCE IS IN MODERATE AGREEMENT WITH COAMPS-TC BEING THE MOST AGGRESSIVE IN THE NEAR TERM SUGGESTING 65 KTS AT TAU 48. HAFS-A AND GFS BOTH DEPICT A MORE GRADUAL INCREASE TO AROUND 40-45 KTS AT TAU 48. GFS AND HAFS-A SLIGHTLY DIFFER ON PEAK INTENSITIES (50 AND 70 KTS RESPECTIVELY) AT TAU 72. THE JTWC INTENSITY FORECAST IS PLACED CLOSER TO HAFS-A AND GFS IN THE NEAR TERM AND SPLITS THE DIFFERENCE WITH A 60 KNOT PEAK WITH MEDIUM CONFIDENCE. FORECAST CONFIDENCE: TRACK 0 - 72 HR: HIGH TRACK 72-120 HR: MEDIUM INTENSITY 0 - 72 HR: MEDIUM INTENSITY 72-120 HR: MEDIUM// NNNN