WDPN31 PGTW 182100 MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) WARNING NR 002// RMKS/ 1. FOR METEOROLOGISTS. 2. 6 HOUR SUMMARY AND ANALYSIS. SUMMARY: INITIAL POSITION: 8.3N 154.6E INITIAL INTENSITY: 25 KTS GEOGRAPHIC REFERENCE: 167 NM EAST-NORTHEAST OF CHUUK MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 05 KTS SIGNIFICANT WAVE HEIGHT: 12 FEET SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL DEPRESSION (TD) 17W REMAINING QUASI-STATIONARY OVER THE PAST SIX HOURS AND MAINTAINING AN ASYMMETRICAL AND SLIGHTLY ELONGATED CIRCULATION, WITH THE NORTHERN SEMICIRCLE OF LOW-LEVEL CLOUD BANDS EXPOSED WHILE THE LOW LEVEL CIRCULATION CENTER (LLCC) REMAINS OBSCURED BY THE PERSISTENT CONVECTION CONFINED TO THE SOUTHERN SEMICIRCLE. ENVIRONMENTAL CONDITIONS ARE ASSESSED AS MARGINALLY FAVORABLE AS 17W IS SITUATED IN WARM (29-30 C) SEA SURFACE TEMPERATURES (SSTS) AND HAS MODERATE RADIAL OUTFLOW ALOFT AS EVIDENCED BY THE CIRRUS FILAMENTS IN EIR IMAGERY. HOWEVER, THE OUTFLOW ON THE POLEWARD SIDE OF 17W IS BEING VISIBLY SUPPRESSED BY 10-15 KTS OF VERTICAL WIND SHEAR (VWS) WHICH, COMBINED WITH AN AREA OF DRY AIR ON THE NORTHERN SEMICIRCLE OF THE SYSTEM, IS OFFSETTING THE OTHERWISE FAVORABLE CONDITIONS. THE INITIAL POSITION IS PLACED WITH LOW CONFIDENCE AS THE LLCC REMAINS OBSCURED BY CONVECTION, BUT IS CLOSE TO THE PGTW FIX. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON AGENCY DVORAK FIXES AND THE SPREAD OF CIMSS OBJECTIVE ESTIMATES. INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND RADII). CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT OF A NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTHEAST AND A SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE NORTH-NORTHEAST AGENCY DVORAK AND AUTOMATED FIXES: PGTW: T1.0 - 25 KTS RJTD: T1.0 - 25 KTS CIMSS SATCON: 29 KTS AT 181730Z CIMSS ADT: 30 KTS AT 181730Z CIMSS AIDT: 22 KTS AT 181730Z CIMSS D-MINT: 25 KTS AT 181548Z CIMSS D-PRINT: 25 KTS AT 181730Z FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE VWS: 10-15 KTS SST: 29-30 CELSIUS OUTFLOW: MODERATE RADIAL OTHER FACTORS: AREA OF DRY AIR SITUATED TO THE NORTH ANALYSIS CONFIDENCE: INITIAL POSITION: LOW 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 17W IS FORECAST TO GAIN TRACK SPEED AND ESCAPE ITS QUASI-STATIONARY MOVEMENT BY TAU 12 AS THE STEERING ENVIRONMENT BECOMES BETTER DEFINED. IN THE SHORT TERM, 17W IS UNDER THE INFLUENCE OF A BUILDING NEAR-EQUATORIAL RIDGE (NER) CENTERED TO THE SOUTHEAST WHICH WILL DRIVE 17W ON A NORTH-NORTHWESTWARD TRACK. AT THE SAME TIME AN EASTERN EXTENSION OF A SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTH-NORTHWEST IS EXPECTED TO RETREAT BY TAU 36, ALLOWING 17W TO MOVE MORE POLEWARD. BEYOND THIS POINT, A WESTWARD EXTENSION OF THE STR CENTERED TO THE NORTHEAST IS FORECAST TO BUILD IN BETWEEN TAUS 60 AND 72, CAUSING 17W TO TURN SLIGHTLY WESTWARD AND CROSS OVER THE NORTHERN MARIANA ISLANDS BETWEEN TAUS 72 AND 96. REGARDING INTENSITY, TD 17W IS EXPECTED TO GRADUALLY INCREASE OUT TO TAU 36 AS ENVIRONMENTAL CONDITIONS REMAIN MARGINALLY FAVORABLE. BEYOND THIS POINT, THE DRY AIR TO THE NORTH IS EXPECTED TO ABATE AND VWS WILL DECREASE AS 17W BECOMES SITUATED WITHIN A MORE FAVORABLE AREA OF POLEWARD OUTFLOW ALOFT. 17W IS FORECAST TO UNDERGO RAPID INTENSIFICATION (RI) FROM TAU 36 TO TAU 84 AS FAVORABLE CONDITIONS TAKE HOLD, EXPECTED TO CROSS THE NORTHERN MARIANAS AT OVER 100 KTS OF INTENSITY. AFTER THIS POINT, THE OUTFLOW OF 17W WILL BECOME SLIGHTLY ISOLATED FROM THE ENHANCED UPPER LEVEL POSITIONING AND LIKELY PLATEAU IN INTENSITY WHILE ITS NEAR-SUPER TYPHOON STRENGTH WILL BE ABLE TO SUSTAIN ITSELF WITHOUT THE OPTIMAL OUTFLOW. MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT OF THE NORTH-NORTHWESTWARD TRACK OF 17W FOR THE FORECAST PERIOD WITH THE EXCEPTION OF JGSM, WHICH IS INTERPOLATED FAR TO THE NORTH. COMPARED TO THE PREVIOUS CYCLE, BOTH OUTLIERS (HAFS-A AND ECMWF DETERMINISTIC) HAVE MOVED INTO BETTER AGREEMENT WITH THE BULK OF GUIDANCE. WHILE ALL ENSEMBLE MEAN TRACKERS ARE WITHIN THE CLOSE GROUPING, OVERALL ENSEMBLE GUIDANCE REMAINS MORE SPREAD WITH GEFS, ECENS, AND ECAIE ALL CONTAINING MEMBERS THAT TRACK AS FAR SOUTH AS ROTA. THE OFFICIAL JTWC TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE, CLOSELY IN LINE WITH BOTH THE MULTI-MODEL CONSENSUS AND EXPERIMENTAL GOOGLE DEEPMIND, WITH UNCERTAINTY ARISING FROM THE LOW CONFIDENCE INITIAL POSITION, WHICH COULD COMPOUND INTO LARGER UNCERTAINTY BY THE TIME THE SYSTEM CROSSES THE NORTHERN MARIANAS. INTENSITY MODEL GUIDANCE IS IN GENERALLY GOOD AGREEMENT ON A SLOW INTENSIFICATION PERIOD OUT TO TAU 36 FOLLOWED RI LASTING FROM A RANGE OF TAUS 72 TO 96. HAFS-A IS NOTABLY THE ONLY MODEL PREDICTING A WEAKENING TREND AFTER TAU 96, LIKELY DUE TO THE ONSET OF AN EYEWALL REPLACEMENT CYCLE (ERC). THE EXPERIMENTAL COAMPS-TC ENSEMBLE RI PROBABILITY PREDICTS 75 TO 90 PERCENT PROBABILITY OF RI FROM TAUS 36 TO 72 WHILE THE GOOGLE DEEPMIND PREDICTS OVER 75 PERCENT RI PROBABILITY BETWEEN TAUS 48 TO 72. THE OFFICIAL JTWC INTENSITY FORECAST IS ASSESSED WITH HIGH CONFIDENCE OUT TO TAU 72, CLOSELY IN LINE WITH THE MULTI-MODEL CONSENSUS AND MEDIUM CONFIDENCE FROM TAU 72 TO 120, DEVIATING HIGHER THAN THE CONSENSUS. FORECAST CONFIDENCE: TRACK 00-72 HR: MEDIUM TRACK 72-120 HR: MEDIUM INTENSITY 00-72 HR: HIGH INTENSITY 72-120 HR: MEDIUM// NNNN