Advisories

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//
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