WDPN31 PGTW 211500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.9N 122.4E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 68 NM NORTHEAST OF TAIPEI, TAIWAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP
CONVECTION OBSCURING THE LOW-LEVEL CIRCULATION CENTER. MODERATE
10-15KT NORTH-NORTHEASTERLY VERTICAL WIND SHEAR HAS ALLOWED THE
CONVECTION TO BECOME MORE CENTERED OVER THE LOW-LEVEL CIRCULATION
CENTER. THE SYSTEM APPEARS TO HAVE STALLED WHILE IN A CONFUSED AND
WEAK STEERING ENVIRONMENT. THE INITIAL POSITION IS PLACED WITH LOW
CONFIDENCE BASED ON A COLOR-ENHANCED 210918Z F-17 SSMIS 37GHZ IMAGE
REVEALS LOW-LEVEL CURVATURE AND THE AFOREMENTIONED EIR. THE INITIAL
INTENSITY OF 25 KTS IS ASSESSED WITH LOW CONFIDENCE BASED ON THE
AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: WEAKLY STEERED BY NORTHEASTERLY
LOW-LEVEL FLOW

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.0 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T1.0 - 25 KTS
   CIMSS ADT: 31 KTS AT 211130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: LOW
   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: TROPICAL STORM (TS) 17W WILL REMAIN IN A WEAK
STEERING ENVIRONMENT BUT EVENTUALLY BE INFLUENCED
WEST-SOUTHWESTWARD BY LOW-LEVEL NORTHEASTERLY FLOW THROUGH THE
TAIWAN STRAIT. WHILE IN A QUASI-STATIONARY STATE, THE SYSTEM WILL
SLIGHTLY INTENSIFY TO 35KTS WHILE IN A MARGINALLY FAVORABLE
ENVIRONMENT WITH WARM SEA SURFACE TEMPERATURES, WEAK OUTFLOW, AND
MODERATE SHEAR THAT WILL PERSIST FOR THE NEXT 36 HOURS. AFTER TAU
24, TS 17W WILL MAKE LANDFALL AND DISSIPATE AFTER INTERACTING WITH
THE TOPOGRAPHY OF MAINLAND CHINA.

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS NOT IN GOOD
AGREEMENT AND BIFURCATES STARTING AT TAU 0. THE MAJORITY OF THE
TRACKERS, ECMWF, GFS, GEFS, AND COAMPS-TC TRACK SOUTHWESTWARD WHILE
UKMET AND THE UKMET ENSEMBLE TRACK NORTHWESTWARD. THE VARIABILITY
OF THE MODEL SOLUTIONS LENDS TO LOW CONFIDENCE IN THE FORECAST
TRACK. THE INTENSITY GUIDANCE FOLLOWS THE VARIETY OF THE TRACK
GUIDANCE AND IS HIGHLY DEPENDENT ON WHERE THE SYSTEM ULTIMATELY
TRACKS. IF IT WERE TO MOVE NORTH OR NORTHEAST, TS 17W WOULD BE MET
WITH MORE FAVORABLE CONDITIONS, SUCH AS WIND SHEAR, THAT IS NOT
REPRESENTED IN THE JTWC INTENSITY FORECAST.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   INTENSITY 0 - 72 HR: LOW//
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