WDPN31 PGTW 220300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SEVENTEEN) WARNING 
NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.0N 119.8E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 93 NM WEST OF TAIPEI, TAIWAN
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 17W WITH CONVECTIVE BURSTS OBSCURING THE LOW-LEVEL
CIRCULATION CENTER (LLCC) AND EXPANDING ACROSS THE ENTIRE TAIWAN
STRAIT. ENVIRONMENTAL ANALYSIS INDICATES THAT 17W IS IN A
MARGINALLY FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE POLEWARD
AND EQUATORWARD OUTFLOW ALOFT, LOW (5-10 KTS) VERTICAL WIND SHEAR,
AND WARM (28-29 C) SEA SURFACE TEMPERATURES OFFSET BY THE CLOSE
PROXIMITY TO LAND. THE INITIAL POSITION IS PLACED WITH LOW
CONFIDENCE BASED ON THE COMPLETELY OBSCURED LLCC SEEN IN THE
ANIMATED MSI. ANIMATED RADAR IMAGERY HELPED POSITION THE SYSTEM BUT
REVEALED THAT THE LLCC MAY NOT BE WELL-DEFINED AND THAT THE
FUNNELING THROUGH THE STRAIT MAY BE THE MAIN SOURCE OF INCREASED
WIND SPEEDS. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE OBJECTIVE INTENSITY ESTIMATES LISTED
BELOW. ADDITIONALLY, NEARBY SHIP OBSERVATIONS REPORT WIND SPEEDS 
UPWARD OF 37 KTS IN THE TAIWAN STRAIT. 

INITIAL WIND RADII BASIS: SURFACE OBSERVATIONS AND MODEL DATA.

CURRENT STEERING MECHANISM: STEERED BY THE NORTHEASTERLY LOW-LEVEL
FLOW.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   DEMS: T1.5 - 25 KTS
   CIMSS ADT: 43 KTS AT 220000Z
   CIMSS AIDT: 38 KTS AT 220000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: PROXIMITY TO LAND

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: 17W IS FORECAST TO CONTINUE TRACKING
SOUTHWESTWARD OVER THE NEXT 12 HOURS AS IT APPROACHES THE COAST OF
CHINA WITH LANDFALL ANTICIPATED TO OCCUR RIGHT AROUND THE TAU 12
MARK. THE ENVIRONMENT IS EXPECTED TO REMAIN RELATIVELY CONDUCIVE AS
17W TRAVERSES THOUGH THE TAIWAN STRAIT AND WILL ALLOW THE SYSTEM TO
MAKE LANDFALL AS BORDERLINE TROPICAL STORM STRENGTH. THE FUNNELING
EFFECT THROUGH THE STRAIT MAY ALSO ALLOW THE INTENSITY TO BRIEFLY
INCREASE BEFORE LANDFALL. AFTER TAU 12, THE SYSTEM IS FORECAST TO
TRACK GENERALLY NORTHWESTWARD THROUGH TAU 24 UNDER THE INFLUENCE OF
THE LOW-LEVEL FLOW AS IS QUICKLY ERODES. DISSIPATION OVER LAND IS
EXPECTED NEAR TAU 24 AS TERRAIN INTERACTION RIPS THE CIRCULATION
APART.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE REMAINS IN POOR
AGREEMENT WITH MANY MODELS LOSING THE VORTEX IN THE TAIWAN STRAIGHT
OR BECOMING ERRATIC VERY QUICKLY. THE JTWC FORECAST CONTINUES WITH
THE SAME PHILOSOPHY AND IS PLACED WITH LOW CONFIDENCE CLOSE TO THE
GFS DETERMINISTIC AND ENSEMBLE MEAN TRACKERS. INTENSITY GUIDANCE IS
IN BETTER AGREEMENT WITH GUIDANCE SUGGESTING 35 KTS NEAR TAU 12
WITH VERY QUICK WEAKENING AFTERWARD. THE JTWC INTENSITY FORECAST IS
PLACED WITH MEDIUM CONFIDENCE, PARTIALLY DUE TO THE MINIMAL TIME
BEFORE LANDFALL.

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