Advisories

WDPN31 PGTW 231500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 021//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 22.6N 139.4E
   INITIAL INTENSITY: 110 KTS
   GEOGRAPHIC REFERENCE: 675 NM EAST-SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 46 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS THE CORE
OF TYPHOON 17W (SAUDEL) CONTINUING TO REORIENT AS THE SYSTEM CONTINUES 
TO UNDERGO AN EYEWALL REPLACEMENT CYCLE (ERC). THE MOST RECENT HIGH 
RESOLUTION MICROWAVE IMAGERY OVER THE SYSTEM WAS A 230811Z WSFM 37 GHZ 
IMAGE, WHICH SHOWED THE INNER EYEWALL CONTINUING TO ERODE AND BEING 
SURROUNDED BY THE MORE DOMINANT OUTER EYEWALL. THE LACK OF MORE RECENT 
MICROWAVE IMAGERY CAUSES THERE TO BE SOME UNCERTAINTY IN THE 
PROGRESSION OF THE ERC. THE EIR DOES SHOW A DISTINCT WEAKNESS IN THE 
NORTHWESTERN QUADRANT OF THE EYEWALL, INDICATING THAT THE INNER 
EYEWALL IS LIKELY BEING CHOKED OFF AND ERODING IN THE LATER STAGES OF 
THE ERC TIMELINE.  THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE 
BASED ON THE EYE IN ANIMATED EIR AND SUPPORTED BY A TIMELY 231143Z 
METOP-B ASCAT PASS. THE INITIAL 34 AND 50 KT WIND RADII ARE PLACED 
WITH HIGH CONFIDENCE BASED ON THE ASCAT DATA. THE INITIAL INTENSITY OF 
110 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK 
FIXES AND CIMSS INTENSITY ESTIMATES LISTED BELOW. ENVIRONMENTAL 
ANALYSIS INDICATES THAT 17W REMAINS PLACED WITHIN A FAVORABLE 
ENVIRONMENT CHARACTERIZED BY STRONG RADIAL OUTFLOW ALOFT, LOW (5-10 
KTS) VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: 231143Z METOP-B ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF AN ELONGATED
SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   RJTD: T5.5 - 102 KTS
   RCTP: T6.0 - 115 KTS
   KNES: T5.0 - 90 KTS
   CIMSS ADT: 104 KTS AT 231200Z
   CIMSS AIDT: 103 KTS AT 231200Z
   CIMSS D-PRINT: 101 KTS AT 231200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: ONGOING EYEWALL REPLACEMENT CYCLE 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TY 17W IS FORECAST TO CONTINUE TRACKING WEST-
NORTHWESTWARD, ALONG THE SOUTHWESTERN PERIPHERY OF THE STR TO THE 
NORTHEAST THROUGH TAU 12. NEAR TAU 12, THE STR IS EXPECTED TO MERGE 
WITH A SEPARATE STR CENTERED OVER SOUTHERN JAPAN, WHICH WILL KEEP 17W 
ON A WEST-NORTHWESTWARD TRAJECTORY THROUGH TAU 60. THE SYSTEM IS 
FORECAST TO SKIRT THE NORTHERN COAST OF OKINAWA BETWEEN TAU 48 AND 60. 
AFTER TAU 60, TRACK SPEEDS ARE EXPECTED TO SLOW THROUGH TAU 96 AS THE 
SYSTEM ENTERS A WEAKNESS IN THE STEERING PATTERN AS THE PRIMARY 
STEERING INFLUENCE TRANSITIONS TO A STR CENTERED OVER EASTERN CHINA. 
17W IS THEN FORECAST TO TRACK WEST-SOUTHWESTWARD, ALONG THE 
SOUTHEASTERN EXTENT OF THAT STR THROUGH THE REMAINDER OF THE FORECAST 
PERIOD. A FINAL LANDFALL IS CURRENTLY FORECAST TO OCCUR JUST BEFORE 
TAU 120, NORTHWEST OF TAIWAN, THOUGH THERE IS SOME UNCERTAINTY IN THE 
TIMING AND LOCATION OF THE LANDFALL. IN TERMS OF INTENSITY, 17W IS 
FORECAST TO MAINTAIN ITS CURRENT INTENSITY OF 110 KTS THROUGH TAU 24 
AS THE SYSTEM ATTEMPTS TO RECOVER FROM THE ONGOING ERC. 17W IS ALSO 
ENTERING WATERS CONTAINING A LOWER OCEAN HEAT CONTENT (OHC), WHICH 
WILL ALSO HINDER THE CHANCES FOR REDEVELOPMENT AFTER THE ERC 
COMPLETES. AROUND TAU 24, THE UPPER-LEVEL TROUGH TO THE WEST OF 17W 
WILL FILL, CAUSING THE POLEWARD OUTFLOW CHANNEL TO WEAKEN. 
ADDITIONALLY, NORTHERLY SHEAR WILL INCREASE, CAUSING 17W TO GRADUALLY 
WEAKEN AS IT APPROACHES OKINAWA. ONCE THE SYSTEM ENTERS THE EAST CHINA 
SEA, NORTHEASTERLY SHEAR INDUCED BY THE STRONG RIDGE OVER CHINA WILL
CAUSE FURTHER WEAKENING THROUGH THE REMAINDER OF THE FORECAST PERIOD. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE REMAINS IN GOOD AGREEMENT
THROUGH TAU 72 WITH ONLY NAVGEM BEING AN OUTLIER, WHICH TRACKS THE 
VORTEX MUCH FURTHER SOUTH. DISCOUNTING NAVGEM, THE CROSS-TRACK SPREAD 
AT TAU 72 IS MEASURED TO ONLY BE ABOUT 90 NM. AFTER TAU 72, THERE IS 
QUITE A BIT MORE SPREAD BETWEEN MODELS. NOTABLY, THE GFS SHOWS A 
SHARPER SOUTHWESTWARD TURN, MAKING LANDFALL IN NORTHERN TAIWAN. ON THE 
OTHER HAND, GALWEM SHOWS NO SOUTHWESTWARD TURN AT ALL AND ACTUALLY 
SHOWS A TURN TO THE NORTHWEST. THE JTWC TRACK FORECAST IS PLACED NEAR 
THE MULTI-MODEL CONSENSUS THROUGH TAU 72 WITH HIGH CONFIDENCE AND THEN 
CLOSER TO THE EXPERIMENTAL GOOGLE DEEPMIND ENSEMBLE MEAN THROUGH TAU 
120 WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE GENERALLY SHOWS A 
STEADY WEAKENING TREND FROM TAU 24 ONWARD, HOWEVER, THERE ARE SOME 
MODELS THAT DIFFER. FIRST, COAMPS-TC SHOWS THE SYSTEM MAINTAINING ITS 
CURRENT INTENSITY THROUGH TAU 60 BEFORE WEAKENING. HAFS-A DEPICTS A 
PERIOD OF REINTENSIFICATION AFTER PASSING OKINAWA AND IS THE ONLY 
MODEL TO DO SO. THE PRIMARY REASON FOR THIS IS ATTRIBUTED TO A 
SIGNIFICANTLY SHARPER SOUTHWESTWARD TURN TOWARD TAIWAN, ALLOWING FOR 
THE VORTEX TO NOT FEEL THE NORTHEASTERLY SHEAR NEARLY AS MUCH. 
OVERALL, THE JTWC INTENSITY FORECAST IS PLACED CLOSE TO THE CONSENSUS 
THROUGH TAU 72 AND THEN WITHIN THE MAIN GUIDANCE ENVELOPE THROUGH TAU 
120 WITH MEDIUM CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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