Advisories

WDPN31 PGTW 240900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 024//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.5N 134.7E
   INITIAL INTENSITY: 105 KTS
   GEOGRAPHIC REFERENCE: 392 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 MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TYPHOON 17W
(SAUDEL) WITH SIGNIFICANTLY WARMING CLOUD TOPS SURROUNDING THE EYEWALL 
OVER THE PAST 6 HOURS. ON THE OTHER HAND, THE CENTRAL DENSE OVERCAST 
(CDO) HAS BECOME MORE SYMMETRIC AND THE EYE HAS WARMED TO 12 C. THIS 
STRUCTURE INDICATES THAT THE EYEWALL REPLACEMENT CYCLE (ERC) HAS 
LIKELY CONCLUDED WITH 17W SHOWING A MORE REGULAR APPEARANCE. THE 
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE DEFINED 
EYE IN ANIMATED MSI. THE INITIAL INTENSITY OF 105 KTS IS ASSESSED WITH 
MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK AND CIMSS INTENSITY 
ESTIMATES LISTED BELOW. THE INITIAL INTENSITY IS SET CLOSER TO THE 
CIMSS ESTIMATES AND AGENCY DATA NUMBERS OF 5.5 RATHER THAN THE CI OF 
6.0. 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: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: THE SOUTHWESTERN FLANK OF A DEEP-LAYER 
SUBTROPICAL RIDGE (STR) WHICH EXTENDS FROM SOUTHERN JAPAN 
SOUTHEASTWARD TO A POINT NORTHEAST OF GUAM.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   RJTD: T6.0 - 115 KTS
   RCTP: T6.0 - 115 KTS
   KNES: T6.0 - 115 KTS
   CIMSS ADT: 109 KTS AT 240600Z
   CIMSS AIDT: 106 KTS AT 240600Z
   CIMSS D-MINT: 101 KTS AT 240445Z
   CIMSS D-PRINT: 105 KTS AT 240600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG RADIAL

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

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 THROUGH TAU 48, PASSING JUST NORTH OF OKINAWA AT AROUND 
TAU 36. NEAR TAU 38, 17W WILL APPROACH A WEAKNESS IN THE STEERING 
RIDGE TO THE NORTH, CAUSING IT TO MARKEDLY SLOW AS THE PRIMARY 
STEERING MECHANISM SHIFTS TO A SUBTROPICAL RIDGE (STR) CENTERED OVER 
EASTERN CHINA. TRACK SPEEDS ARE EXPECTED TO REMAIN SLOW THROUGH TAU 72 
BEFORE THE STEERING GRADIENT BECOMES STRONGER. AFTER TAU 72, THE SPEED 
OF ADVANCE OF 17W IS FORECAST TO INCREASE WITH A SOUTHWESTWARD 
TRAJECTORY AS IT TRACKS ALONG THE SOUTHEASTERN PERIPHERY OF THE STR 
OVER CHINA THROUGH THE REMAINDER OF THE FORECAST PERIOD. LANDFALL OVER 
EASTERN CHINA IS CURRENTLY FORECAST TO OCCUR NEAR TAU 96. IN TERMS OF 
INTENSITY, 17W IS FORECAST TO MAINTAIN ITS CURRENT INTENSITY FOR ONLY 
ANOTHER 12 HOURS BEFORE IT STARTS TO WEAKEN UNDER THE EFFECTS OF 
INCREASED NORTHWESTERLY SHEAR AND DRY AIR ENTRAINMENT. ADDITIONALLY, 
THE UPPER-LEVEL TROUGH CURRENTLY TO THE WEST OF THE SYSTEM IS EXPECTED 
TO MOVE WESTWARD AND FILL, REDUCING OUTFLOW ALOFT. AROUND TAU 72, 
NORTHEASTERLY SHEAR WILL GREATLY INCREASE, INDUCED BY THE STRONG RIDGE 
OVER EASTERN CHINA CAUSING A MORE RAPID RATE OF WEAKENING AS THE 
SYSTEM APPROACHES THE COAST. AFTER LANDFALL, RUGGED TERRAIN 
INTERACTION WILL CAUSE THE VORTEX TO RAPIDLY WEAKEN.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN VERY GOOD AGREEMENT 
THROUGH TAU 48 WITH A CROSS-TRACK SPREAD OF ONLY 50 NM. AFTER TAU 48, 
TWO OUTLIERS ARISE IN GFS AND GALWEM. THE GFS SHOWS A SIGNIFICANTLY 
SHARPER TURN TO THE SOUTHWEST, MAKING LANDFALL IN NORTHERN TAIWAN 
WHILE, GALWEM SHOWS NO SOUTHWESTWARD TURN AT ALL. DISCOUNTING THOSE 
TWO OUTLIERS, MODELS GENERALLY AGREE ON A TRAJECTORY SIMILAR TO THE 
JTWC OFFICIAL FORECAST. THE FORECAST IS PLACED NEAR THE MULTI-MODEL 
CONSENSUS THROUGH TAU 72 AND THEN CLOSER TO THE ECMWF AND EXPERIMENTAL 
GOOGLE DEEPMIND ENSEMBLE MEAN THROUGH TAU 120. INTENSITY GUIDANCE 
AGREES ON STEADY WEAKENING THROUGHOUT THE ENTIRE FORECAST PERIOD WITH 
NO NOTABLE OUTLIERS PRESENT. THE JTWC INTENSITY FORECAST IS PLACED 
SLIGHTLY HIGHER THAN THE CONSENSUS WITH OVERALL 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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