Advisories

WDPN31 PGTW 220900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.6N 145.8E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 204 NM NORTH OF SAIPAN
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 34 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TYPHOON 17W
(SAUDEL) WITH A QUICKLY DEVELOPING 20 NM EYE. THERE IS A WEAKNESS
WITHIN THE NORTHEASTERN PORTION OF THE EYEWALL, LIKELY DUE TO THE
WEDGE OF DRY AIR INTRUDING INTO THE VORTEX ABOUT 6 HOURS AGO. THE
SYSTEM HAS BEEN ANALYZED TO HAVE NOW UNDERGONE RAPID INTENSIFICATION, 
STRENGTHENING FROM 60 KTS 24 HOURS AGO TO 95 KTS NOW. A 220617Z F18 
SSMIS 91 GHZ MICROWAVE IMAGE REVEALED A DEFINED BAND OF DEEP 
CONVECTION NEARLY ENTIRELY SURROUNDING THE CENTER WITH AN OUTER BAND 
WRAPPING AROUND THE WESTERN PERIPHERY. THE 37 GHZ VERSION OF THE SAME 
SSMIS PASS SHOWED A SYMMETRICAL MICROWAVE EYE FEATURE AT THE LOWER 
LEVELS THAT NOW STACKS VERTICALLY WITH HEIGHT. THE TIMELY SSMIS PASS 
LENDS HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY 
OF 95 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED PRIMARILY ON THE 
AGENCY DVORAK ESTIMATES OF T5.0-5.5. THE CIMSS ADT ESTIMATE IS 
CURRENTLY TOO LOW DUE TO THE RAPID DEVELOPMENT OF THE EYE AND 
CONSTRAINTS ON THE CI OUTPUT. ENVIRONMENTAL ANALYSIS INDICATES THAT 
17W IS NOW IN A HIGHLY FAVORABLE ENVIRONMENT CHARACTERIZED BY STRONG 
POLEWARD OUTFLOW ALOFT, LOW (5-10 KTS) VERTICAL WIND SHEAR, AND VERY 
WARM (30-31 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: 212336Z METOP-C ASCAT SCATTEROMETER DATA
AND OBJECTIVE BEST TRACK

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   RJTD: T4.5 - 77 KTS
   RCTP: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   CIMSS ADT: 59 KTS AT 220600Z
   CIMSS AIDT: 67 KTS AT 220600Z
   CIMSS D-MINT: 93 KTS AT 220616Z
   CIMSS D-PRINT: 83 KTS AT 220600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD

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 TRACK WEST-NORTHWESTWARD
ALONG THE SOUTHWESTERN PERIPHERY OF THE SOUTHWESTERN PERIPHERY OF
THE SUBTROPICAL RIDGE TO THE NORTHEAST THROUGH TAU 48. NEAR TAU 48, 
THE STR IS EXPECTED TO MERGE WITH ANOTHER STR POSITIONED OVER SOUTHERN 
JAPAN. THE COMBINED STEERING INFLUENCE OF THE RIDGING COMPLEX WILL 
KEEP 17W ON A WEST-NORTHWESTWARD TRAJECTORY THROUGH TAU 96. LANDFALL 
OVER OKINAWA IS CURRENTLY FORECAST TO OCCUR JUST BEFORE TAU 96. 
AFTERWARD, TRACK SPEEDS ARE EXPECTED TO SLOW THROUGH TAU 120 AS THE 
PRIMARY STEERING MECHANISM TRANSITIONS TO A SUBTROPICAL RIDGE 
POSITIONED OVER EASTERN CHINA. IN TERMS OF INTENSITY, 17W IS FORECAST 
TO CONTINUE INTENSIFYING THROUGH TAU 36 TO A PEAK OF AROUND 130 KTS AS 
THE ENVIRONMENT REMAINS HIGHLY CONDUCIVE. AFTER TAU 36, THE POLEWARD 
OUTFLOW CHANNEL WILL WEAKEN AND NORTHERLY SHEAR WILL BEGIN TO 
INCREASE, CAUSING 17W TO START ON A WEAKENING TREND AS IT APPROACHES 
OKINAWA. THE LANDFALL INTENSITY IS FORECAST TO BE ABOUT 90 KTS AS IT 
PASSES OVER THE ISLAND. FURTHER WEAKENING THROUGH TAU 120 IS EXPECTED 
DUE TO GREATLY INCREASING NORTHERLY SHEAR CAUSED BY THE STRONG RIDGE 
OVER EASTERN CHINA. 

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH
TAU 72 BESIDES TWO MINOR OUTLIERS IN NAVGEM AND JGSM, WHICH MAKE UP
THE SOUTHERNMOST MODELS. WHEN DISCOUNTING THE TWO SOUTHERN OUTLIERS, 
THE CROSS-TRACK SPREAD AT TAU 72 IS MEASURED TO BE ABOUT 125 NM. AFTER 
TAU 72, CROSS-TRACK SPREAD INCREASES TO 170 NM WITH NAVGEM AND JGSM 
REMAINING THE ONLY MODELS SHOWING A TRACK WELL SOUTH OF OKINAWA. THE 
JTWC TRACK FORECAST IS PLACED SLIGHTLY NORTH OF THE MULTI-MODEL 
CONSENSUS THROUGHOUT THE FORECAST PERIOD WITH HIGH CONFIDENCE THROUGH 
TAU 72 AND MEDIUM CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS SPLIT 
BETWEEN THE RAPID INTENSIFICATION (RI) AIDS AND THE NON-RI AIDS. THE 
RI AIDS SHOW A PEAK OF 140-150 KTS AT TAU 36 WHILE HAFS-A AND THE 
EXPERIMENTAL GOOGLE DEEPMIND ONLY SHOW A PEAK OF 105-110 KTS. THE JTWC 
INTENSITY FORECAST IS PLACED CLOSER TO THE RI AID DEPICTION DUE TO THE 
EXPECTED HIGHLY FAVORABLE ENVIRONMENT. MODELS THEN AGREE ON STEADY 
WEAKENING FROM TAU 48 ONWARD AND THE JTWC INTENSITY FORECAST FOLLOWS 
SUIT.

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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