Advisories

WDPN31 PGTW 222100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 018//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.4N 143.6E
   INITIAL INTENSITY: 115 KTS
   GEOGRAPHIC REFERENCE: 943 NM EAST-SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 45 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
RAPIDLY DETERIORATING AND SHRINKING EYE FEATURE, WITH DEEP
CONVECTION CONFINED TO THE SOUTHWESTERN QUADRANT AND WARMING
CLOUD-TOPS ALONG THE NORTHEASTERN FLANK. THE SYSTEM HAS UNDERGONE A
REMARKABLE TRANSFORMATION IN JUST THE PAST TWO HOURS, FROM HAVING A
WELL-DEFINED, IF A BIT RAGGED, EYE FEATURE, TO NOW BARELY HOLDING
ONTO AN EXTREMELY SMALL AND ILL-DEFINED EYE, WITH WANING
CONVECTION. A 221550Z AMSR2 89GHZ MICROWAVE IMAGE PROVIDES SOME
INSIGHT INTO THE MOST LIKELY CAUSE, NAMELY THE ONSET OF AN ERC. THE
AMSR2 IMAGE DEPICTS A WELL-DEFINED INNER EYEWALL, CONSISTENT WITH
THE EIR IMAGERY AT THAT TIME, AND A STRONG SECONDARY EYEWALL
EVIDENT ABOUT 60NM OUTWARD FROM THE INNER. A MOAT IS ALSO EVIDENT
WRAPPING UP THE EASTERN FLANK OF THE CORE, ALL OF WHICH IS
CONSISTENT WITH AN ERC. THE CIMSS M-PERC ERC ONSET PROBABILITY
JUMPED UP TO 45 PERCENT BASED ON THIS RECENT DATA. A 221850Z SSMIS
91GHZ IMAGE DEPICTS THE SAME OVERALL SCENARIO, WITH AN EVEN BETTER
DEFINED MOAT, AND SECONDARY EYEWALL AND A MORE DEGRADED INNER
EYEWALL. THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE
BASED ON THE ERODING EYE IN THE EIR. THE INITIAL INTENSITY IS
ASSESSED WITH HIGH CONFIDENCE, BASED ON THE OVERALL AGREEMENT OF
NEARLY ALL SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED
BELOW. ENVIRONMENTAL CONDITIONS ARE FAVORABLE OVERALL, WITH WARM
SSTS, HIGH (100 KJ PER CM2 OR GREATER) OHC, LOW DEEP-LAYER EASTERLY
SHEAR AND STRONG RADIAL OUTFLOW. HOWEVER, MODERATE MID-LEVEL SHEAR
AND THE ERC WILL SERVE TO OFFSET THE OVERALL FAVORABLE CONDITIONS
IN THE NEAR-TERM.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   RJTD: T5.0 - 90 KTS
   RCTP: T6.0 - 115 KTS
   KNES: T6.0 - 115 KTS
   CIMSS SATCON: 114 KTS AT 221800Z
   CIMSS ADT: 112 KTS AT 221800Z
   CIMSS AIDT: 101 KTS AT 221800Z
   CIMSS D-MINT: 118 KTS AT 221610Z
   CIMSS D-PRINT: 116 KTS AT 221800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL
   OTHER FACTORS: NORTHWESTWARD OUTFLOW CHANNEL; MODERATE MID-LEVEL
SHEAR; EYEWALL REPLACEMENT CYCLE (ERC). 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TYPHOON (TY) 17W (SAUDEL) IS TRACKING
NORTHWESTWARD ALONG THE SOUTHWESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST, CONNECTING WITH
A SECOND STR CENTERED OVER SOUTHERN JAPAN. NO SIGNIFICANT CHANGE IN
THE TRACK IS EXPECTED FOR THE FIRST 60 HOURS OF THE FORECAST.
BEGINNING AT TAU 60 HOWEVER, THE GLOBAL MODELS BEGIN TO DIVERGE ON
HOW THEY HANDLE THE RIDGING PATTERN, SPECIFICALLY IN REGARDS TO THE
PASSAGE OF A TROUGH TO THE NORTH WILL IMPACT THE RIDGE. THE GFS
DEPICTS A MUCH STRONGER AND DEEPER TROUGH WHICH BREAKS DOWN THE
RIDGE FROM TAU 72 TO TAU 84, WHICH RESULTS IN A RAPID SLOWDOWN OF
TY 17W AS IT CROSSES THE RYUKYUS BEFORE TURNING WEST THEN SOUTHWEST
AS A RIDGE OVER CHINA BECOMES THE DOMINANT STEERING INFLUENCE AFTER
TAU 96. THE ECMWF ON THE OTHER HAND DEPICTS A MUCH WEAKER, NEARLY
NON-EXISTENT, TROUGH WHICH DOES NOT BREAK THE RIDGE, RESULTING IN A
FASTER TRACK WHICH EVENTUALLY CURVES SOUTHWESTWARD BY THE END OF
THE FORECAST PERIOD. THE EXPERIMENTAL AI MODELS ARE TRENDING MORE
TOWARDS THE GFS SCENARIO. TY 17W WILL SLOW DOWN AFTER TAU 60 AS IT
PASSES SOUTH OF THE WEAKNESS IN THE STEERING RIDGE, BEFORE PASSING
THROUGH THE RYUKYU CHAIN IN THE VICINITY OF OKINAWA SHORTLY AFTER
TAU 72. THE SYSTEM COMES UNDER THE INFLUENCE OF THE RIDGE OVER
NORTHERN CHINA BY TAU 96, AND BEGINS A TURN WEST-SOUTHWESTWARD TO
THE NORTHEAST OF TAIWAN. REGARDING INTENSITY, THE LIKELY ONSET OF AN
ERC WILL RESULT IN A SMALL AMOUNT OF WEAKENING OR MAINTENANCE OF
THE CURRENT INTENSITY THROUGH TAU 12. ONCE THE ERC IS COMPLETE, THE
SYSTEM WILL RESUME INTENSIFICATION AS IT PASSES OVER A POOL OF HIGH
OHC WATERS, WHILE MAINTAINING STRONG POLEWARD OUTFLOW ALOFT. BY TAU
36, THE SYSTEM IS EXPECTED TO BECOME MORE ISOLATED IN TERMS OF THE
UPPER-LEVEL PATTERN, WITH A RESULTANT REDUCTION IN OUTFLOW ALOFT.
BY TAU 48, UPPER-LEVEL NORTHERLY FLOW ALONG THE WESTERN FLANK OF
THE SYSTEM WILL RESULT IN A DRAMATIC INCREASE IN SHEAR, ACCELERATING
THE WEAKENING TREND BEGUN 12 HOURS EARLIER. THE SYSTEM IS FORECAST
TO STEADILY WEAKEN FROM THIS POINT ONWARDS, THOUGH IT WILL REMAIN A
STRONG TYPHOON AS IT PASSES OKINAWA. ONCE THE SYSTEM MOVES INTO THE
EAST CHINA SEA, CONDITIONS IMPROVE SLIGHTLY, ALLOWING TY 17W TO
MAINTAIN INTENSITY THROUGH TAU 120. 

MODEL DISCUSSION: TRACK GUIDANCE, WITH THE EXCEPTION OF NAVGEM, IS
IN GOOD OVERALL AGREEMENT THROUGH TAU 72, WITH A CROSS-TRACK SPREAD
OF 155NM BETWEEN JGSM ON THE SOUTHERN EDGE AND THE CLUSTER OF GFS,
GEFS AND THE AI MODELS ON THE NORTHERN EDGE. THE JTWC FORECAST IS
POSITIONED CLOSE TO THE CONSENSUS MEAN (MODIFIED TO DISREGARD
NAVGEM), AND THE ECMWF DETERMINISTIC TRACKER, JUST SOUTH OF THE
AI-GFS CLUSTER NOTED ABOVE, WITH HIGH CONFIDENCE. IN THE LONG-RANGE
FORECAST, THE MODELS BEGIN TO DIVERGE MORE SIGNIFICANTLY, PRIMARILY
IN THE ALONG-TRACK DIRECTION. ECMWF IS BY FAR THE FASTEST OF ALL
THE MODEL GUIDANCE, SHOWING LANDFALL IN SOUTHEASTERN CHINA BY TAU
120. THE AI-GFS CLUSTER ARE THE SLOWEST, REMAINING EAST OF 124E
LONGITUDE BY TAU 120. THE JTWC FORECAST IS POSITIONED CLOSE TO THE
MODIFIED CONSENSUS WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS IN
GOOD AGREEMENT, THE ONLY OUTLIER IS THE HAFS-A DEPICTING STEADY
WEAKENING AFTER TAU 12 TO TAU 96 BEFORE FLATTENING OUT. THE
REMAINDER OF THE GUIDANCE IS TIGHTLY CLUSTERED ABOUT THE MEAN, AND
THE JTWC FORECAST. OF NOTE, THE RIDE, RI25 AND RI30 RAPID
INTENSIFICATION (RI) AIDS ARE TRIGGERED WITH THIS RUN, THOUGH WITH
THE ONSET OF THE ERC, APPEAR UNLIKELY AT THIS TIME. 

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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