Tropical Storm NANMADOL Advisory 9月18日

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 16W (NANMADOL) WARNING NR 021//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 28.5N 131.4E
   INITIAL INTENSITY: 115 KTS
   GEOGRAPHIC REFERENCE: 226 NM EAST-NORTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 48 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY CLEARLY SHOWS
THAT TY 16W HAS EMBARKED ON A FAIRLY RAPID WEAKENING TREND. AT THE
1800Z HOUR, THE RAGGED EYE WAS MEASURED AT 9-NM IN THE EIR BUT BY
THE 1900Z HOUR THE EYE HAD FILLED AND NO LONGER DISCERNABLE.
ANIMATED RADAR DATA INDICATES THE LOW LEVEL EYE REMAINS
WELL-DEFINED, THOUGH THE INNER EYEWALL IS WEAKENING AND SHRINKING
AND IS NOW OPEN ON THE WESTERN SIDE. THE SECONDARY EYEWALL THAT
DEVELOPED OVER THE PAST 24 HOURS DURING THE EYEWALL REPLACEMENT
CYCLE (ERC) IS ALSO BEGINNING TO BECOME MORE RAGGED. ALL THIS TO
SAY, TY 16W HAS DEFINITELY PASSED ITS PRIME. THE INITIAL POSITION
IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE 9-NM EYE IN THE EIR
AS WELL AS RJTD RADAR FIX POSITIONS AND A 171824Z GMI 89GHZ
MICROWAVE IMAGE. THE INITIAL INTENSITY IS LOWERED TO 115 KNOTS WITH
HIGH CONFIDENCE BASED ON CONCURRENCE OF THE BULK OF AGENCY AND
OBJECTIVE ESTIMATES. THE SATCON REMAINS UNREASONABLY HIGH DUE TO AN
UNLIKELY 145 KNOT AMSU-B SOUNDER ESTIMATE. THE SYSTEM IS GENERALLY
MOVING NORTHWESTWARD AT A STEADY 9 KNOT THROUGH THE 1800Z HOUR,
BEFORE RADAR INDICATED A PRONOUNCED SLOW-DOWN AND WOBBLE TO THE RIGHT.
THE ENVIRONMENT IS BECOMING STEADILY LESS FAVORABLE, FOR WHILE BULK
SHEAR IS LOW, AND OUTFLOW IS IMPROVING TO POLEWARD, THE SYSTEM HAS
TRACKED OVER A PATCH OF LOW OHC WATER, IS STILL STRUGGLING WITH THE
ERC AND AS SHOWN IN THE NAZE 1200Z SOUNDING, A FIST OF ELEVATED
MID-LEVEL WESTERLY FLOW IS IMPARTING SOME CONVERGENT FLOW ON THE
WEST SIDE. THIS IS CONFIRMED BY LATE RECEIPT OF A 171921Z SSMIS
91GHZ MICROWAVE IMAGE SHOWING SIGNIFICANT EROSION OF THE OUTER
CONVECTIVE BANDS ON THE WEST SIDE.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE EAST.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T6.0 - 115 KTS
   RJTD: T5.5 - 102 KTS
   KNES: T6.0 - 115 KTS
   CIMSS SATCON: 128 KTS AT 171727Z
   CIMSS ADT: 115 KTS AT 171740Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL
   OTHER FACTORS: INCREASING POLEWARD OUTFLOW, STRONG MID-LEVEL
WESTERLY FLOW.
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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 16W IS BEGINNING TO ROUND THE AXIS OF THE
STEERING RIDGE TO THE EAST AND WILL GRADUALLY START TO TURN
NORTHWARD TOWARDS THE WESTERN COAST OF KYUSHU OVER THE NEXT 12
HOURS. THE RECENT SLOWDOWN AND SHARP WOBBLE TO THE EAST SEEN IN THE
RADAR IMAGERY ARE CONCERNING BUT AT THIS POINT THE TRACK FORECAST
REMAINS NORTH-NORTHWEST TURNING TO THE NORTH BY TAU 24. THE SYSTEM
IS FORECAST TO PASS CLOSE BY YAKUSHIMA ISLAND, THEN MAKE LANDFALL
ALONG THE SOUTHERN TIP OF KYUSHU BY TAU 12, THEN SKIRT THE WEST
COAST OF KYUSHU TO A POINT NEAR NAGASAKI BY TAU 24. AT THIS POINT
THE SYSTEM HAS REACHED THE RIDGE AXIS AND WILL TURN SHARPLY
NORTHEASTWARD AHEAD OF A FAST-APPROACHING UPPER-LEVEL TROUGH AND
ACCOMPANYING SURFACE COLD FRONT DRAPED ACROSS THE SEA OF JAPAN. BY
TAU 36 THE CENTER OF THE RAPIDLY WEAKENING SYSTEM WILL BE FOUND
NEAR IWAKUNI, ACCELERATING QUICKLY OFF TO THE NORTHEAST ALONG THE
JAPANESE ALPS. THE CENTER OF THE SYSTEM IS FORECAST TO REMAIN
GENERALLY TO THE SOUTH AND EAST OF THE SPINE OF THE JAPANESE ALPS,
AND REEMERGE INTO THE PACIFIC OCEAN BY TAU 72, AND QUICKLY SCOOTING
OUT INTO THE NORTH PACIFIC BY THE END OF THE FORECAST PERIOD. EVEN
THOUGH THE SYSTEM IS BEGINNING TO TAP INTO SOME STRONG POLEWARD
OUTFLOW, TY 16W IS UNLIKELY TO RECOVER FROM THE EFFECTS OF THE
ONGOING ERC AND THE IMPACTS OF THE WESTERLY MID-LEVEL CONVERGENCE
AND FUTURE TERRAIN INFLUENCES, AND THE FORECAST CALLS FOR STEADY
WEAKENING DOWN TO 90 KNOTS BY TAU 24. ONCE THE SYSTEM MAKES ITS TURN
TO THE NORTHEAST, IT WILL QUICKLY BE TORN APART BY THE COMBINATION OF
THE ELEVATED TERRAIN AND VERY STRONG SHEAR INDUCED BETWEEN DIRECTLY
OPPOSING LOW-LEVEL AND UPPER-LEVEL FLOW. WHAT REMAINS OF THE LOW
LEVEL CIRCULATION WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT) BY TAU
48 AS THE SYSTEM SLIDES NORTHEAST ALONG THE FRONTAL BOUNDARY. FULL
ETT IS EXPECTED NO LATER THAN TAU 96, THOUGH THE SYSTEM IS EXPECTED
TO RELATIVELY WEAK BY THIS POINT.
MODEL DISCUSSION: REGARDLESS OF THE RECENTLY OBSERVED WOBBLE, MODEL
GUIDANCE IS IN VERY TIGHT AGREEMENT TO TAU 12, WITH ALL
DETERMINISTIC AND ENSEMBLE MEMBERS SUPPORTING THE JTWC FORECAST
TRACK. IN LINE WITH PREVIOUS FORECAST, THINGS GET SPOTTY AFTER TAU
24 AS THE MODELS DIVERGE SIGNIFICANTLY IN HOW THEY HANDLE THE
BAROCLINIC ZONE INTERACTION AND THE RATE OF WEAKENING AND THEN
ENSUING TRACK. GENERALLY, THE MODELS SPLIT INTO TWO CAMPS AFTER TAU
36; THE FIRST CONSISTS OF THE FAST MODELS INCLUDING GALWEM, UKMET,
UKMET ENSEMBLE AND JGSM. THESE MODELS SHOW THE SYSTEM RACING TO THE
NORTHEAST ON THE NORTH SIDE OF THE JAPANESE ALPS, EVEN TAKING THE
SYSTEM OVER WATER IN THE SOJ FOR A WHILE, THEN CROSS THE COAST
AGAIN NEAR MISAWA AND FOR THE MOST PART DISSIPATE THE SYSTEM BY TAU
72 FAR TO THE NORTHEAST OF HOKKAIDO. THE SECOND GROUP OF SLOWER
MODELS CONSISTS OF GFS, NAVGEM, ECMWF AND THEIR ENSEMBLES AND THE
HWRF. IN THIS GROUP, THE SYSTEM SLIDES TO THE NORTHEAST, AT A
SLOWER PACE AND REMAINS SOUTH OF THE ALPS BEFORE MOVING INTO THE
PACIFIC TO A POINT EAST OF HOKKAIDO BY TAU 96. THE ECMWF IS THE
EXTREME OUTLIER OF THIS GROUP, INSISTING ON EARLY DISSIPATION AND A
TRACK TO THE SOUTH OF TOKYO BY TAU 72. THE RESULTING ALONG-TRACK
SPREAD IN THE GUIDANCE PACKAGE IS EXTREME, WELL OVER 1000NM. THE
JTWC FORECAST REMAINS GENERALLY CONSISTENT WITH THE PREVIOUS,
THOUGH IT DOES SHIFT THE TRACK SOUTH OF THE ALPS. CONFIDENCE IS LOW
THROUGH THE FORECAST PERIOD. INTENSITY GUIDANCE IS BETTER BEHAVED
FOR THE MOST PART, WITH ALL MODELS SHOWING A SHARP WEAKENING
THROUGH THE PERIOD. THE JTWC FORECAST LIES ON THE HIGH EDGE OF THE
PACKAGE THROUGH THE FORECAST WITH MEDIUM CONFIDENCE.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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