WDPN33 PGTW 130300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 08W (AMPIL) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.5N 138.2E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 653 NM SOUTH OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A STILL
VERTICALLY TILTED VORTEX, WITH THE LLCC LOCATED NEAR THE NORTHERN
EDGE OF THE MASS OF DEEP THUNDERSTORMS. VISIBLE IMAGERY SUGGESTS
THAT THE TILT IS ABOUT 40 NM, AND THAT THE VORTEX IS NOW
PRECESSING, WITH THE TILT NOW MORE SOUTHWARD COMPARED TO
SOUTHWESTWARD SIX HOURS AGO. THE PRECESSION COULD BE CONTRIBUTING
TO THE TEMPORARY SLOWDOWN IN FORWARD MOTION OVER THE PAST FEW
HOURS. THE OVERALL SATELLITE PRESENTATION HAS NOT CHANGED MUCH, SO
THE INTENSITY IS HELD AT 40 KT. THE CENTER OF CIRCULATION IS
PASSING WEST OF IWO TO, WHERE OBSERVED SUSTAINED WINDS HAVE NOT YET
EXCEEDED 16 KT. HEALTHY EQUATORWARD OUTFLOW ALOFT IS NOTED, WITH
POLEWARD OUTFLOW A LITTLE WEAKER THAN SIX HOURS AGO.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TROPICAL STORM 08W (AMPIL) IS TRACKING
NORTHEASTWARD ALONG THE SOUTHERN RIM OF THE LARGE-SCALE MONSOON
GYRE PATTERN IN THE NORTHWEST PACIFIC. AMPIL'S CIRCULATION IS
VERTICALLY TILTED DUE TO LIGHT NORTHEASTERLY SHEAR, CAUSED BY
NEAR-CALM UPPER-LEVEL WINDS OVERLAYING SOUTHWESTERLY LOWER-LEVEL
BACKGROUND FLOW. LIGHT TO MODERATE NORTHEASTERLY OR NORTHERLY SHEAR
IS EXPECTED TO PERSIST FOR 36-48 HOURS, WHICH MAY DAMPEN THE
STORM'S RATE OF ORGANIZATION, BUT IS NOT STRONG ENOUGH TO PREVENT
INTENSIFICATION. THE GFS, HAFS-A, AND ECMWF MODELS INITIALIZED THE
VORTEX TILT WELL, AND AGREE ON GRADUAL ALIGNMENT OF THE VORTEX
DURING THE NEXT 24-48 HOURS. THE EXACT TIMING OF THIS ALIGNMENT
WILL DETERMINE THE PACE OF INTENSIFICATION FOR THE FIRST 48 HOURS,
WITH QUICKER ALIGNMENT LEADING TO QUICKER INTENSIFICATION AND VICE
VERSA. THE JTWC FORECAST FOLLOWS A CLIMATOLOGICAL INTENSIFICATION
RATE IN THE SHORT-TERM, BRINGING AMPIL TO 75 KT IN 48 HOURS AS IT
TRACKS WITHIN 130 NM NORTHWEST OF THE ISLAND OF IWO TO. AFTER 48
HOURS, AMPIL WILL INTERACT WITH A DECAYING UPPER-LEVEL LOW (ULL)
SOUTHEAST OF HONSHU IN A WAY THAT WILL REDUCE VERTICAL SHEAR AND
POTENTIALLY CAUSE FAVORABLE SUPERPOSITION OF THE ULL WITH THE
STORM. THIS ENVIRONMENTAL EVOLUTION, COUPLED WITH PASSAGE OVER
30-31 DEGREE CELSIUS SEA SURFACE TEMPERATURES THAT ARE RUNNING 2-3
CELSIUS WARMER THAN CLIMATOLOGY, SHOULD LEAD TO AN INCREASE IN THE
INTENSIFICATION RATE BETWEEN 48 AND 72 HOURS. THE JTWC FORECAST
BRINGS AMPIL TO A PEAK INTENSITY OF 100 KT BY 72 HOURS. DURING THE
36-72 HOUR PERIOD, THE TRACK IS EXPECTED TO BEND TOWARD THE NORTH
AS AMPIL PRESSES AGAINST THE WESTERN PERIPHERY OF A DEEP-LAYER
STEERING RIDGE EAST OF JAPAN. MODELS ARE IN GOOD AGREEMENT THAT
THIS RIDGE WILL BE CENTERED NEAR THE LATITUDE OF TOKYO DURING THIS
TIME, LEADING TO A NEAR-NORTHWARD STEERING CURRENT UNTIL THE STORM
REACHES THE SAME LATITUDE. THIS COULD BRING AMPIL CLOSE TO OR OVER
HONSHU BEFORE IT TURNS NORTHEASTWARD AROUND THE RIDGE AND INTO THE
JET STREAM. THERE REMAINS SIGNIFICANT UNCERTAINTY AS TO WHETHER
AMPIL WILL MAKE LANDFALL IN JAPAN, LARGELY DUE TO TRACK UNCERTAINTY
IN THE FIRST 48 HOURS CAUSED BY THE VORTEX ALIGNMENT PROCESS. THE
JTWC FORECAST TAKES AMPIL CLOSE TO THE COASTLINE OF CHIBA
PREFECTURE AS A STRONG TYPHOON. PLEASE NOTE THAT THE POINT OF
CLOSEST APPROACH TO JAPAN CURRENTLY FALLS BETWEEN TWO FORECAST
POINTS NEAR THE APEX OF TRACK CURVATURE, AND THUS THE STORM MAY
PASS CLOSER TO JAPAN THAN THE INTERPOLATED FORECAST TRACK DEPICTS.
AFTER 96 HOURS, AMPIL WILL INTERACT WITH THE MID-LATITUDE JET AND
BEGIN EXTRATROPICAL TRANSITION WHILE TURNING NORTHEASTWARD OVER
COOLER WATERS.

MODEL DISCUSSION: DYNAMICAL MODELS ARE GRADUALLY COMING INTO CLOSER
AGREEMENT ON THE TRACK THROUGH 72 HOURS. THE JTWC FORECAST LEANS
CLOSE TO A CONSENSUS OF THE GFS, GFS ENSEMBLE MEAN, ECMWF, ECMWF
ENSEMBLE MEAN, AND HAFS-A. THE JTWC INTENSITY FORECAST IS CLOSE TO
THE MULTI-MODEL CONSENSUS THROUGH 48 HOURS, BUT HIGHER THAN THE
CONSENSUS BETWEEN 48 AND 96 HOURS, SHOWING A PEAK INTENSITY CLOSER
TO THE PREDICTIONS OF HAFS-A, HWRF, COAMPS-TC, AND GFS THAN SOME OF
THE WEAKER STATISTICAL MODELS.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
NNNN