WDPN33 PGTW 140300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 08W (AMPIL) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 26.0N 140.2E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 559 NM SOUTH OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A CENTRAL
MASS OF DEEP CONVECTION WITH LARGE SPIRAL BANDS TO THE SOUTH, EAST,
AND NORTHEAST OF THE EMBEDDED LOW-LEVEL CIRCULATION CENTER (LLCC).
BASED ON AVAILABLE DATA, THE VORTEX NOW APPEARS TO BE VERTICALLY
ALIGNED FOR THE FIRST TIME. NORTHERLY SHEAR OF 10-15 KT PERSISTS,
RESULTING IN LIMITED SPIRAL BANDING AND SOME DRY AIR ENTRAINMENT IN
THE NORTHWEST QUADRANT. THIS SHEAR IS BEGINNING TO ABATE SLIGHTLY,
AND POLEWARD OUTFLOW IS EXPANDING IN THE NORTHEAST QUADRANT.
HEALTHY EQUATORWARD OUTFLOW CONTINUES TOWARD THE SOUTH. THE INITIAL
INTENSITY IS SET AT 60 KT AS A BLEND OF AGENCY DVORAK AND OBJECTIVE
SATELLITE ESTIMATES RANGING FROM 55 TO 65 KT. A 132352Z ASCAT-B
SCATTEROMETER PASS SHOWED THAT THE CORE WIND FIELD IS EXPANDING IN
SIZE AS THE STORM INTENSIFIES. THE ISLAND OF IWO TO LOCATED 93 NM
SOUTHEAST OF THE LLCC HAS BEEN REPORTING SUSTAINED WINDS OF 25-30
KT GUSTING TO 40-45 KT OVER THE PAST FEW HOURS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T4.0 - 65 KTS
   KNES: T3.5 - 55 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: DRY AIR TO THE NORTHWEST

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL STORM 08W (AMPIL) CONTINUES TO TRACK
NORTHEASTWARD, BUT IS EXPECTED TO IMMINENTLY TURN NORTHWARD AS IT
PRESSES AGAINST THE WESTERN PERIPHERY OF A DEEP-LAYER SUBTROPICAL
RIDGE EAST OF JAPAN. THIS RIDGE WILL GUIDE AMPIL NORTHWARD FOR
ABOUT 48 HOURS, BRINGING THE STORM CLOSE TO HONSHU BEFORE TURNING
NORTHEASTWARD INTO THE MID-LATITUDE WESTERLIES. DYNAMICAL MODEL
GUIDANCE CONTINUES TO COME INTO TIGHTER AGREEMENT ON THIS PORTION
OF THE TRACK, WHICH HAS SHIFTED SLIGHTLY EASTWARD DURING THE PAST
12 HOURS. THE JTWC FORECAST FOLLOWS SUIT WITH A SLIGHT NUDGE TO THE
RIGHT THROUGH 72 HOURS, BUT IS SLIGHTLY LEFT OF THE MULTI-MODEL
CONSENSUS DURING THE 48-72 HOUR PERIOD, LEANING TOWARD THE ECMWF
ENSEMBLE MEAN AND GFS ENSEMBLE MEAN. THIS TRACK BRINGS AMPIL WITHIN
ABOUT 30 NM OF THE COASTLINE OF CHIBA PREFECTURE, JAPAN. AN
OFFSHORE TRACK IS NOT YET GUARANTEED, BUT ITS ODDS HAVE INCREASED A
BIT TODAY, WITH NOW APPROXIMATELY 75 PERCENT OF ECMWF AND GFS
ENSEMBLE MEMBERS STAYING OFFSHORE COMPARED WITH APPROXIMATELY 50
PERCENT 24 HOURS AGO. NOTE THAT THE STORM'S CORE WIND FIELD IS
STILL EXPECTED TO IMPACT PORTIONS OF EASTERN HONSHU EVEN AS THE
CIRCULATION CENTER PASSES OFF THE COAST. REGARDING THE INTENSITY
FORECAST, AMPIL IS LIKELY TO STEADILY INTENSIFY DURING THE NEXT 24
HOURS, THOUGH PERHAPS NOT RAPIDLY DUE TO ONGOING LIGHT NORTHERLY
SHEAR AND ASSOCIATED DRY AIR ENTRAINMENT. AFTER 24 HOURS, WIND
SHEAR RELAXES AS AN UPPER LOW NORTH OF THE STORM DECAYS, LEAVING
AMPIL IN A NEARLY IDEAL ENVIRONMENT WITH LOW SHEAR AND SEA SURFACE
TEMPERATURES OF 30-31 DEGREES CELSIUS, SIGNIFICANTLY WARMER THAN
CLIMATOLOGY. THE JTWC FORECAST THEREFORE SHOWS RAPID
INTENSIFICATION DURING THE 24-48 HOUR PERIOD TO A PEAK INTENSITY OF
110 KT PRIOR TO ITS CLOSEST APPROACH TO JAPAN. THIS FORECAST PEAK
REMAINS BELOW SOME OF THE MORE AGGRESSIVE MODELS SUCH AS HAFS-A AND
THE COAMPS-TC ENSEMBLE. AFTER 48 HOURS, A TURN NORTHEASTWARD OVER
SIGNIFICANTLY COOLER WATERS WILL RESULT IN STEADY OR RAPID
WEAKENING. THE STORM WILL THEN INTERACT WITH THE MID-LATITUDE
WESTERLIES AND UNDERGO EXTRATROPICAL TRANSITION DURING THE 96-120
HOUR PERIOD.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS CLOSE TO THE MODEL
CONSENSUS THROUGH 36 HOURS, THEN SLIGHTLY NORTHWEST OF THE
CONSENSUS DURING THE 48-72 HOUR PERIOD, LEANING CLOSER TO THE ECMWF
ENSEMBLE MEAN AND GFS ENSEMBLE MEAN. ALONG-TRACK SPREAD GROWS
SIGNIFICANTLY AFTER 48 HOURS AS MODELS DIFFER ON THE STORM'S
FORWARD SPEED WHILE ROUNDING THE SUBTROPICAL RIDGE, LEADING TO
LOWER CONFIDENCE AFTER 72 HOURS. THE JTWC INTENSITY FORECAST IS
ABOVE THE MULTI-MODEL CONSENSUS THROUGH 72 HOURS DUE TO THE
STATISTICAL SHIPS MODEL BEING THROWN OUT AS A WEAK OUTLIER. THE
INTENSITY FORECAST IS OF OVERALL MEDIUM CONFIDENCE.

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