WDPN33 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 08W (AMPIL) WARNING NR 
002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.4N 137.9E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 661 NM SOUTH OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A TILTED
VORTEX, WITH THE LOW-LEVEL CIRCULATION CENTER (LLCC) LOCATED NEAR
THE NORTHEASTERN EDGE OF A DEEP CONVECTIVE CANOPY ASSOCIATED WITH A
CIRCULAR REGION OF PERSISTENT DEEP CONVECTION TO THE SOUTHWEST. A
121613Z AMSR-2 MICROWAVE IMAGE CONFIRMED THAT THE MID-LEVEL CENTER
IS OFFSET SOUTHWEST OF THE LLCC BY APPROXIMATELY 45 NM, LOCATED
WITHIN THE DEEP CONVECTION. THIS IS A SIMILAR TILT MAGNITUDE TO THE
GFS AND HAFS-A MODEL INITIALIZATIONS. THIS TILT IS CURRENTLY BEING
MAINTAINED BY LIGHT NORTHEASTERLY VERTICAL SHEAR OF AROUND 10 KTS. A
POLEWARD OUTFLOW CHANNEL IS OBSERVED INTO THE EASTERN FLANK OF AN
UPPER-LEVEL LOW TO THE NORTHWEST MOVING OVER THE RYUKYU ISLANDS,
AND EQUATORWARD OUTFLOW IS PRESENT AS WELL. THE INITIAL INTENSITY
IS SET AT 40 KTS BASED ON EARLIER 121240Z ASCAT SCATTEROMETER DATA
SHOWING A BAND OF AROUND 40 KTS ON THE SOUTHEASTERN SIDE OF THE
LLCC, SUPPORTED BY AGENCY DVORAK ESTIMATES AS HIGH AS T2.5 AND
OBJECTIVE SATELLITE INTENSITY ESTIMATES OF 35-39 KT FROM CIMSS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: 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 POLEWARD AND 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. THIS SHEAR IS EXPECTED TO PERSIST FOR 36-48 HOURS,
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 AN AVERAGE INTENSIFICATION RATE, BRINGING AMPIL TO
75 KT IN 48 HOURS AS IT TRACKS NORTH 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 THIS
TIME, 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, WHICH
COULD BRING THE STORM CLOSE TO OR OVER HONSHU BEFORE IT TURNS
NORTHEASTWARD AGAIN. 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 KEEPS AMPIL JUST OFFSHORE, AND IS
SLIGHTLY EAST OF THE PREVIOUS FORECAST BY ABOUT 25 NM DURING THE
72-96 HOUR PERIOD. 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: THE JTWC TRACK FORECAST LEANS TOWARD A CONSENSUS
OF THE ECMWF, GFS, GFS ENSEMBLE MEAN, UKMET, AND HAFS-A MODELS.
TRACK SPREAD REMAINS SUBSTANTIAL AFTER 72 HOURS, WITH A SIGNIFICANT
MINORITY OF GFS ENSEMBLE AND ECMWF ENSEMBLE MEMBERS DEPICTING A
TRACK OVER MAINLAND JAPAN, WHILE THE MAJORITY OF THE MEMBERS REMAIN
OFFSHORE. THE JTWC INTENSITY FORECAST IS CLOSE TO THE MULTI-MODEL
CONSENSUS THROUGH 48 HOURS, AND HIGHER THEREAFTER, CLOSER TO THE
HAFS-A, HWRF, AND GFS FORECASTS.

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