WDPN33 PGTW 260900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING 
NR 012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 34.6N 146.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 352 NM EAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
11W (NEPARTAK) IS A SUBTROPICAL STORM THAT CONTINUES TO CONSIST OF
A LARGE, SPRAWLING CIRCULATION WITH AN ASYMMETRIC WIND FIELD.
DURING THE LAST 18 HOURS, JTWC HAD BEEN TRACKING THE CENTROID OF
MULTIPLE VORTICITY MAXIMA WITHIN THE CIRCULATION ENVELOPE. IN
RECENT HOURS, THE CIRCULATION HAS COALESCED INTO A SINGLE DOMINANT
LOW-LEVEL CENTER (LLC) IN ANIMATED MULTISPECTRAL SATELLITE IMAGERY,
THOUGH STILL ELONGATED FROM WEST-SOUTHWEST TO EAST-NORTHEAST. THE
INITIAL CENTER POSITION IS THUS PLACED AT THE CENTER OF THIS
DOMINANT CIRCULATION WITH MORE CONFIDENCE THAN IN THE LAST FEW
WARNINGS. THE LLC REMAINS EXPOSED, WITH DEEP CONVECTION LIMITED TO
THE NORTHERN SEMICIRCLE, WRAPPING AROUND NOW TO THE WEST OF THE LLC
AS DEEP-LAYER MOISTURE GETS ADVECTED CYCLONICALLY AROUND THE
CIRCULATION. CONTRASTINGLY, DRY AIR IS WRAPPING AROUND THE EASTERN
SEMICIRCLE, AND IS BEGINNING THE PROCESS OF PINCHING THE CYCLONE
OFF FROM THE MONSOONAL FLOW BELT TO ITS SOUTHEAST. THE INITIAL
INTENSITY IS HELD AT 35 KT BASED ON EARLIER ASCAT SCATTEROMETER
PASSES BETWEEN 252200Z AND 252300Z.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: UPPER-LEVEL LOW TO THE SOUTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   CIMSS ADT: 37 KTS AT 260200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE WESTWARD
   OTHER FACTORS: DRY AIR WRAPPING AROUND EASTERN SEMICIRCLE

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: 11W (NEPARTAK) IS A SUBTROPICAL CYCLONE
UNDERGOING A PROCESS SIMILAR TO MID-LATITUDE SECLUSION, WHERE THE
LLC IS BECOMING CUT OFF FROM THE CONVEYOR BELT OF MONSOONAL
MOISTURE TO ITS SOUTHEAST AS DRY AIR WRAPS AROUND THE EASTERN
SEMICIRCLE AND THE LLC BECOMES ALIGNED NEARLY DIRECTLY BENEATH AN
UPPER-LEVEL LOW CENTERED JUST TO THE SOUTH. THIS UPPER-LEVEL LOW
WILL CONTINUE HOOKING NEPARTAK WESTWARD DURING THE NEXT 12-24
HOURS. SUPERPOSITION OF NEPARTAK'S VORTEX WITH THIS UPPER LOW IS 
EXPECTED TO RESULT IN A MORE COMPACT CIRCULATION BY THIS TIME AND A 
SLIGHT INCREASE IN MAXIMUM SURFACE WINDS TO 40 KT. AFTER 24 HOURS, A
BUILDING LOW TO MID-LEVEL RIDGE TO THE EAST WITH A WESTWARD
EXTENSION OVER KYUSHU WILL TURN THE CYCLONE NORTHWESTWARD, WITH
NEPARTAK MAKING LANDFALL IN NORTHERN HONSHU IN JUST UNDER 36 HOURS
WITH MAXIMUM WINDS OF 35 KT. IT IS EXPECTED THAT NEPARTAK WILL NOT
HAVE TIME TO TRANSITION INTO A FULLY TROPICAL CYCLONE PRIOR TO
LANDFALL. A TRACK ACROSS HONSHU AND INTO THE SEA OF JAPAN WITH
SLOWING FORWARD MOTION IS FORECAST THROUGH 96 HOURS AS THE STEERING
RIDGE OVER KYUSHU WEAKENS. NEPARTAK IS EXPECTED TO WEAKEN FOLLOWING
LAND INTERACTION WITH JAPAN, AND DISSIPATE AROUND 96 HOURS AS IT
DRIFTS NORTHWARD INTO SEA SURFACE TEMPERATURES COOLER THAN 26
DEGREES CELSIUS AND VERTICAL SHEAR INCREASES.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS SIMILAR TO A BLEND OF
ECMWF, GALWEM, AND THE GFS ENSEMBLE MEAN. THE INTENSITY FORECAST IS
BASED ON THE RAW WIND FIELD OUTPUT FROM ECMWF, GFS, AND JGSM.

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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