WDPN33 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.2N 150.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 595 NM EAST-SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 27 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A HIGHLY
DISORGANIZED SUBTROPICAL SYSTEM WITH TWO DISTINCT VORTICITY MAXIMA
SPACED ABOUT 350 NM APART, COMPRISING A SPRAWLING, ELONGATED
CIRCULATION ORIENTED SOUTH-SOUTHWEST TO NORTH-NORTHEAST. ASSIGNING
A PRECISE CENTER POSITION IS CURRENTLY DIFFICULT. THE INITIAL
POSITION HAS BEEN RELOCATED TO THE APPROXIMATE CENTROID OF THE
LARGER CIRCULATION ENVELOPE, BETWEEN THE TWO VORTICITY MAXIMA. THE
CURRENT FORWARD SPEED IS UNREPRESENTATIVE DUE TO THIS RELOCATION,
AND WILL BE REEVALUATED IN THE NEXT WARNING. THE CYCLONE REMAINS
ENTANGLED WITH A LARGE UPPER-LEVEL TROUGH TO ITS WEST, CAUSING AN
ASYMMETRIC DISTRIBUTION OF CONVECTION ALMOST EXCLUSIVELY COVERING
THE EASTERN SEMICIRCLE. THE WIND FIELD IS SIMILARLY ASYMMETRIC,
WITH EARLIER SCATTEROMETER DATA FROM 242300Z ASCAT-A IMAGE SHOWING A 
FIELD OF 30-35 KT WINDS SPANNING HUNDREDS OF MILES EAST OF THE 
ESTIMATED CIRCULATION CENTER. THIS BROAD, ASYMMETRIC STRUCTURE IS 
TYPICAL OF A SUBTROPICAL CYCLONE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: UPPER-LEVEL TROUGH TO THE WEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   CIMSS ADT: 33 KTS AT 250540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30+ KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EASTWARD
   OTHER FACTORS: DRY AIR IN THE WESTERN SEMICIRCLE

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: THE MESSY, LARGE CIRCULATION IS
CURRENTLY INTERACTING STRONGLY WITH THE UPPER-LEVEL TROUGH TO ITS
WEST. THIS INTERACTION IS BAROCLINIC IN NATURE, AND COULD INDUCE A
SLIGHT INCREASE IN MAXIMUM SURFACE WINDS DESPITE AN ENVIRONMENT
THAT WOULD BE HOSTILE TO A TRULY TROPICAL CYCLONE, CHARACTERIZED
BY HIGH VERTICAL SHEAR AND NEARBY DRY AIR. THE EVOLUTION OF THE
CYCLONE DURING THE NEXT 48 HOURS WILL BE SIMILAR TO THAT OF
MID-LATITUDE OCCLUSION AS 11W (NEPARTAK) HOOKS TO THE NORTHWEST
AROUND THE UPPER-LEVEL TROUGH AND THEN BECOMES VERTICALLY ALIGNED
BENEATH IT WHILE APPROACHING JAPAN. MAXIMUM WINDS ARE FORECAST TO
INCREASE SLIGHTLY TO 40 KTS DURING THIS PROCESS. ONCE ALIGNMENT
OCCURS, THE CYCLONE WILL BECOME SEPARATED FROM THE MONSOONAL FLOW
IN WHICH IT IS CURRENTLY EMBEDDED, AND THE WIND FIELD IS FORECAST
TO SHRINK IN SIZE. DRY AIR ASSOCIATED WITH THE UPPER-LEVEL TROUGH
MAY ENVELOPE THE CIRCULATION AT THIS POINT, AND SLIGHT WEAKENING TO
35 KT IS FORECAST AS THE CYCLONE NEARS HONSHU. SOME UNCERTAINTY
EXISTS IN THIS PORTION OF THE INTENSITY FORECAST, AS DYNAMICAL
MODELS DISAGREE SOMEWHAT ON HOW DRY (AND THUS CONVECTIVELY ACTIVE)
THE VORTEX WILL BE. NEPARTAK IS EXPECTED TO BE STEERED
NORTHWESTWARD ACROSS NORTHERN HONSHU BY A WEAK RIDGE OVER KYUSHU,
MAKING LANDFALL JUST AFTER 48 HOURS. A SLOWING OF FORWARD MOTION IS
EXPECTED ONCE THE CYCLONE EMERGES IN THE SEA OF JAPAN AS THE
STEERING RIDGE OVER KYUSHU WEAKENS, AND NEPARTAK IS FORECAST TO BE
NEARLY STATIONARY BY 120 HOURS. SOME REINTENSIFICATION OF THE
CYCLONE IS POSSIBLE OVER THE SEA OF JAPAN, WHERE SEA SURFACE
TEMPERATURES ARE HIGHLY ANOMALOUSLY WARM AT 27-28C, WHICH COULD
ALLOW NEPARTAK TO TRANSITION TO A FULLY TROPICAL CYCLONE. THE JTWC
INTENSITY FORECAST KEEPS NEPARTAK AT A MODEST INTENSITY OF 30 KT IN
THE SEA OF JAPAN, WITH LOWER THAN AVERAGE CONFIDENCE.

MODEL DISCUSSION: THE JTWC TRACK FORECAST FOLLOWS A BLEND OF THE
GFS, ECMWF, AND HWRF MODELS. THE INTENSITY FORECAST IS CLOSE TO THE
CONSENSUS OF DYNAMICAL GLOBAL MODEL GUIDANCE THROUGH 72 HOURS, AND
THEN SLIGHTLY ABOVE CONSENSUS AFTER 72 HOURS.

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