WDPN33 PGTW 241500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.3N 150.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 715 NM SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS THAT THE
LOW-LEVEL CENTER (LLC) REMAINS HIGHLY ELONGATED FROM SOUTHWEST TO
NORTHEAST, AND PLACING AN EXACT CENTER LOCATION IS DIFFICULT. DEEP
CONVECTION REMAINS LARGELY DISPLACED TO THE NORTHEAST OF THE LLC
DUE TO WESTERLY SHEAR AND DRY AIR ASSOCIATED WITH A STRONG
UPPER-LEVEL TROUGH TO THE NORTHWEST. BAROCLINIC INTERACTION WITH
THIS TROUGH IS LEADING TO THE DEVELOPMENT OF A LARGE, ASYMMETRIC
WIND FIELD. A 241001Z ASCAT-A PASS SHOWED THAT A FIELD OF 30-35 KT
WINDS EXTENDS TO AT LEAST 300 NM EAST OF THE ESTIMATED CENTER
POSITION. MAXIMUM WINDS ALSO REMAIN FAR FROM THE CENTER
(APPROXIMATELY 135 NM). THIS STRUCTURE IS CONSISTENT WITH THAT OF A
SUBTROPICAL CYCLONE. THE INITIAL INTENSITY IS HELD AT 35 KT BASED
ON THE AFOREMENTIONED SCATTEROMETER DATA.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: UPPER-LEVEL TROUGH TO THE NORTHWEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   CIMSS ADT: 27 KTS AT 241140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EASTWARD
   OTHER FACTORS: DRY AIR OVER LLC AND WESTERN SEMICIRCLE

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

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,
CHARACTERIZED BY A BROAD, ASYMMETRIC WIND FIELD WRAPPING MONSOONAL
MOISTURE AROUND THE EASTERN SIDE OF AN EXPOSED LOW-LEVEL CENTER.
THE CYCLONE IS BEGINNING THE PROCESS OF BAROCLINIC INTERACTION WITH
THE STRONG UPPER-LEVEL LOW TO THE NORTHWEST, AND THIS INTERACTION
WILL DOMINATE THE EVOLUTION OF NEPARTAK DURING THE FORECAST PERIOD.
WITH THE MID-LATITUDE JET DISPLACED WELL NORTH OF JAPAN, THE
AFOREMENTIONED UPPER-LEVEL LOW IS EXPECTED TO MOVE LITTLE OVER THE
NEXT 48 HOURS, ALLOWING NEPARTAK TO PINWHEEL NORTHWARD AND THEN
NORTHWESTWARD AROUND ITS PERIPHERY, EVENTUALLY REACHING JAPAN. THIS
EVOLUTION WILL SHARE SIMILARITIES WITH BAROCLINIC OCCLUSION OF
MID-LATITUDE CYCLONES, AND IS LIKELY TO CAUSE SOME INTENSIFICATION
OF NEPARTAK DESPITE THE ELEVATED LEVELS OF VERTICAL SHEAR AND DRY
AIR THAT ARE PRESENT. NUMERICAL MODEL GUIDANCE IS IN GENERAL
AGREEMENT ON THIS POINT, AND THE JTWC INTENSITY FORECAST CALLS FOR
MAXIMUM WINDS TO INCREASE TO 45 KT BY 24-36 HOURS. THEREAFTER, THE
OCCLUSION PROCESS WILL COMPLETE, CONSISTING OF NEPARTAK BECOMING
VERTICALLY ALIGNED BENEATH THE UPPER-LEVEL LOW. THIS EVOLUTION IS
LIKELY TO CAUSE A CONTRACTION OF THE CYCLONE'S WIND FIELD AS DRY
AIR WRAPS AROUND THE CIRCULATION, AND ALSO SOME WEAKENING AS
NEPARTAK LOSES THE BAROCLINIC SUPPORT THAT THE UPPER TROUGH WAS
PROVIDING PRIOR TO VERTICAL ALIGNMENT. THUS, SLIGHT WEAKENING TO 40
KT IS FORECAST PRIOR TO LANDFALL, WHICH IS FORECAST TO OCCUR
BETWEEN 72 AND 96 HOURS. THE TRACK FORECAST BECOMES OF LOWER
CONFIDENCE NEAR JAPAN, AS STEERING CURRENTS WILL WEAKEN ONCE
NEPARTAK IS STACKED BENEATH THE UPPER-LEVEL LOW, MAROONED WELL TO
THE SOUTH OF THE MID-LATITUDE JET. NUMERICAL MODEL GUIDANCE IS
SPREAD BETWEEN A SLOW WESTWARD DRIFT ACROSS HONSHU AND A TURN
NORTHWARD TO THE EAST OF HONSHU WITHOUT MAKING LANDFALL. THE JTWC
FORECAST IS CLOSE TO THE MULTI-MODEL CONSENSUS AND THE PREVIOUS
FORECAST, SHOWING A TRACK ACROSS HONSHU TO THE SOUTH OF MISAWA
WHILE FORWARD MOTION SLOWS. THE LONG-TERM INTENSITY FORECAST IS
ALSO OF LOWER CONFIDENCE, AS LAND INTERACTION WITH JAPAN AND DRY
AIR WRAPPED IN BY THE UPPER-LEVEL LOW COULD CAUSE DISSIPATION OF
THE CYCLONE IN THE VICINITY OF HONSHU. HOWEVER, IF A COHERENT
CIRCULATION EMERGES IN THE SEA OF JAPAN, ANOMALOUSLY WARM SST OF
27-28C THERE COULD ALLOW THE CYCLONE TO SLOWLY UNDERGO TROPICAL
TRANSITION AND POTENTIALLY RESTRENGTHEN. THE JTWC FORECAST SHOWS
NEPARTAK REGAINING TROPICAL STORM INTENSITY BY 120 HOURS, BUT
REMAINS CONSERVATIVE, AS THIS PORTION OF THE FORECAST IS CURRENTLY
OF LOW CONFIDENCE. 

MODEL DISCUSSION: DYNAMICAL MODEL TRACK SPREAD HAS INCREASED AFTER
72 HOURS. THE JTWC TRACK FORECAST REMAINS CLOSE TO THE MULTI-MODEL
CONSENSUS, AND IS SIMILAR TO THE PREVIOUS FORECAST. THE JTWC
INTENSITY FORECAST REMAINS SLIGHTLY ABOVE THE MULTI-MODEL CONSENSUS
THROUGH 36 HOURS DUE TO THE ANTICIPATION THAT STRENGTHENING WILL 
OCCUR BY NON-TROPICAL PROCESSES THAT ARE NOT ACCOUNTED FOR IN SOME 
OF THE INTENSITY GUIDANCE.

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