WDPN33 PGTW 272100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (NEPARTAK) WARNING 
NR 018//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 37.8N 142.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 160 NM SOUTH-SOUTHEAST OF MISAWA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY, JMA RADAR
FIXES, AND AGENCY FIXES ALL SHOW POLEWARD MOVEMENT AND ACCELERATION
ALONG TRACK DURING THE PAST SIX HOURS. A NEARLY-FULLY EXPOSED LOW
LEVEL CIRCULATION CENTER LENDS GOOD CONFIDENCE TO THE POSITION. THE
INTENSITY IS BASED ON A JTWC DVORAK ASSESSMENT OF T2.5 USING THE
SUB-TROPICAL TECHNIQUE AND IS SUPPORTED BY A 271157Z ASCAT PASS. A
271657Z AMSR2 SERIES SHOWS LITTLE CONVECTION REMAINING OVER THE
SOUTHEASTERN SEMICIRCLE AND ONLY FRAGMENTED DEEP CONVECTION OVER
THE NORTHWEST QUADRANT. AN EARLY MORNING SCATTEROMETRY PASS SHOWS
AN ASYMMETRY TO THE SYSTEM WITH SIGNIFICANTLY WEAKER WINDS ON THE
POLEWARD SIDE. COASTAL WIND REPORTS HAVE NOT YET INCREASED TO 20
KNOTS BUT WINDS OVER ISHINOMAKI ARE BEGINNING TO BACK TOWARDS THE
NORTHWEST. 

INITIAL WIND RADII BASIS: PERSISTENCE FROM 271157Z ASCAT.

CURRENT STEERING MECHANISM: A LOW TO MID-LEVEL HI-AMPLITUDE RIDGE
TO THE EAST OF HONSHU. 


AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T1.5 - 25 KTS
   CIMSS ADT: 43 KTS AT 271740Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 0-5 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: WEAK RADIAL
   OTHER FACTORS: A COLD CORE UPPER LEVEL LOW OVER THE SURFACE
CIRCULATION IMPEDES INTENSIFICATION.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TROPICAL STORM NEPARTAK REMAINS A SUB-TROPICAL 
CYCLONE. AN UPPER LEVEL CYCLONE HAS MOVED DIRECTLY OVER THE SURFACE
CIRCULATION AND IS SUPPRESSING FURTHER DEVELOPMENT. TROPICAL STORM 
11W HAS LOCKED INTO A NORTH-NORTHWESTWARD TRACK AND IS HONING IN ON 
MIYATO ISLAND, AT THE NORTHERN FRINGES OF SENDAI BAY. THE STORM WILL 
GO ASHORE WITHIN THE NEXT SIX HOURS AND MOVE INLAND. THE DEEP 
VERTICAL STACK OF THE SYSTEM MAY ALLOW THE VORTEX TO REMAIN INTACT 
ACROSS THE RUGGED TERRAIN OF THE JAPANESE ALPS. IF THE VORTEX 
REMAINS INTACT, WARM WATERS OVER THE SEA OF JAPAN WILL SUSTAIN THE 
SYSTEM FOR ANOTHER 12 TO 24 HOURS BEFORE HIGH VERTICAL WIND SHEAR 
TEARS IT APART. 


MODEL DISCUSSION: MODEL GUIDANCE HAS BEEN OUTSTANDING, ESPECIALLY
CONSIDERING THE SUB-TROPICAL NATURE OF THE SYSTEM AND ITS LACK OF
INTENSITY. FOUR DAYS AGO TRACK GUIDANCE INDICATED A LANDFALL AT
SENDAI AND A PEAK INTENSITY OF LESS THAN 50 KNOTS THROUGHOUT THE
ENTIRE LIFE CYCLE. THE TRACK FORECAST STAYS CLOSE TO CONSENSUS. THE
GREATEST UNCERTAINTY ASSOCIATED WITH MODEL GUIDANCE IS WHETHER OR
NOT THE VORTEX MAKES IT ACROSS HONSHU AND INTO THE SEA OF JAPAN.
SEVERAL INTENSITY AIDS ONTO THE SYSTEM THROUGH TAU 36, HOWEVER.
THE JTWC FORECAST STAYS CLOSE TO THE INTENSITY CONSENSUS BUT
DISSIPATES THE SYSTEM OVER WATER 12 HOURS MORE QUICKLY THAN
CONSENSUS.  

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