WDPN32 PGTW 281500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 029//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 30.6N 130.1E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 151 NM SOUTH OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 32 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TYPHOON (TY) 11W HAS SLOWED IN ITS POLEWARD TRAJECTORY OVER THE
PAST SIX HOURS, WITH TROCHOIDAL (WOBBLE) TRACK MOTION STILL
EVIDENT IN ANIMATED RADAR IMAGERY. TRACK SPEEDS REMAIN ERRATIC AND
BELOW CLIMATOLOGICAL AVERAGES AS THE SYSTEM ENTERS WEAK STEERING
FLOW IN A COL REGION BETWEEN SUBTROPICAL RIDGING ON BOTH SIDES,
EAST AND WEST. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY
DEPICTS A RAGGED EYE BECOMING MORE OCCLUDED AS THE SYSTEM CONTINUES
TO WEAKEN SLIGHTLY. DESPITE NEAR-RADIAL OUTFLOW AND LOW VERTICAL
WIND SHEAR, TY 11W HAS A PRONOUNCED DRY SLOT THAT APPEARS POLEWARD
OF THE NORTHERN EYEWALL DUE TO THE INTRODUCTION OF LAND INTERACTION
AS THE SYSTEM TRAVERSES THE RYUKYU ISLAND CHAIN. THE CONVECTION
WRAPPING AROUND THE EYE HAS DEMONSTRATED CONSISTENT FRAGMENTATION
AND A TREND TOWARDS WARMING CLOUD TOP TEMPERATURES IN ALL
QUADRANTS. BASED ON THE EIR IMAGERY AND HOURLY RADAR FIXES, THERE
IS HIGH CONFIDENCE IN THE INITIAL POSITION. SURFACE OBSERVATIONS
FROM YAKU-SHIMA, APPROXIMATELY 30 NM EAST-SOUTHEAST, INDICATE
MAXIMUM SUSTAINED SURFACE WINDS OF ABOUT 47 KNOTS (ADJUSTED TO
1-MINUTE AVERAGE). THE INITIAL INTENSITY IS ASSESSED WITH HIGH
CONFIDENCE BASED ON A BLEND OF THE AGENCY SUBJECTIVE DVORAK
ESTIMATES AND THE CIMSS OBJECTIVE INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE BUT THE 50-KNOT AND 64-KNOT
WIND RADII WERE ADJUSTED SLIGHTLY BASED ON SURFACE OBSERVATIONS
FROM YAKU-SHIMA. 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
POSITIONED TO THE EAST AND WEST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   KNES: T5.0 - 90 KTS
   RJTD: T5.5 - 102 KTS
   RCTP: T5.0 - 90 KTS
   CIMSS ADT: 102 KTS AT 281130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE RADIAL

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: TY 11W IS FORECAST TO TRACK NORTHWARD THROUGH
TAU 24 TURNING SHARPLY EASTWARD AFTER IT MAKES LANDFALL OVER
WESTERN KYUSHU. TY 11W WILL WEAKEN GRADUALLY AS IT APPROACHES
KYUSHU DUE TO INTERACTION WITH LAND AND WILL RAPIDLY WEAKEN AS IT
TRACKS INLAND THROUGH THE FORECAST PERIOD. TRACK SPEEDS WILL REMAIN
VERY SLOW (3-5 KNOTS) ESPECIALLY AFTER TAU 24 WHEN THE MIDLATITUDE
FLOW TO THE NORTH BECOMES MORE ZONAL LEADING TO A WEAK STEERING
ENVIRONMENT WHERE TY 11W WILL BE TRAPPED IN A COL AREA BETWEEN
SUBTROPICAL RIDGING TO THE EAST AND WEST. AFTER TAU 96, A MAJOR
SHORTWAVE TROUGH WILL EMERGE OVER THE SEA OF JAPAN AND WHILE STILL
POLEWARD OF TY 11W, WILL BEGIN USHERING THE SYSTEM TO THE NORTHEAST
AT A QUICKENING RATE THROUGH THE END OF THE FORECAST PERIOD.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 24 WITH A 20-NM CROSS-TRACK SPREAD AT TAU 24. HOWEVER, 
DETERMINISTIC GUIDANCE DIVERGES SIGNIFICANTLY WITH LOW CONFIDENCE IN 
THE JTWC TRACK FORECAST AFTER TAU 24. WITH THE EXCEPTION OF THE GALWEM 
AND JGSM TRACKERS, WHICH SHOW UNREALISTICALLY FAST TRACK SPEEDS OVER 
NORTHERN HONSHU, THE BULK OF THE GUIDANCE SUPPORTS THE GENERAL TRACK 
PHILOSOPHY, WITH A BIT MORE AGREEMENT FAVORING TY 11W TRACKING 
SOUTHWARD OF SHIKOKU ISLAND WHERE PREVIOUSLY THERE WAS VERY LITTLE 
AGREEMENT. HOWEVER, GUIDANCE HAS BEEN INCONSISTENT FROM RUN-TO-RUN. 
CONSEQUENTLY, THE JTWC FORECAST REMAINS CONSISTENT WITH THE PREVIOUS 
FORECAST. THE 280600Z ECMWF ENSEMBLE (EPS) SUPPORTS THE JTWC TRACK 
FORECAST WITH ABOUT 80 PERCENT OF THE SOLUTIONS SHOWING A SLOW, 
MEANDERING EASTWARD TRACK THROUGH TAU 72 THEN A SLOW NORTHEASTWARD 
TRACK THROUGH TAU 120. THE 280600Z GFS ENSEMBLE (GEFS) SHOWS MORE 
ERRATIC, SLOWER SOLUTIONS OVER THE WESTERN PORTION OF JAPAN. 

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