WDPN32 PGTW 272100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 026//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 29.0N 130.1E
   INITIAL INTENSITY: 115 KTS
   GEOGRAPHIC REFERENCE: 247 NM SOUTH OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 35 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FAIRLY
STEADY CORE STRUCTURE WITH A CLEAR 30 NM EYE AND A COMPLETE EYEWALL
CONSISTING OF CLOUD TOPS AS COLD AS -70 DEGREES CELSIUS. RADAR
IMAGERY FROM JAPAN SHOWS THAT THE EYEWALL IS WEAK ON THE WEST SIDE,
PERHAPS OWING TO ONGOING NORTHWESTERLY SHEAR OF 15-20 KT.
UPPER-LEVEL OUTFLOW IS ALSO CONSTRICTED IN THE WESTERN SEMICIRCLE
DUE TO THIS SHEAR, BUT IS STRONG TOWARDS THE EAST. THE INITIAL
INTENSITY IS SET AT 115 KT BASED ON A BLEND OF AGENCY DVORAK
ESTIMATES AND OBJECTIVE SATELLITE ESTIMATES IN THE RANGE OF 110 TO
120 KT.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   RJTD: T6.0 - 115 KTS
   RCTP: T5.5 - 102 KTS
   KNES: T6.0 - 115 KTS
   CIMSS SATCON: 110 KTS AT 271650Z
   CIMSS ADT: 112 KTS AT 271730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG EASTWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS FORECAST IS SIGNIFICANTLY SLOWER
THAN THE PREVIOUS ONE ESPECIALLY AFTER 72 HOURS DUE TO CONTINUING
SLOWING IN MODEL GUIDANCE. THIS FORECAST IS 5 HOURS SLOWER AT CPA
TO SASEBO, JAPAN AND 13 HOURS SLOWER AT CPA TO IWAKUNI, JAPAN
RELATIVE TO THE PREVIOUS FORECAST.

FORECAST DISCUSSION: TYPHOON 11W (SHANSHAN) CONTINUES TO MOVE
NORTH-NORTHWESTWARD AT A SNAIL'S PACE AS IT NEARS THE RYUKYU ISLAND
CHAIN. STEERING CURRENTS ARE WEAK AS THE TYPHOON IS MIRED JUST
SOUTHEAST OF A COL IN THE DEEP-LAYER STEERING FLOW NEAR THE
SOUTHERN KOREAN PENINSULA. AN UPPER-LEVEL TROUGH OVER THE SEA OF
JAPAN IS EXPECTED TO ALLOW SHANSHAN TO CONTINUE MAKING PROGRESS
NORTH-NORTHWESTWARD DURING THE NEXT 36 HOURS, BUT WILL THEN PULL
OUT TO THE NORTHEAST, LEAVING THE STORM IN EVEN WEAKER STEERING
CURRENTS. AT THIS POINT, MODELS GENERALLY AGREE THAT A VERY SLOW
TURN TOWARDS THE EAST WILL TAKE PLACE AS A WEAK RIDGE BUILDS IN
SOUTHEAST OF THE TYPHOON, BRINGING SHANSHAN INLAND OVER KYUSHU IN
APPROXIMATELY 48 HOURS. DURING THIS TURN, WIND SHEAR IS EXPECTED TO
BE LOW, BUT OCEAN HEAT CONTENT VALUES DECREASE FROM NEAR 80 KJ PER
SQUARE CENTIMETER WHERE SHANSHAN IS NOW TO LESS THAN 30 IN THE
WATERS SOUTH OF SASEBO. COMBINED WITH A SLOW RIGHTWARD TURN, THIS
WILL RESULT IN SIGNIFICANT OCEANIC COOLING BENEATH THE STORM,
LIKELY LEADING TO SUBSTANTIAL WEAKENING PRIOR TO LANDFALL, POSSIBLY
AT A RAPID PACE. THE JTWC FORECAST SHOWS WEAKENING TO 80 KT PRIOR
TO LANDFALL, BUT THE LANDFALL INTENSITY WILL BE SENSITIVE TO
EXACTLY HOW LONG IT TAKES THE TYPHOON TO TRANSIT THE EAST CHINA
SEA. AFTER LANDFALL, MODELS BEGIN TO DISAGREE SIGNIFICANTLY ON THE
STORM'S TRACK DUE TO CONTINUING WEAK STEERING CURRENTS, WITH THE
JET STREAM STAYING WELL NORTH OF THE STORM. A SECOND UPPER-LEVEL
TROUGH IS EXPECTED TO BRIEFLY DIG INTO NORTHEASTERN CHINA AND
SOUTHEASTERN RUSSIA IN ABOUT 72 HOURS, WHICH SOME MODELS EXPECT TO
BE STRONG ENOUGH TO USHER SHANSHAN NORTHEASTWARD INTO HONSHU.
HOWEVER, THIS TROUGH ALSO EXITS QUICKLY, AND SOME MODELS LEAVE
SHANSHAN STUCK SOUTH OF THE JETSTREAM EVEN AFTER THIS SECOND TROUGH
PASSES, EITHER STALLING OR MOVING SLOWLY NORTHEASTWARD. THE OVERALL
TREND IN MODEL GUIDANCE HAS BEEN SLOWER FOLLOWING LANDFALL IN
KYUSHU, WITH A CRAWLING PACE PERSISTING THROUGHOUT THE 5-DAY
FORECAST. THE JTWC FORECAST MAINTAINS A SLOW EAST-NORTHEASTWARD OR
NORTHEASTWARD MOTION NEAR SHIKOKU DURING THE 72-120 HOUR PERIOD.
THERE IS HIGHER THAN AVERAGE UNCERTAINTY IN THE TRACK AFTER 48
HOURS, AND ERRATIC STORM MOTION IS POSSIBLE IN THE VICINITY OF
KYUSHU AND SHIKOKU. THERE IS ALSO LOWER CONFIDENCE IN THE INTENSITY
DURING THIS PORTION OF THE TRACK, AS THE STORM COULD REMAIN OVER
LAND OR MOVE BACK OVER WATER SOUTH OF JAPAN FOR A TIME. HOWEVER,
WEAKENING IS GENERALLY EXPECTED AFTER THE INITIAL LANDFALL DUE TO
CONTINUING LAND INTERACTION AND THE PRESENCE OF DRY AIR AROUND THE
STORM DEPOSITED BY THE PASSING UPPER-LEVEL TROUGHS.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS SLIGHTLY SLOWER THAN
THE MULTI-MODEL CONSENSUS, DISCARDING SOME OF THE FASTEST OUTLIERS
SUCH AS JGSM. ALONG-TRACK SPREAD REMAINS GIGANTIC BEYOND 48 HOURS.
THE JTWC INTENSITY FORECAST IS SLIGHTLY ABOVE THE MULTI-MODEL
CONSENSUS, WITH MODEL SPREAD GROWING AFTER 48 HOURS IN PART DUE TO
DIFFERENCES IN THEIR TRACK FORECASTS.

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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