WDPN32 PGTW 262100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 022//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 28.0N 131.1E
   INITIAL INTENSITY: 95 KTS
   GEOGRAPHIC REFERENCE: 198 NM EAST-NORTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 34 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A ROBUST
AND SYMMETRIC TYPHOON STRENGTH TC WITH A RECENTLY CLEARED 11NM EYE.
A 261708Z ATMS 183GHZ MICROWAVE IMAGE DEPICTS A WELL DEFINED
MICROWAVE EYE ENCIRCLED BY A UNIFORM SINGULAR BAND OF DEEP
CONVECTION. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON EIR AND ATMS IMAGERY. WIND RADII FIDELITY WAS SUPPORTED BY
A RECENT 261457Z SCATTEROMETERY PASS. THE INITIAL INTENSITY OF 95
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE PREPONDERANCE
OF INTENSITY ESTIMATES INCLUDING AGENCY DVORAK, CIMSS SATCON, ADT
AND AIDT ALL CONGREGATING AROUND 95KTS WITH MINIMAL DEVIATION.

INITIAL WIND RADII BASIS: 261457Z SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   RJTD: T4.5 - 77 KTS
   RCTP: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   CIMSS SATCON: 102 KTS AT 261644Z
   CIMSS ADT: 94 KTS AT 261800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TYPHOON 11W (SHANSHAN) IS FORECASTED TO
CONTINUE TRACKING GENERALLY NORTHWESTWARD UNDER THE STEERING
INFLUENCE OF THE STR TO THE NORTHEAST. THE FUTURE OF 11W IS
ESSENTIALLY TIED TO THE STRENGTH AND ORIENTATION OF TWO
METEOROLOGICAL FEATURES, THE SUBTROPICAL RIDGE TO THE NORTHEAST AND
A SERIES OF MID LATITUDE TROUGHS THAT DIG DOWN OVER THE KOREAN
PENINSULA IN THE COMING DAYS. THE MAJORITY OF CURRENT MODEL
GUIDANCE STILL ECHOES THE CURRENT JTWC TRACK FORECAST, WHICH
INDICATES THAT 11W WILL ROUND THE WESTERN EDGE OF THE STR AND
INTERACT WITH A PASSING MID-LATITUDE TROUGH WHICH INCREASES ITS
POLEWARD ACCELERATION AND EVENTUALLY INITIATES EXTRA-TROPICAL
TRANSITION (ETT). IN THE PREVIOUS SCENARIO HOWEVER, IF THE STR WAS
FURTHER EASTWARD AND THE MID-LATITUDE TROUGH DIDNT DIG AS FAR SOUTH
AS GFS IS NOW INDICATING, IT WOULD RESULT IN THE STORM BEING HALTED
AS IT PASSES OVER SHIKOKU AND 11W WOULD BE TRAPPED AND FORCES
BACKWARDS TEMPORARILY UNTIL THE RIDGE BREAKS DOWN. AS PREVIOUSLY
STATED, THE CURRENT GFS OUTCOME IS PROBABILISTICALLY LESS LIKELY
BUT STILL NONE ZERO, SO IT WARRANTS FURTHER INTERROGATION.  IN
TERMS OF INTENSITY, OVER THE NEXT 24-36 HOURS, FUELED BY VERY WARM
SSTS AND FAVORABLE OUTFLOW, 11W IF FORECASTED TO INTENSIFY TO
AROUND 105KTS AND POSSIBLY HIGHER. AS THE SYSTEMS ROUNDS THE RIDGE
AXIS AND BEGINS TRACKING POLEWARD AND TOWARDS SASEBO NEAR TAU 48,
TRACK SPEEDS SLOW TO AROUND 3KTS. THIS DECELERATION ALLOWS FOR
SIGNIFICANT UPWELLING TO OCCUR, WHICH IS FURTHER EXACERBATED BY
INCREASING VERTICAL WIND SHEAR WHICH WILL ROB THE SYSTEM OF ENERGY
AND RESULT IN A LANDFALL INTENSITY NEAR 80KTS BY TAU 72. AS THE
SYSTEM INTERACTS WITH THE MID-LATITUDE TROUGH AND ISLANDS OF JAPAN
IT WILL QUICKLY WEAKEN TO AROUND 50KTS BY TAU 96. BY TAU 120, THE
SYSTEM IS FORECASTED TO BEGIN ETT AS IT SPEEDS POLEWARD CLINGING TO
THE LAST VESTIGES OF STORM STRENGTH.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN FAIR AGREEMENT THAT 11W
WILL ROUND THE STR AXIS AND PASS SOUTH OF SASEBO BEFORE CURVING
POLEWARD THOUGHT THE FORECAST PERIOD. WHILE THE CROSS TRACK
DISCREPANCY ISNT TOO BAD, THE ALONG TRACK SPREADING QUICKLY WORSENS
AFTER TAU 48. FOR THIS REASON THE JTWC INITIAL TRACK FORECAST IS
PLACED WITH MEDIUM CONFIDENCE AND WITH LOW CONFIDENCE THEREAFTER.
WITH THE EXCEPTION OF HAFS AND GFS, THE BULK OF RELIABLE MODEL
INTENSITY GUIDANCE ECHOES THE JTWC FORECAST INTENSITY WITH MILD
VARIABILITY. THE INTENSITY GUIDANCE IS THEREFORE PLACED WITH
SIMILAR CONFIDENCE.  

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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