WDPN32 PGTW 270300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 023//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 28.2N 130.8E
   INITIAL INTENSITY: 105 KTS
   GEOGRAPHIC REFERENCE: 299 NM SOUTH OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 36 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
AS THE SUN RISES OVER 11W, ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) DEPICTS A RECENTLY CLOUD FILLED EYE SURROUNDED BY MOSTLY
SYMMETRIC DEEP CONVECTION. A 262209Z SSMIS 91GHZ MICROWAVE IMAGE
SHOWS A WELL DEFINED MICROWAVE EYE WITH A RELATIVELY SMALL
CONVECTIVE COVER BEYOND WHICH A LARGE FIELD OF LOW LEVEL CLOUD
LINES IS VISIBLE. A VERY RECENT 270023Z ASCAT-B SCATTEROMETER PASS
REVEALS A MOSTLY SYMMETRIC WIND FIELD WITH A SLIGHT DISTENTION TO
THE EAST OF THE LLCC. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON MSI AND SSMIS IMAGERY. THE INITIAL INTENSITY OF
105 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
PREPONDERANCE OF INTENSITY ESTIMATES INCLUDING AGENCY DVORAK, CIMSS
ADT AND AIDT ALL CONGREGATING AROUND 105KTS WITH MINIMAL DEVIATION.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   RJTD: T5.5 - 102 KTS
   RCTP: T5.5 - 102 KTS
   KNES: T5.5 - 102 KTS
   CIMSS ADT: 109 KTS AT 262340Z

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

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

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 TRACK OF 11W IS
ESSENTIALLY TIED TO THE STRENGTH AND ORIENTATION OF THREE NOTABLE
METEOROLOGICAL FEATURES, THE STEERING SUBTROPICAL RIDGE TO THE
NORTHEAST, AN UPPER LEVEL LOW TO THE EAST 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). OVER THE PAST
SIX HOURS HOWEVER, NUMERICAL AND ENSEMBLE MODEL GUIDANCE IS
INCREASINGLY INDICATING A DIFFERENT SCENARIO IS POSSIBLE. IN THIS
SECOND SCENARIO, THE STEERING STR IS FURTHER EASTWARD AND THE
MID-LATITUDE TROUGH DOESNT DIG AS FAR SOUTH AS GFS IS NOW
INDICATING, WHICH WOULD RESULT IN THE STORM BEING HALTED AS IT
PASSES OVER SHIKOKU AND 11W WOULD BE TRAPPED AND BECOME
QUASI-STATIONARY IF NOT FORCED BACKWARDS TEMPORARILY UNTIL THE
RIDGE BREAKS DOWN. IN TERMS OF INTENSITY, OVER THE NEXT 12 HOURS,
FUELED BY VERY WARM SSTS AND FAVORABLE OUTFLOW, 11W IS FORECASTED
TO INTENSIFY TO AROUND 110KTS. AS THE SYSTEMS ROUNDS THE RIDGE AXIS
AND BEGINS TRACKING POLEWARD AND TOWARDS SASEBO NEAR TAU 48, AN
UPPER LEVEL LOW IS FORECASTED TO BREAK DOWN THE STEERING RIDGE
RESULTING IN WEAKER STEERING FLOW AND THEREFOR MUCH SLOWER TRACK
SPEEDS. 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 LOWER
LANDFALL INTENSITY NEAR TAU 60. 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 CLINGS TO THE LAST VESTIGES OF STORM STRENGTH.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN POOR TO 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 OVERALL
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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