WDPN32 PGTW 292100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (SHANSHAN) WARNING 
NR 034//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.7N 131.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 61 NM SOUTHWEST OF IWAKUNI, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
SHALLOWING LOW-LEVEL CIRCULATION CENTER (LLCC) WITH WARMING CLOUD
TOPS RELATED TO THE WEAKENING THAT HAS OCCURRED OVER THE LAST TWELVE
HOURS. WATER VAPOR IMAGERY REVEALS STRONG POLEWARD OUTFLOW FEEDING
INTO AN UPPER-LEVEL JET TO THE NORTHEAST, WHICH INTRODUCES CLEAR
DRY AIR IMPINGEMENT ON THE WESTERN PERIPHERY WHILE THE CORE APPEARS
TO REMAIN MOIST. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON ANIMATED EIR AND RADAR IMAGERY REVEALING
CONCENTRIC CLOUD BANDING DESPITE TOPOGRAPHIC EFFECTS ERODING THE
TROPICAL STORM STRUCTURE WHILE OVER LAND. THE INITIAL INTENSITY OF
40 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE AGENCY AND
OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: PINNED IN A COL REGION BETWEEN TWO
COMPETING SUBTROPICAL RIDGES (STR) OF SIMILAR STRENGTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T3.0 - 45 KTS
   CIMSS ADT: 41 KTS AT 291930Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE JTWC TRACK FORECAST IS EXPECTED
TO TAKE A NORTHWARD TURN BETWEEN TAU 48 AND 72.

FORECAST DISCUSSION: TROPICAL STORM (TS) 11W IS FORECAST TO TRACK
EAST-NORTHEASTWARD OVER THE NEXT 12 HOURS TOWARDS IWAKUNI WHILE
CAPTURED WITHIN THE COL REGION. THE TWO RIDGES STALL BETWEEN TAU
12-24, RESULTING IN A QUASI-STATIONARY PERIOD BETWEEN TAU 24-48.
THE MOTION DURING THAT TIME IS UNCERTAIN AND FORECAST WITH LOW
CONFIDENCE, AND MINOR CHANGES WILL CAUSE THE SYSTEM TO EITHER
REMAIN OVER THE EASTERN SHIKOKU DISTRICT OR TAKE TS 11W OUT OVER
THE WARM WATERS (26-28C) OF KII-SUIDO. AN UPPER-LEVEL TROUGH
PASSING OVER THE SEA OF JAPAN BETWEEN TAU 24-36 WILL DEGRADE THE
STR TO THE EAST AND ALLOW THE TROPICAL STORM TO BE PICKED UP IN A
MORE NORTH-NORTHEASTWARD DIRECTION AFTER TAU 48. THE TRACK
INTENSITY IS CONFIDENT WITHIN THE FIRST 24 HOURS AS THE SYSTEM
TRACKS BACK OVER LAND IN THE VICINITY OF IWAKUNI. BETWEEN TAU 24
AND 48, THE SYSTEM IS EXPECTED TO CONTINUE TO WEAKEN DUE TO LAND
INTERACTION, HOWEVER THE VARIABILITY IN THE TRACK FORECAST WHILE
QUASI-STATIONARY MAY BRING THE SYSTEM OUT OVER WARMER WATERS AND
MAY PROVIDE THE SYSTEM ENERGY TO MAINTAIN THE CIRCULATION AND
INTENSITY LONGER THAN REFLECTED IN THE JTWC INTENSITY FORECAST. 

MODEL DISCUSSION: THE DETERMINISTIC MODEL GUIDANCE IS IN GOOD
AGREEMENT REGARDING THE GENERAL TRACK DIRECTION DURING THE FORECAST
PERIOD, HOWEVER THERE ARE VARIED TRACK SPEED ACROSS THE FORECAST
PERIOD LEADING TO A 400NM ALONG-TRACK SPREAD. THE SPREAD CAN BE
ATTRIBUTED TO DIFFERENT RATES OF WEAKENING REFLECTED ACROSS THE
MODELS, ESPECIALLY BETWEEN TAU 24-48 WHEN VARIOUS MODELS EITHER
TAKE THE SYSTEM OUT OVER WATER ALLOWING THE INTENSITY TO BE
MAINTAINED OR WILL KEEP THE CIRCULATION OVER THE MOUNTAINOUS
SHIKOKU RESULTING IN POTENTIALLY WEAKER DISSIPATION. THE ENSEMBLE
GUIDANCE DEPICTS JUST AS VARIED SOLUTIONS IN BOTH ECENS AND GEFS,
WITH ECENS SPECIFICALLY ALLOWING MORE TIME OVER WATER AND
MAINTAINING AN INTENSITY ABOVE WARNING THRESHOLD FOR MUCH LONGER.

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