WDPN32 PGTW 282100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 030//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.3N 130.2E
   INITIAL INTENSITY: 85 KTS
   GEOGRAPHIC REFERENCE: 111 NM SOUTH-SOUTHEAST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
COLLAPSING UPPER-LEVEL STRUCTURE FROM WHAT WAS PREVIOUSLY AN EYE
FEATURE DUE TO MODERATE 15-20KT VERTICAL WIND SHEAR (VWS) ERODING
THE VORTEX FROM THE WEST. DRY AIR IS ALSO APPARENT BORDERING THE
NORTHWESTERN PERIPHERY OF THE SYSTEM, HOWEVER IT IS NOT PENETRATING
THE VORTEX AND THE INNER CORE OF TYPHOON 11W REMAINS MOIST. ON
WATER VAPOR IMAGERY, STRONG POLEWARD OUTFLOW IS IDENTIFIABLE
FEEDING IN TO AN UPPER-LEVEL JET EXTENDING ACROSS THE NORTHERN
PACIFIC OCEAN. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON ANIMATED EIR AND RADAR IMAGERY. THE INITIAL INTENSITY OF
85 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY AND
OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: A WEAK STEERING ENVIRONMENT LEADING TO
SLOW MOTION BETWEEN A SUBTROPICAL RIDGE (STR) TO THE EAST AND A STR
TO THE WEST CAPPED BY ZONAL FLOW TO THE NORTH PREVENTING THE SYSTEM
FROM TRACKING NORTHEASTWARD

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   RJTD: T5.0 - 90 KTS
   RCTP: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   CIMSS SATCON: 83 KTS AT 281722Z
   CIMSS ADT: 79 KTS AT 281730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: TY 11W WILL BE NEARLY
QUASI-STATIONARY BETWEEN TAU 48-96 DUE TO A POOR STEERING
ENVIRONMENT WITHIN A COL REGION BETWEEN TWO RIDGES. DURING THIS
TIME FRAME, THE TRACK WILL BE HIGHLY VARIABLE WITH A POTENTIAL FOR
A LOOPING TRACK, AND EVEN BACKTRACKING AT TIMES. 

FORECAST DISCUSSION: TY 11W WILL REMAIN IN A VARIABLE AND WEAK
STEERING ENVIRONMENT PINNED WITHIN TWO RIDGES AND ZONAL FLOW TO THE
NORTH. AS THE RIDGES SLOWLY DRIFT EASTWARD, THE TYPHOON WILL
CONTINUE MOVING IN THAT DIRECTION, WITH A QUASI-STATIONARY PERIOD
BETWEEN TAU 48-96 BEFORE BEING DRAWN IN A NORTHEASTWARD DIRECTION
BETWEEN TAU 96 AND 120 BY AN UPPER-LEVEL TROUGH THAT BREAKS THROUGH
THE BLOCKING FLOW TO THE NORTH. TY 11 WILL CONTINUE TO WEAKEN TO
45KTS THROUGH TAU 36 DUE TO LAND INTERACTION BEFORE STAGNATING AT A
45KT INTENSITY THROUGH TAU 72 WHILE QUASI-STATIONARY CLOSE TO
WATER. AFTER TAU 72, THE SYSTEM WILL CONTINUE TO WEAKEN WHILE
INTERMITTENTLY OVER LAND AND WATER THROUGH THE END OF THE FORECAST
PERIOD DESPITE OTHER FAVORABLE FEATURES OF LOW VWS, WARM SEA
SURFACE TEMPERATURES, AND GOOD UPPER-LEVEL OUTFLOW UNTIL TAU 72 WHEN
AN UPPER-LEVEL TROUGH WILL INTERACT WITH THE SYSTEM.

MODEL DISCUSSION: THE JTWC MULTI-MODEL CONSENSUS PRODUCED VARIED
SOLUTIONS LEADING TO UNCERTAINTY IN THE TRACK FORECAST. GALWEM AND
GSM WERE CONSIDERED OUTLIERS BASED ON UNREALISTIC TRACK SPEED, BUT
THE OTHER JTWC MEMBERS GENERALLY AGREED ON A SLOWER TRACK SPEED.
THE TRACK GUIDANCE ALSO SHOWS SIGNIFICANT VARIABILITY IN THE JTWC
MULTI-MODEL CONSENSUS BETWEEN TAU 24-72 DURING A QUASI-STATIONARY
PERIOD REPRESENTING SEVERAL POTENTIALLY ERRATIC LOOPS AND PERIODS
OF BACKTRACKING WHILE IN A COL REGION BETWEEN THE TWO RIDGES. THE
INTENSITY GUIDANCE IS IN GOOD AGREEMENT THAT THE SYSTEM WILL
GRADUALLY WEAKEN DUE TO INTERMITTENT LAND-SEA INTERACTION AS TY 11W
APPROACHES JAPAN. 

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