WDPN32 PGTW 290300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 031//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.9N 130.2E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 76 NM SOUTH-SOUTHEAST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS DEEP
CONVECTION OBSCURING A LOW-LEVEL CIRCULATION (LLCC) SYSTEM.
WEST-SOUTHWESTERLY SHEAR ERODES THE NORTHWESTERN SIDE OF THE
TYPHOON AND INTRODUCES DRY AIR; HOWEVER, THE VORTEX REMAINS
VERTICALLY STACKED AND THE ENVIRONMENT REMAINS MOIST. WATER VAPOR
IMAGERY REVEALS STRONG POLEWARD OUTFLOW FED INTO AN UPPER-LEVEL JET
LOCATED TO THE NORTH. A 292119Z RCM-2 SYNTHETIC APERTURE RADAR
(SAR) IMAGE REVEALS LARGER WIND RADII THAN REFLECTED IN THE
PREVIOUS WARNING AND A LARGE SWATH OF 75-80KT WINDS TO THE WEST.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON RADAR
AND ANIMATED MSI. THE INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH
HIGH CONFIDENCE BASED ON THE AFOREMENTIONED SAR IMAGERY
SUPPLEMENTED BY THE AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: A WEAK STEERING ENVIRONMENT BETWEEN TWO
SUBTROPICAL RIDGES (STR) TO THE EAST AND WEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   RJTD: T4.5 - 77 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE WIND RADII ARE LARGER THAN
PREVIOUSLY FORECASTED BASED ON AN RCM-2 292119Z SAR PASS. AS A
RESULT, SPECIFICALLY THE NORTHWESTERN AND SOUTHWESTERN WIND RADII
ARE LARGER IN THE NEAR-TERM FORECAST.

FORECAST DISCUSSION: TYPHOON (TY 11W) IS FORECAST TO REMAIN IN A
COMPETING STEERING ENVIRONMENT BETWEEN TWO RIDGES. FROM TAU 24-36,
TY 11W WILL DRIFT EASTWARD WHILE CAPTURED BY THE COL. WHILE IN A
WEAK ENVIRONMENT, THE SYSTEM HAS THE POTENTIAL TO MOVE ERRATICALLY
DURING A QUASI-STATIONARY PERIOD WITH POSSIBLE LOOPS AND
BACKTRACKING NOT REFLECTED ON THE WARNING GRAPHIC BETWEEN TAU
36-72. AN UPPER-LEVEL TROUGH WILL BREAK DOWN THE RIDGE TO THE EAST,
ALLOWING THE TYPHOON TO TRACK NORTHEASTWARD OVER LAND BETWEEN TAU
96-120. AS THE SYSTEM INTERMITTENTLY INTERACTS WITH THE TERRAIN OF
SOUTHERN JAPAN, IT WILL WEAKEN SIGNIFICANTLY OVER THE FIRST 36
HOURS TO 40KTS, WHICH IT IS EXPECTED TO STAGNATE BETWEEN TAU 36-72
WHILE IN A QUASI-STATIONARY PERIOD. AS THE SYSTEM TRACKS BACK OVER
LAND BETWEEN TAU 96-120, IT IS EXPECTED TO CONTINUE WEAKENING TO
30KTS. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE VARIES GREATLY
WITHIN MEMBERS OF THE JTWC MULTI-MODEL CONSENSUS. GFS TRACKS IN A
SOUTHWESTERLY DIRECTION WHILE ALL OF THE OTHER GUIDANCE TRACKS
NORTHEASTWARD. THE MAJORITY OF THE CONSENSUS MEMBERS ALSO HAVE A
SLOW 5-10KT TRACK SPEED WITH THE EXCEPTION OF ECMWF, GSM, AND
GALWEM. THE SLOWER TRACK SOLUTIONS REPRESENT MULTIPLE DIFFERENT
TIMES AND POSITIONS WHERE LOOPING AND BACKTRACKING IS POSSIBLE
BETWEEN TAU 36-72, WHICH IS WHY THE JTWC FORECAST SLOWS
SIGNIFICANTLY DURING THAT PERIOD, BUT MAY NOT CAPTURE THE MINUTE
MOTION WITHIN THE FORECAST. THE INTENSITY GUIDANCE IS IN GOOD
AGREEMENT FOR A FAST RATE OF WEAKENING WITHIN THE FIRST 24-36 HOURS
AND THEN A MORE GRADUAL WEAKENING AS THE SYSTEM INTERACTS WITH LAND
AND WATER THROUGH THE END OF THE FORECAST. HOWEVER, THE TRACK
VARIABILITY AND MEDIUM TRACK CONFIDENCE WILL IMPACT THE RATE OF
WEAKENING, THEREBY DECREASING THE FAR-TERM INTENSITY FORECAST
CONFIDENCE AS WELL.

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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