WDPN32 PGTW 230900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (SHANSHAN) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8N 141.2E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 420 NM SOUTH OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 11W (SHANSHAN) AS A COMPACT SYSTEM WITH STRONG CONVECTIVE
BANDING FEATURES ALONG THE SOUTHERN PERIPHERY. APPARENT DRY AIR IS
INTRUDING ON THE WESTERN PORTION OF 11W, HINDERING CONVECTIVE
DEVELOPMENT IN THAT REGION. ADDITIONALLY, THE TUTT CELL POSITIONED
TO THE NORTH OF 11W IS CAUSING INCREASED WESTERLY SHEAR. THESE TWO
FACTORS ARE CAUSING THE WESTERN SEMICIRCLE OF THE DEFINED LOW-LEVEL
CIRCULATION CENTER (LLCC) TO BE PARTIALLY EXPOSED. ENVIRONMENTAL
ANALYSIS INDICATES THAT 11W IS IN A MARGINALLY FAVORABLE
ENVIRONMENT FOR FURTHER INTENSIFICATION CHARACTERIZED BY MODERATE
POLEWARD AND EQUATORWARD OUTFLOW ALOFT, MODERATE WESTERLY VERTICAL
WIND SHEAR, THE PRESENCE OF DRY AIR, AND WARM SEA SURFACE
TEMPERATURES. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON THE ANIMATED MSI IMAGERY AND A 230625Z SSMIS 91 GHZ
MICROWAVE IMAGE SHOWING THE LOW-LEVEL CLOUD LINES JUST WEST OF THE
DEEP CONVECTION. THE INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE OBJECTIVE AND SUBJECTIVE INTENSITY
ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: SMAP DATA 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SOUTHWARD
EXTENDING RIDGE TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.0 - 65 KTS
   CIMSS SATCON: 58 KTS AT 230400Z
   CIMSS ADT: 74 KTS AT 230600Z
   CIMSS AIDT: 66 KTS AT 230600Z
   CIMSS DPRINT: 45 KTS AT 230600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: DRY AIR TO THE WEST AND A TUTT CELL POSITIONED TO
THE NORTH OF THE SYSTEM.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: 11W IS FORECAST TO CONTINUE TRACKING GENERALLY
NORTHWARD THROUGH TAU 24, ALONG THE WESTERN PERIPHERY OF THE
RIDGING TO ITS EAST. AFTER TAU 24, A BUILDING SUBTROPICAL RIDGE
(STR) POSITIONED EAST OF HONSHU WILL TURN THE SYSTEM NORTHWESTWARD
THROUGH TAU 72. AFTER TAU 72, 11W IS EXPECTED TO ROUND THE STR AND
MAKE ITS WAY NORTHEASTWARD TOWARDS HONSHU BEFORE MAKING LANDFALL
NEAR TAU 108. REGARDING INTENSITY, 11W IS FORECAST TO STEADILY
INTENSIFY TO 80 KTS AT TAU 48 DUE TO THE UNFAVORABLE PLACEMENT OF
THE TUTT CELL. AFTER TAU 48, THE TUTT CELL WILL HAVE PROPAGATED FAR
ENOUGH SOUTHWESTWARD TO ALLOW VERTICAL WIND SHEAR TO DRASTICALLY
DROP WHICH WILL INTENSIFY 11W AS A RESULT. AN INTENSITY OF 115 KTS
IS FORECASTED AT TAU 96, BUT THE PEAK INTENSITY OF THE SYSTEM MAY
OCCUR AFTERWARDS, JUST BEFORE LANDFALL. AS THE VORTEX MAKES
LANDFALL IN HONSHU, TERRAIN INTERACTION WILL QUICKLY BEGIN WEAKENING
THE SYSTEM TO AROUND 65 KTS NEAR THE END OF THE FORECAST PERIOD. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 96, WITH A CROSS-TRACK SPREAD OF 118 NM AT THAT TIME.
HOWEVER, AFTER TAU 96, MODELS BEGIN TO SLIGHTLY DISAGREE ON THE
SHARPNESS OF THE NORTHEASTWARD TURN. ECMWF AND GFS DETERMINISTIC
MODELS BOTH TURN THE SYSTEM SHARPLY, TRACKING NEAR TOKYO AT TAU
120. THE ENSEMBLE MEAN VARIANTS AS WELL AS HAFS-A AND NAVGEM, ALL
TRACK THE SYSTEM MORE NORTH-NORTHEASTWARD AS IT ROUNDS THE STR. THE
JTWC TRACK FORECAST IS PLACED CLOSE TO CONSENSUS THROUGH TAU 96,
AND CLOSER TO THE BULK OF GUIDANCE AFTERWARDS. IF FUTURE MODEL RUNS
BEGIN DRIFTING CLOSER TO TOKYO, A SHIFT OF THE TRACK SOUTHEASTWARD
COULD ENSUE. OVERALL, THERE IS MEDIUM CONFIDENCE IN THE JTWC TRACK
THROUGH TAU 72 AND LOW THEREAFTER. INTENSITY GUIDANCE IS IN GOOD
AGREEMENT THROUGH TAU 48, DEPICTING A GRADUAL INTENSIFICATION
TREND. AFTER TAU 48, MODELS BEGIN TO HIGHLY DISAGREE ON THE PEAK
INTENSITY OF THE SYSTEM. HAFS-A DEPCITS A PEAK OF AROUND 140 KTS
AND COAMPS-TC OF 115 KTS. THE STATISTICAL DYNAMICAL MODELS, AS WELL
AS GFS, ALL DEPICT A PEAK INTENSITY OF AROUND 90 KTS. THE JTWC
INTENSITY FORECAST CALLS FOR A PEAK OF 115 KTS, THOUGH THE ACTUAL
PEAK INTENSITY OF 11W COULD BE EVEN HIGHER BEFORE LANDFALL IS MADE.
AS A RESULT, THE JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM
CONFIDENCE THROUGH TAU 72 AND LOW THEREAFTER.

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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