WDPN32 PGTW 230300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (SHANSHAN) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8N 141.0E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 314 NM WEST-NORTHWEST OF TINIAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 11W (SHANSHAN) WITH CONTINUED FLARING CONVECTION, OBSCURING
THE LOW-LEVEL CYCLONIC CIRCULATION (LLCC). HOWEVER, DRY AIR HAS
NOTICEABLY BEGUN WRAPPING INTO THE WESTERN PERIPHERY OF THE LLC,
ALLOWING FOR A LOOK AT THE CURVED LOW-LEVEL CLOUD LINES THROUGHOUT
THE WESTERN QUADRANT. OVER THE LAST SIX HOURS, TS 11W HAS SLOWLY
MEANDERED TO THE NORTHEAST IN THE EVIDENT WEAK STEERING
ENVIRONMENT. ALTHOUGH WEAK TO MODERATE VERTICAL WIND SHEAR (VWS) OF
15-20 KTS REMAINS OVER THE SYSTEM, WARM SEA SURFACE TEMPERATURES
(29-30 C) AND MODERATE POLEWARD OUTFLOW CONTINUE TO STEADILY
INTENSIFY SHANSHAN. THE INITIAL POSITION IS PLACED WITH LOW
CONFIDENCE BASED ON A PARTIAL 222124Z 91 GHZ SSMIS F16 IMAGE
SHOWING LOW-LEVEL CLOUD LINES WRAPPING INTO THE ASSESSED CENTER
FROM THE WESTERN PERIPHERY OF THE LLCC. THE INITIAL INTENSITY OF 55
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY AND
OBJECTIVE ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: THE PERIPHERY OF A SOUTHWESTWARD
EXTENDING RIDGE FROM THE DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE
EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   CIMSS ADT: 49 KTS AT 230100Z

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

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 11W IS FORECASTED TO REMAIN IN A RELATIVELY
WEAK STEERING ENVIRONMENT OVER THE NEXT 6 HOURS AS RIDGING FROM THE
EAST BUILDS WESTWARD OVER THE NEXT 12 HOURS. AFTER THE STR BUILDS
WESTWARD, TS 11W WILL TRACK NORTHWARD INTO TAU 36. AFTER TAU 36,
THE RIDGE WILL CONTINUE TO BUILD WESTWARD, FORCING THE SYSTEM TO
TRACK NORTHWESTWARD FOR 36 HOURS AS SHANSHAN NEARS THE STR AXIS
BETWEEN TAU 72 AND TAU 96. AFTER TAU 72, AN EASTWARD PROGRESSING
LONGWAVE TROUGH EXTENDING SOUTHWARD OVER THE KOREAN PENINSULA WILL
DRIVE THE SYSTEM TO THE NORTH-NORTHEAST TOWARD WAKAYAMA, JAPAN JUST
BEFORE MAKING LANDFALL BEFORE TAU 120. REGARDING INTENSITY, TS 11W
IS EXPECTED TO STEADILY INTENSIFY AS PERSISTENT VWS AND DRY AIR
WILL REMAIN PREVALENT THROUGHOUT THE NEXT 36 HOURS. AFTER TAU 36,
VWS DECREASES TO LESS THAN 15 KTS AS THE SYSTEM APPROACHES THE STR
AXIS, BECOMING MORE CONDUCIVE FOR FURTHER DEVELOPMENT BY TAU 72. AT
TAU 72, THE AFOREMENTIONED LONGWAVE TROUGH WILL ADD AN ADDITIONAL
INTENSIFICATION MECHANISM WITH ENHANCED POLEWARD OUTFLOW AS TS 11W
NEARS THE WARM KUROSHIO CURRENT, LEADING TO AMPLIFIED
INTENSIFICATION RIGHT BEFORE LANDFALL NEAR WAKAYAMA, JAPAN PRIOR TO
TAU 120.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN MODERATE AGREEMENT
THAT TS 11W WILL GENERALLY TRACK NORTH-NORTHWESTWARD OVER THE NEXT
48 HOURS. GFS REMAINS THE OUTLIER AS THE TRACK BRINGS THE SYSTEM
FURTHER EASTWARD PRIOR TO THE RECURVE NORTH-NORTHEASTWARD BETWEEN
TAU 72 AND TAU 96. THE CROSS-TRACK SPREAD AT TAU 72 IS 132 NM AND
EXTENDS TO 156 NM NEAR THE NORTH-NORTHEASTWARD TURN. THE GFS (GEFS)
AND ECMWF (EPS) ENSEMBLES SUPPORT THE SYSTEMS RECURVE TRACK BETWEEN
TAU 72 AND TAU 96, WITH THE BULK OF SOLUTIONS SHOWING A
NORTH-NORTHEASTWARD TRACK AFTER TAU 96. COAMPS-TC INTENSITY
GUIDANCE ILLUSTRATES A WINDOW OF RAPID INTENSIFICATION BEGINNING AT
TAU 48, WITH PROBABILITIES INCREASING STEADILY FROM 10 PERCENT TO
70 PERCENT BETWEEN TAU 48 AND TAU 96, LEADING TO ENHANCED
INTENSITIES BEFORE MAKING LANDFALL IN WAKAYAMA, JAPAN. THE JTWC
TRACK IS PLACED CLOSE TO CONSENSUS WITH MEDIUM CONFIDENCE.
INTENSITY GUIDANCE IS IN LOW AGREEMENT, WITH NUMERICAL MODELS
VARYING ON INTENSITY GUIDANCE AFTER TS 11W ROUNDS THE STR AXIS.
OVERALL, THE JTWC INTENSITY FORECAST REMAINS IN CLOSE ALIGNMENT
WITH THE CONSENSUS DURING THE FIRST 60 HOURS OF THE FORECAST
PERIOD, WHILE INCREASING ABOVE CONSENSUS AS THE ENVIRONMENT BECOMES 
MORE CONDUCIVE FOR DEVELOPMENT BETWEEN TAU 72 AND TAU 120 AS TS 11W 
APPROACHES MAINLAND JAPAN.

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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