WDPN32 PGTW 221500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (SHANSHAN) WARNING 
NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.0N 140.9E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 296 NM WEST-NORTHWEST OF TINIAN
   MOVEMENT PAST 6 HOURS: WESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 11W (SHANSHAN) WITH A LARGE (ABOUT 250 NM IN DIAMETER)
SYMMETRICAL CENTRAL DENSE OVERCAST (CDO) OBSCURING THE LOW-LEVEL
CIRCULATION CENTER (LLCC). THE SYSTEM HAS REMAINED QUASI-STATIONARY
FOR THE PAST TWELVE HOURS AS THE CDO HAS CONTINUED TO EXPAND OVER
THE STORM. ENVIRONMENTAL ANALYSIS INDICATES THAT 11W IS IN A
FAVORABLE ENVIRONMENT FOR FURTHER INTENSIFICATION CHARACTERIZED BY
MODERATE EQUATORWARD OUTFLOW ALOFT, LOW VERTICAL WIND SHEAR, AND
WARM SEA SURFACE TEMPERATURES. EVEN THOUGH THE CDO HAS ENLARGED,
THE OVERALL INTENSITY OF THE SYSTEM HAS BEEN SIMILAR OVER THE PAST
12 HOURS, AND THE CONSISTENT DVORAK FIXES OF AROUND T3.0 HELP
SUPPORT THAT NOTION. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON A 221134Z METOPB AMSUB 89 GHZ MICROWAVE IMAGE
REVEALING THE LOW-LEVEL CLOUD LINES WRAPPING INTO THE SYSTEM. THE
INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE OBJECTIVE AND SUBJECTIVE INTENSITY ESTIMATES LISTED
BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 48 KTS AT 221030Z
   CIMSS ADT: 53 KTS AT 221200Z
   CIMSS AIDT: 46 KTS AT 221200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: 11W IS FORECAST TO STAY WITHIN A WEAK STEERING
ENVIRONMENT FOR THE NEXT 12 HOURS OR SO AS RIDGING TO THE EAST
BEGINS TO BUILD. ONCE THE RIDGE HAS SUFFICIENTLY FORMED, 11W WILL
TRACK NORTHWARD ALONG THE WESTERN PERIPHERY OF THE RIDGE THROUGH
TAU 48. NEAR TAU 48, A SUBTROPICAL RIDGE (STR) IS EXPECTED TO FORM
EAST OF HONSHU AND DRIVE THE SYSTEM NORTHWESTWARD THROUGH TAU 96.
AS 11W BEGINS TO ROUND THE STR, IT WILL TRACK NORTHWARD AS IT
APPROACHES THE KII CHANNEL BEFORE MAKING LANDFALL AROUND TAU 120.
REGARDING INTENSITY, 11W IS FORECAST TO STEADILY INTENSIFY TO
AROUND 65 KTS, AT TAU 36 AS LOW VERTICAL WIND SHEAR AND OUTFLOW REMAIN
CONDUCIVE. AFTER TAU 36, A TUTT CELL IS EXPECTED TO PROPAGATE
SOUTHWESTWARD WHICH WILL HINDER THE OUTFLOW OF THE SYSTEM AND
INCREASE THE VERTICAL WIND SHEAR TO AROUND 25 KTS. THIS WILL CAUSE
A STAGNANT INTENSITY THROUGH TAU 72. AFTER TAU 72, THE TUTT CELL
WILL HAVE PASSED BY, ALLOWING OUTFLOW AND SHEAR TO IMPROVE. 11W IS
THEN FORECAST TO CONTINUE INTENSIFYING TO AROUND 105 KTS BEFORE IT
MAKES LANDFALL IN JAPAN. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN RELATIVELY GOOD
AGREEMENT WITH THE EXCEPTION OF GALWEM, WHICH TRACKS THE SYSTEM
MUCH FURTHER WEST THROUGH TAU 24 THAN THE REST OF THE GUIDANCE. THE
CROSS-TRACK SPREAD AT TAU 48 IS AROUND 125 NM, WHICH OPENS UP TO
AROUND 200 NM AT TAU 96. ECMWF CONTINUES TO TRACK 11W MORE SHARPLY
WESTWARD IN RESPONSE TO THE STR NEAR TAU 48 AND BECOMES THE
WESTERNMOST MEMBER. GFS AND THE GFS ENSEMBLE MEAN ARE BOTH LESS
AFFECTED BY THE STR AND MAKE UP THE EASTERNMOST MEMBERS. THE JTWC
TRACK FORECAST IS PLACED SLIGHTLY EAST OF CONSENSUS, TO OFFSET
GALWEM, WITH MEDIUM CONFIDENCE. THE INTENSITY GUIDANCE HAS IMPROVED
OVER THE PAST 6 HOURS WITH MANY OF THE RAPID INTENSIFICATION (RI)
AIDS NO LONGER TRIGGERING. HAFS-A IS SOMEWHAT OF AN OUTLIER WITH NO
INTENSIFICATION THROUGH TAU 72. THE PEAK INTENSITY OCCURS AROUND
TAU 108, JUST BEFORE LANDFALL, IN NEARLY ALL THE GUIDANCE. THE PEAK
RANGES FROM 85 KTS (GFS) TO 120 KTS (HAFS-A), WITH THE JTWC
INTENSITY FORECAST CALLING FOR 105 KTS AS THE SYSTEM MAKES
LANDFALL, THOUGH THE PEAK WILL LIKELY OCCUR JUST BEFOREHAND.
OVERALL, THERE IS MEDIUM CONFIDENCE IN THE NEAR TERM REGARDING THE
JTWC INTENSITY FORECAST WHICH DROPS TO LOW THEREAFTER DUE TO THE
WIDE RANGES OF PEAK INTENSITY VALUES. 

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