WDPN32 PGTW 251500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (SHANSHAN) WARNING NR 017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 26.6N 136.6E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 276 NM WEST-NORTHWEST OF IWO TO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 33 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A
MEDIUM-SIZED SYSTEM THAT CONTINUED TO SLOWLY CONSOLIDATE AS
EVIDENCED BY COOLING CONVECTIVE TOPS OVER THE COLD DENSE OVERCAST
(CDO) THAT HAS RECENTLY DEVELOPED A PINHOLE EYE. A ROBUST POLEWARD
OUTFLOW, A TUTT CELL AND DRY AIR INTRUSION ALONG THE WESTERN FLANK
ARE ALSO EVIDENT ON THE EIR LOOP. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON THE PINHOLE EYE, ADJUSTED FOR A
SLIGHT FORWARD TILT AND LINED UP WITH A MICROWAVE EYE ON THE
250857Z SSMIS IMAGE. THE INITIAL INTENSITY OF 75KTS IS ALSO
ASSESSED WITH HIGH CONFIDENCE BASED ON THE HIGH END OF AGENCY AND
OBJECTIVE DVORAK ESTIMATES AND REFLECTS THE IMPROVED 6-HR EIR
SIGNATURE. ANALYSIS INDICATES A FAVORABLE ENVIRONMENT WITH WARM
ALONG-TRACK SST AND A STRONG, EFFICIENT VENTILATION ALOFT OFFSET BY
MODERATE RELATIVE VWS, SUBSIDENCE FROM THE TUTT CELL TO THE WEST,
AND COLD DRY AIR INTRUSION AT THE LOWER LEVELS.

INITIAL WIND RADII BASIS: PARTIAL SCATTEROMETER DATA.

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO
THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T4.0 - 65 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.5 - 77 KTS
   CIMSS SATCON: 72 KTS AT 251032Z
   CIMSS ADT: 79 KTS AT 251130Z
   CIMSS AIDT: 76 KTS AT 250530Z
   CIMSS DPRNT: 64 KTS AT 251030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TYPHOON SHANSHAN WILL CONTINUE ON A FISHBONE
NORTHWESTWARD TRACK AS THE STEERING STR BUILDS. AFTER TAU 36, IT
WILL TURN MORE NORTHWARD AS IT CRESTS THE STR AXIS AND AFTER TAU
48, ACCELERATE NORTHEASTWARD ON THE POLEWARD SIDE OF THE STR AND BY
TAU 72, MAKE LANDFALL OVER SHIKOKU THEN TRACK ACROSS HONSHU AND
EXIT INTO THE SEA OF JAPAN (SOJ) AROUND TAU 90. BY TAU 120, IT WILL
MAKE LANDFALL OVER THE SOUTHERN TIP OF HOKKAIDO. THE FAVORABLE
ENVIRONMENT WILL FUEL A GRADUAL INTENSIFICATION TO A PEAK OF 110KTS
BY TAU 48 AS THE POLEWARD OUTFLOW, ENHANCED BY THE STRONG WESTERLIES,
IS AMPLIFIED. AFTERWARD, LAND INTERACTION, INCREASING VWS, AND COOLER 
SST IN THE SOJ WILL RAPIDLY ERODE THE SYSTEM DOWN TO 45KTS BY TAU 
120. CONCURRENTLY BY TAU 72, TY 11W WILL BEGIN EXTRA-TROPICAL 
TRANSITION (ETT) AS IT ENTERS THE COLD BAROCLINIC ZONE AND BECOME A 
STRONG GALE-FORCE COLD CORE LOW WITH AN EXPANDING WIND FIELD BY TAU 
120.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT WITH A
GRADUAL AND EVEN SPREAD TO A MERE 100NM BY TAU 72, LENDING HIGH
CONFIDENCE TO THE NEAR- TO MID-PORTION OF THE JTWC TRACK FORECAST
AND MEDIUM CONFIDENCE TO THE INTENSITY FORECAST. AFTERWARD, THE
MODEL ENVELOPE SPREADS OUT UNEVENLY TO 234NM BY TAU 120. THIS WIDE
RANGE PLUS THE VARIABILITY ASSOCIATED WITH LAND INTERACTION AND ETT
LEND LOW CONFIDENCE TO THE EXTENDED PORTION OF THE TRACK AND
INTENSITY FORECASTS. 

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