WDPN32 PGTW 220900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (SHANSHAN) WARNING 
NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.0N 141.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 286 NM WEST-NORTHWEST OF TINIAN
   MOVEMENT PAST 6 HOURS: WESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM 11W (SHANSHAN) WITH PERSISTENT AND VIGOROUS CONVECTIVE BURSTS
OVERRAN OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC). 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. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON LOW-LEVEL CLOUD TRACING IN A 220639Z F18 SSMIS 91 GHZ
MICROWAVE IMAGE. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE OBJECTIVE AND SUBJECTIVE INTENSITY
ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T3.0 - 45 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 47 KTS AT 220060Z
   CIMSS AIDT: 40 KTS AT 220600Z
   CIMSS DPRINT: 39 KTS AT 220600Z
   CIMSS DMINT: 39 KTS AT 220433Z

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: 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 DRIFT NORTHWESTWARD AND
STAY WITHIN A WEAK STEERING ENVIRONMENT FOR THE NEXT 24 HOURS AS
RIDGING TO THE EAST STARTS TO BUILD AND BECOMES THE STEERING
MECHANISM FOR THE SYSTEM. AFTER TAU 24, THE RIDGING TO ITS EAST
WILL DRIVE 11W NORTH-NORTHWESTWARD THROUGH TAU 48. AFTER TAU 48, A
BUILDING SUBTROPICAL RIDGE (STR) IS EXPECTED TO FORM EAST OF HONSHU
AND WILL TURN THE SYSTEM TO A MORE NORTHWESTWARD TRACK THROUGH TAU
96. AS 11W BEGINS TO ROUND THE STR, IT IS FORECAST TO TRACK NEARLY
STRAIGHT NORTHWARD FOR THE REMAINDER OF THE FORECAST PERIOD AS IT
APPROACHES THE KII CHANNEL. REGARDING INTENSITY, 11W IS FORECAST TO
CONTINUE INTENSIFYING OVER THE NEXT 36 HOURS TO AROUND 70 KTS DUE
TO THE LOW VERTICAL WIND SHEAR AND OUTFLOW REMAINING CONDUCIVE.
AFTER TAU 36, A TUTT CELL IS EXPECTED TO PROPAGATE SOUTHWESTWARD
AND WILL HINDER THE OUTFLOW OF THE SYSTEM THROUGH TAU 72 CAUSING A
STAGNANT INTENSITY. AFTER TAU 72, THE TUTT CELL WILL HAVE PASSED
BY, ALLOWING OUTFLOW TO IMPROVE. 11W IS THEN FORECAST TO CONTINUE
INTENSIFYING TO AROUND 100 KTS BEFORE IT MAKES LANDFALL. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN MODERATE AGREEMENT
WITH A 150 NM CROSS-TRACK SPREAD AT TAU 36. THIS IS LIKELY DUE TO
THE WEAK STEERING ENVIRONMENT OVER THAT TIME. THE TIMING OF THE
POLEWARD TURN WILL GIVE MORE CONFIDENCE TO THE FORECAST, AS MODELS
AGREE AFTERWARDS. ECMWF TURNS MORE SHARPLY NORTHWESTWARD AS THE STR
BUILDS AROUND TAU 72, MAKING IT THE WESTERNMOST MEMBER FROM TAU 96
ONWARD. GFS AND THE GFS ENSEMBLE MEAN BOTH SEEMINGLY LESS AFFECTED
BY THE STR AND MAKE UP THE EASTERNMOST MEMBERS. THE JTWC TRACK
FORECAST IS PLACED CLOSE TO CONSENSUS WITH OVERALL MEDIUM
CONFIDENCE. INTENSITY GUIDANCE IS IN WORSE AGREEMENT, WITH RI AIDS
TRIGGERING. THE RI AIDS HIGHLY DISAGREE WITH MESOSCALE MODELS.
HAFS-A AND HWRF BOTH CALL FOR MINIMUM INTENSIFICATION THROUGH TAU
72. GFS ALSO CALLS FOR INTENSIFICATION TO AROUND 55 KTS AT TAU 72.
COAMPS-TC FALLS BETWEEN THE RI AND PREVIOUSLY MENTIONED MODELS. 
HOWEVER, AFTER TAU 72, MODELS DO AGREE ON INTENSIFICATION WITH 
CONSENSUS NEAR THE 100 KNOT MARK AT TAU 120. THE JTWC INTENSITY 
FORECAST IS PLACED HIGHER THAN THE MESOSCALES MODELS, BUT WITHOUT 
RAPID INTENSIFICATION THROUGH TAU 72 AND CLOSE TO CONSENSUS 
THEREAFTER. THE PERSISTENCE OF RI AIDS COULD CHANGE THE UPCOMING 
FORECASTS, AS ENVIRONMENT CONDITIONS ARE OVERALL FAVORABLE. DUE TO 
THE LARGE SPREAD IN GUIDANCE, THE JTWC INTENSITY FORECAST IS PLACED 
WITH OVERALL LOW CONFIDENCE.

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