WDPN32 PGTW 031500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (BAILU) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 35.8N 145.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 275 NM EAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 20 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED PROXY VISIBLE (PROXYVIS) SATELLITE IMAGERY DEPICTS
FRAGMENTED BANDS OF SHALLOW CONVECTION WRAPPING INTO A GENERALLY
MORE DEFINED LOW LEVEL CIRCULATION CENTER (LLCC). THIS EVENINGS
ASCAT PASSES MISSED THE CENTER OF TROPICAL STORM (TS) 13W, BUT THE
TRENDS IN THE VISIBLE AND THEN PROXYVIS IMAGERY SUGGESTS THAT THE
TWO CIRCULATIONS SEEN SIX HOURS AGO HAVE INDEED MERGED AND THE
SYSTEM MAY IN FACT BE CONSOLIDATING INTO A SINGLE VORTEX. THERE ARE
HINTS THERE MAY STILL BE A SECONDARY CIRCULATION OFF TO THE WEST. A
LACK OF RECENT MICROWAVE AND SCATTEROMETER DATA COVERING THE CENTER
PRECLUDES MORE COMPLEX ANALYSIS. SO, THE INITIAL POSITION IS
ASSESSED WITH LOW CONFIDENCE, WHILE THE INITIAL INTENSITY IS
ASSESSED WITH MEDIUM CONFIDENCE, BASED ON THE OVERALL AVERAGE OF
THE AVAILABLE SUBJECTIVE AND OBJECTIVE ESTIMATES NOTED BELOW.
ENVIRONMENTAL CONDITIONS ARE MARGINAL, WITH LOW VWS, MARGINAL SSTS,
STRONG POLEWARD OUTFLOW INTO A 200MB JET MAX PASSING EAST OF
HOKKAIDO, AND A FAIRLY DEEP LAYER OF DRY AIR LYING OUT TO THE WEST
SIDE OF THE CENTER.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   CIMSS SATCON: 42 KTS AT 031120Z
   CIMSS ADT: 34 KTS AT 031140Z
   CIMSS AIDT: 41 KTS AT 031140Z
   CIMSS D-PRINT: 34 KTS AT 031120Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10-15 KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY MID-LEVEL AIR, PARTICULARLY ON THE WESTERN
PORTION OF THE CIRCULATION. 

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 13W HAS BEEN TRAVELING ON A STEADY
NORTHEASTWARD COURSE OVER THE LAST DAY, ALONG THE NORTHWESTERN SIDE
OF THE STR. GLOBAL MODELS SUGGEST THE RIDGE IS ABOUT TO COMMENCE
ITS RAPID REPOSITIONING TO THE SOUTHWEST AND ONCE THIS PROCESS
KICKS OFF, TS 13W WILL QUICKLY SHIFT TO A MORE EASTWARD TRACK ALONG
THE NORTH SIDE OF THE RIDGE. AS THE RIDGE AND TS 13W MOVE FURTHER
APART FROM ONE ANOTHER, THE STEERING GRADIENT WILL WEAKEN, AND TS
13W BEGINS TO SLOW DOWN AS EARLY AS TAU 24. AROUND TAU 48, A COL
REGION DEVELOPS SOUTH OF TS 13W, BETWEEN THE STR THAT BY THEN IS
POSITIONED SOUTH OF TOKYO AND A BUILDING STR MOVING IN FROM THE
EAST, POSITIONED NEAR THE DATELINE. AS TS 13W PASSES NORTH OF THIS
COL REGION, TRACK SPEEDS SLOW SIGNIFICANTLY AS THERE WILL BE LITTLE
IN THE WAY OF STEERING GRADIENT AT THIS TIME. TAU 48, AS THE
EASTERN RIDGE BUILDS IN, TS 13W SHIFTS TO A MORE NORTHEASTWARD
TRACK AND BEGINS TO PICK UP SPEED ONCE AGAIN, CONTINUING TO
ACCELERATE NORTHEASTWARD THROUGH THE REMAINDER OF THE FORECAST.
WHILE THE SYSTEM APPEARS TO HAVE CONSOLIDATED INTO A SINGLE VORTEX,
AT LEAST FOR THE MOMENT, AND SHEAR AND OUTFLOW CONDITIONS ARE
SUPPORTIVE FOR INTENSIFICATION, THE COOL SSTS AND INFLUENCE OF DRY
AIR WILL EFFECTIVELY OFFSET THE OTHERWISE FAVORABLE UPPER-LEVEL
CONDITIONS. TS 13W IS FORECAST TO MAINTAIN INTENSITY THROUGH TAU 48
BEFORE WEAKENING DUE TO DRAMATICALLY INCREASING SHEAR AND DRY AIR
ENTRAINMENT. THE SYSTEM WILL BEGIN EXTRATROPICAL TRANSITION (ETT)
BY TAU 72 AND IS EXPECTED TO COMPLETE ETT NO LATER THAN TAU 96 AND
LIKE AS EARLY AS TAU 84. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE REMAINS IN GOOD
AGREEMENT, WITH 115NM CROSS-TRACK UNCERTAINTY AT TAU 72, INCREASING
SLIGHTLY BY TAU 96. ALONG-TRACK SPREAD INCREASES TO 100NM AT TAU 72
AND 150NM AT TAU 96. GFS REMAINS PARKED ON THE RIGHT SIDE OF THE
ENVELOPE AND IS THE FASTEST OF THE CONSENSUS AIDS, WHILE THE GALWEM
AND ECWMF ARE POSITIONED ON THE LEFT SIDE AND SLOWEST. THE JTWC
FORECAST IS PLACED A BIT RIGHT OF AND SLIGHTLY FASTER THAN THE
CONSENSUS MEAN, HEDGED TOWARDS THE GFS, WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH ALL OF THE
CONSENSUS MEMBERS SHOWING MINOR FLUCTUATIONS BETWEEN 35 AND 40
KNOTS THROUGH TAU 60, FOLLOWED BY A RAPID DROP-OFF. THE JTWC
FORECAST CONSISTENT WITH THE CONSENSUS MEAN TO TAU 48 THEN DEVIATES
ABOUT 10 KNOTS HIGHER DUE TO MODEL TENDENCY TO WEAKEN TROPICAL
CYCLONES TOO QUICKLY DURING ETT. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: HIGH
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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