WDPN33 PGTW 010900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (ROKE) WARNING NR 
013//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 33.1N 146.5E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 362 NM EAST-SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 19 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A SOMEWHAT
RAGGED BUT OVERALL SYMMETRICAL CENTRAL DENSE OVERCAST (CDO)
OBSCURING A COMPACT LOW LEVEL CIRCULATION CENTER (LLCC), WITH A
COLD FRONTAL-LIKE CONVECTIVE BAND EXTENDING SOUTHWARD FROM THE
LLCC. ANIMATED WATER VAPOR IMAGERY DEPICTS EXTREMELY STRONG
POLEWARD OUTFLOW STREAMING OUTWARD FROM THE SYSTEM ALL THE WAY TO
THE ALEUTIAN ISLANDS. A 010534Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS
THAT THE SYSTEM HAS MAINTAINED A COMPACT INNER CORE AND A SMALL 
MICROWAVE EYE FEATURE WITH DEEP CONVECTIVE BANDS WRAPPING INTO THE 
CORE OF THE SYSTEM. THE INITIAL POSITION IS ASSESSED WITH HIGH 
CONFIDENCE BASED ON THE MICROWAVE EYE FEATURE AND AGREEMENT WITH 
AGENCY FIX POSITIONS. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM 
CONFIDENCE BASED ON THE HIGHER END OF THE AGENCY FIX INTENSITIES, 
WHICH ALIGN THE RECENT SATELLITE CONSENSUS ESTIMATES. ENVIRONMENTAL 
CONDITIONS ARE GENERALLY UNFAVORABLE WITH HIGH VWS, AND MARGINAL SSTS 
(25-26C) OFFSET BY VERY STRONG POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: PERSISTENCE AND OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) EXTENSION CENTER TO THE EAST-SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   CIMSS SATCON: 57 KTS AT 010101Z
   CIMSS ADT: 82 KTS AT 010540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30+ KTS
   SST: 25-26 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE SOUTHWEST

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TROPICAL STORM ROKE IS MOVING NORTHEASTWARD
ALONG THE WEST SIDE OF THE SUBTROPICAL RIDGE TO THE EAST AND THIS
GENERAL MOTION IS EXPECTED TO CONTINUE FOR THE NEXT 12 TO 24 HOURS.
AROUND TAU 24 A FAST-MOVING TRANSITORY ANTICYCLONE WILL MOVE OFF
THE COAST OF HOKKAIDO AND MOVE INTO A POSITION TO THE NORTH OF TS
20W. AS THIS OCCURS, STRONG NORTHEASTERLY FLOW WILL BLOCK TS ROKE
FROM MOVING FURTHER TO THE NORTHEAST, AND THE SYSTEM WILL SLOW TO
JUST 5 KNOTS OR LESS BY TAU 36. TROPICAL STORM ROKE HAS ALREADY
BEGUN THE SUBTROPICAL TRANSITION (STT) PROCESS, WITH AUTOMATED AND
MANUAL PHASE CLASSIFICATIONS INDICATING A HYBRID SYSTEM,
CHARACTERIZED AS HAVING BOTH TROPICAL AND SUBTROPICAL
CHARACTERISTICS. OVER THE PAST SIX HOURS THE SYSTEM HAS ACTUALLY
BECOME BETTER ORGANIZED WITH THE DEEP CONVECTIVE BANDS, THOUGH
LIMITED IN AREAL EXTENT, WRAPPING INTO A BETTER DEFINED AND MORE
SYMMETRICAL LLCC, WHICH IS SURPRISING GIVEN THE VERY STRONG SHEAR
(APPROACHING 40 KNOTS). HOWEVER THE SHEAR VECTOR IS ALMOST PERFECTLY
ALIGNED WITH STORM MOTION, SO EFFECTIVE SHEAR IS LESS AND IS BORN
OUT BY THE SATELLITE APPEARANCE AND THE FACT THAT THE SYSTEM HAS
BEEN ABLE TO MAINTAIN A COMPACT INNER CORE. BUT THE SYSTEM HAS
MOVED NORTH OF THE 26C ISOTHERM AND FROM HERE ON OUT THE WATER
TEMPS WILL GET PROGRESSIVELY COOLER, ROBBING TS 20W OF THAT SOURCE
OF ENERGY. HOWEVER, IN THE NEAR-TERM, THE SYSTEM IS LIKELY TO
BRIEFLY INTENSIFY AS IT TAPS INTO SOME FAIRLY STRONG POSITIVE
VORTICITY ADVECTION AHEAD OF A 500MB TROUGH CURRENTLY JUST TO THE
WEST AND THE STRONGLY DIVERGENT OUTFLOW AHEAD OF THE 200MB TROUGH
A BIT FURTHER TO THE WEST. BY TAU 24 HOWEVER, PERSISTENTLY STRONG
VWS AND ENTRAINMENT OF DRY AIR FROM THE WEST, ALONG WITH COOL SSTS,
WILL CONSPIRE TO SHARPLY WEAKEN THE SYSTEM THROUGH THE REMAINDER OF
THE FORECAST. COMPLETION OF STT IS EXPECTED NO LATER THAN TAU 36
WHEN THE SYSTEM BECOMES VERTICALLY STACKED WITH A 500MB COLD-CORE
LOW. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT WITH MINIMAL
CROSS-TRACK SPREAD THROUGH THE FORECAST PERIOD. ALONG-TRACK SPREAD
INCREASES AFTER TAU 24, BUT REMAINS MODEST AT 140NM. THE JTWC TRACK
LIES CLOSE TO THE MULTIMODEL CONSENSUS WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE IS MIXED WITH DECAY-SHIPS MODELS INDICATING
INTENSIFICATION FROM THE START, WHILE THE MESOSCALE MODELS
INCLUDING HWRF, TWRF AND BOTH COAMPS-TC VERSION INDICATE
INTENSIFICATION THROUGH THE NEXT 12-18 HOURS, TO A PEAK AROUND 72
KNOTS, FOLLOWED BY RAPID WEAKENING THEREAFTER. THE COAMPS-TC
ENSEMBLE SHOWS AN 85 PERCENT CHANCE OF MODEST INTENSIFICATION
THROUGH TAU 12 AND EVEN THE ECENS AND GEFS INDICATE NEAR-TERM
INTENSIFICATION PROBABILITIES EXCEEDING 50 PERCENT. THUS THE JTWC
FORECAST CALLS FOR SOME INTENSIFICATION THROUGH TAU 12, LIES ABOVE
THE CONSENSUS MEAN BUT BELOW THE MORE AGGRESSIVE HWRF AND COAMPS-TC
GUIDANCE WITH MEDIUM CONFIDENCE. 

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