WDPN33 PGTW 011500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (ROKE) WARNING NR 
014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 34.1N 148.5E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 440 NM EAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 19 KTS
   SIGNIFICANT WAVE HEIGHT: 27 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM 20W (ROKE) CONTINUES TO DEFY THE ODDS AND HAS
MANAGED TO INTENSIFY OVER THE PREVIOUS FEW HOURS. ANIMATED
ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES THAT TS ROKE
HAS RETAINED A SMALL CORE WITH DEEP CONVECTIVE TOWERS FIRING ON THE
WESTERN SIDE OF THE OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC)
AND ATTEMPTING TO WRAP AROUND TO THE SOUTHERN SIDE. A 011232Z GMI
89GHZ MICROWAVE IMAGE DEPICTED A 12-NM WIDE EYE FEATURE, WITH A
NEARLY COMPLETE EYEWALL OF STRONG CONVECTION, WHICH SOMEWHAT
SURPRISINGLY, IS ACTUALLY MUCH STRONGER THAN WHAT WAS SEEN IN THE
THE PREVIOUS MICROWAVE IMAGE FROM ABOUT FOUR HOURS EARLIER. A
NEARLY COINCIDENT 011139Z ASCAT-B BULLSEYE PASS SHOWED THAT THE
RADIUS OF MAXIMUM WINDS HAS REMAINED VERY SMALL, AROUND 20-NM, AND
SHOWED MAXIMUM WINDS OF 55 KNOTS, WHICH WHEN CONSIDERING THE LOW
BIAS AT THIS SPEED, EASILY SUPPORTS A 60 KNOT INITIAL INTENSITY.
BOTH THE INITIAL POSITION AND INTENSITY WERE ASSESSED WITH HIGH
CONFIDENCE BASED PRIMARILY ON THE MICROWAVE AND SCATTEROMETER DATA
RESPECTIVELY. THE ENVIRONMENT IS IN OVERALL UNFAVORABLE, WITH THE
SYSTEM NOW HAVING MOVED NORTH OF THE 26C ISOTHERM INTO COOLER
WATERS, VERY STRONG (40-50 KNOTS) SOUTHWESTERLY VWS AND
SIGNIFICANTLY DRY AIR INTRUSION ON THE WEST SIDE. HOWEVER,
EXTREMELY ROBUST POLEWARD OUTFLOW INTO THE BASE OF A 150 KNOT
SUBTROPICAL JET MAX IS OFFSETTING THE UNFAVORABLE SHEAR FOR THE
MOMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   CIMSS SATCON: 67 KTS AT 011136Z
   CIMSS ADT: 82 KTS AT 011140Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL STORM 20W HAS COMMENCED SUBTROPICAL
TRANSITION (STT), WITH AUTOMATED AND MANUAL PHASE CLASSIFICATION
WORKSHEETS INDICATING A HYBRID SYSTEM WITH CHARACTERISTICS OF BOTH
A TROPICAL AND SUBTROPICAL CYCLONE. OVERALL TRACK MOTION HAS
REMAINED STEADY OVER THE PAST SIX HOURS AS THE SYSTEM MOVES QUICKLY
NORTHEASTWARD ALONG THE WESTERN SIDE OF THE STEERING RIDGE. OVER
THE NEXT 12 HOURS, FORWARD SPEED WILL BEGIN TO SLOW DOWN AS THE
TRACK SHIFTS MORE EASTWARD AS THE STEERING RIDGE WEAKENS AND MOVES
TO THE SOUTHEAST, WEAKENING THE STEERING GRADIENT. AT THE SAME TIME
A TRANSITORY ANTICYCLONE WILL MOVE QUICKLY ACROSS NORTHERN JAPAN
AND INTO THE WESTERN PACIFIC, INDUCING STRONG NORTHEASTERLY FLOW TO
THE NORTH OF TS 20W, WHICH WILL BLOCK ADDITIONAL POLEWARD MOVEMENT
AND THE SYSTEM WILL TURN EASTWARD, SLOWING DOWN TO LESS THAN 10
KNOTS BY THE END OF THE FORECAST PERIOD. AS MENTIONED EARLIER, STT
HAS BEGUN AS THE SYSTEM HAS STARTED TO INTERACT WITH A DEEP
SUBTROPICAL TROUGH MOVING OFF THE EAST COAST OF HONSHU. TS 20W HAS
BEEN ABLE TO TAP INTO THE ASSOCIATED POSITIVE VORTICITY ADVECTION
AND THE STRONG POLEWARD OUTFLOW AND THUS INTENSIFY DURING THIS
PERIOD OF MAXIMUM BAROCLINIC FORCING. BUT THAT PARTY WILL SOON BE
OVER. THE SYSTEM WILL MAINTAIN INTENSITY FOR ANOTHER 12 HOURS, BUT
AFTER THAT THE COMBINATION OF VERY COOL SSTS, ADVECTION OF COOLER,
DRIER AIR INTO THE CORE AND THE DEVELOPMENT OF UPPER-LEVEL
CONVERGENCE AS THE SYSTEM SETTLES UNDER A MID-LEVEL COLD-CORE LOW,
WILL CONSPIRE TO RAPIDLY WEAKEN THE SYSTEM. HOWEVER, TS 20W IS
EXPECTED TO COMPLETE STT NO LATER THAN TAU 36, AND AS EARLY AS TAU
24, INTO A GALE-FORCE SUBTROPICAL LOW WITH AN EXPANDING WIND FIELD.


MODEL DISCUSSION: TRACK GUIDANCE IS IN MODEST AGREEMENT, WITH
CROSS-TRACK SPREAD INCREASING TO 150NM AT TAU 36. ALONG-TRACK
SPREAD HAS DECREASED, WITH A SPREAD OF ONLY 70-NM BETWEEN THE
CONSENSUS MEMBERS AT TAU 36. THE JTWC TRACK IS FLATTER THAN THE
PREVIOUS FORECAST BUT OTHERWISE IS NOT SIGNIFICANTLY DIFFERENT AND
LIES NEAR THE CONSENSUS MEAN. INTENSITY GUIDANCE REMAINS MIXED,
WITH THE HWRF AND COAMPS-TC (NAVGEM) WANTING TO INTENSIFY THE
SYSTEM FURTHER, TO A PEAK OF 72 KNOTS BY TAU 12, WHILE THE
CONSENSUS MEAN AND COAMPS-TC (GFS) INDICATE A FLAT TREND FOR THE
FIRST 12 HOURS FOLLOWED BY STEADY WEAKENING. THE FINAL GROUP
CONSISTING OF GFS AND THE DECAY-SHIPS SHOW RAPID WEAKENING FROM TAU
00. THE JTWC FORECAST CLOSELY FOLLOWS THE COAMPS-TC (GFS) THROUGH
THE FORECAST PERIOD WITH MEDIUM CONFIDENCE. 

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