Tropical Storm ROKE Advisory Sat Oct 01

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (ROKE) WARNING NR
012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 31.8N 144.4E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 315 NM SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 20 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN
ASYMMETRIC CENTRAL DENSE OVERCAST CLOUD LAYER THAT CONTINUES TO BE
SHEARED NORTHEASTWARD ACROSS THE NORTHEASTERN AND EASTERN SEMICIRCLE,
WITH A LOW LEVEL CIRCULATION THAT HAS BECOME OBSCURED BY DEEP
CONVECTION. AN EARLIER 302114Z SSMIS 91GHZ MICROWAVE IMAGE REVEALS
A DEEP CURVED CONVECTIVE BAND OVER THE NORTHWEST QUADRANT WRAPPING
INTO A WELL DEFINED LOW LEVEL CIRCULATION. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED ON THE VISIBLE IMAGERY ALONG
WITH THE AFOREMENTIONED SSMIS MICROWAVE IMAGE. THE INITIAL
INTENSITY OF 55 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A
COMBINATION OF PGTW AND RJTD DVORAK CURRENT INTENSITY ESTIMATES
ALONG WITH OBJECTIVE DVORAK ESTIMATES FROM CIMSS ADT, SATCON, AND
DMN.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: NORTHWEST PERIPHERY OF MID-LEVEL
SUBTROPICAL RIDGE (STR)
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   CIMSS SATCON: 61 KTS AT 301642Z
   CIMSS ADT: 79 KTS AT 302340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 25-30 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
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: TS 20W IS CURRENTLY ACCELERATING NORTHEASTWARD
AS THE SYSTEM TRANSITS ALONG THE NORTHWEST PERIPHERY OF THE MID-LEVEL
SUBTROPICAL RIDGE (STR) TO THE EAST-SOUTHEAST. THE SYSTEM WILL
CONTINUE TO TRANSIT NORTHEASTWARD QUICKLY OVER THE NEXT 24 HOURS, DUE
TO THE SYSTEM'S POSITION WITHIN THE SUBTROPICAL WESTERLIES. AFTER
WHICH, TS 20W'S ACCELERATED TRACK MOTION IS FOLLOWED BY A SIGNIFICANT
DECELERATION AND SLOWED POLEWARD PROGRESSION, AS A LOW-LEVEL HIGH
BUILDS IN TO THE NORTH-NORTHWEST, THROUGH TAU 48. BY TAU 24, A
SUBTROPICAL SHORTWAVE TROUGH WILL INTERACT WITH THE SYSTEM, WHICH
WILL BEGIN INITIATING A SUBTROPICAL TRANSITION, AND IS FORECAST TO
COMPLETE SUBTROPICAL TRANSITION BY TAU 48. AS THIS SUBTROPICAL
TRANSITION OCCURS, THE SYSTEMS WIND FIELD WILL ALSO EXPAND INTO A
MORE BROAD GALE FORCE WIND FIELD. TS 20W IS EXPECTED TO MAINTAIN ITS
CURRENT INTENSITY BEFORE A STEADY WEAKENING TREND OCCURS AS THE
SYSTEM TRANSITS IN A MARGINALLY FAVORABLE ENVIRONMENT. THESE
MARGINALLY FAVORABLE ENVIRONMENTAL FACTORS ARE CHARACTERIZED BY WARM
(28-29C) SEA SURFACE (SST), STRONG POLEWARD OUTFLOW, OFFSET BY HIGH
(25-30KTS) VERTICAL WIND SHEAR.
MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT WITH A
GRADUAL CROSS TRACK SPREAD OF 150 NM BY TAU 48. THIS SPREAD IS
RELATIVELY LOW EVEN THOUGH THE SYSTEM WILL BE INFLUENCED BY A
RELATIVELY COMPLEX SYNOPTIC STEERING PATTERN. BOTH THE 301800Z GFS
ENSEMBLE (GEFS) AND 301800Z ECMWF ENSEMBLE (EPS) ARE IN GOOD
AGREEMENT ON THE NORTHEAST TRACK FORECAST ALONG WITH A SLOWED STORM
MOTION BY THE END OF THE FORECAST BY TAU 48. THEREFORE, THERE IS
HIGH CONFIDENCE IN THE JTWC TRACK FORECAST THROUGH TAU 48. THERE IS
MEDIUM CONFIDENCE IN THE JTWC INTENSITY FORECAST AND FOLLOWS CLOSE
TO THE MULTI-MODEL CONSENSUS. AS GFS AND COAMPS-TC BOTH SHOW A PEAK
INTENSITY OF 65 KTS, WITH GFS DECAY SHIPS (DSHA) REPRESENTING THE
LOW END OF INTENSITY GUIDANCE WITH A STEADY DECREASE FROM THE
INITIAL INTENSITY THROUGH OUT THE FORECAST.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   INTENSITY 0 - 72 HR: MEDIUM//
NNNN
NNNN