Tropical Storm ROKE Advisory Cu, 30.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (ROKE) WARNING NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 27.2N 134.1E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 564 NM SOUTH-SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TY 20W IS ROUNDING THE SUBTROPICAL RIDGE AXIS AND IS STEADILY
WEAKENING AS IT ENCOUNTERS THE MIDLATITUDE WESTERLIES AND
INCREASING VERTICAL WIND SHEAR (25 TO 30 KNOTS). ANIMATED ENHANCED
INFRARED (EIR) SATELLITE IMAGERY DEPICTS A MORE ASYMMETRIC CENTRAL
DENSE OVERCAST WITH WARMING CLOUD TOP TEMPERATURES REFLECTING THE
GENERAL WEAKENING TREND. A 291628Z AMSR2 89GHZ MICROWAVE IMAGE
REVEALS TIGHTLY-CURVED BANDING WRAPPING INTO A WELL-DEFINED
LOW-LEVEL CIRCULATION CENTER WITH THE BULK OF THE DEEP CONVECTION
SHEARED OVER THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON THE TIMELY AMSR2 MICROWAVE IMAGE. THE
INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE KNES AND RJTD DVORAK CURRENT INTENSITY ESTIMATES AS
WELL AS THE CIMSS ADT ESTIMATE.
INITIAL WIND RADII BASIS: WIND RADII HAVE BEEN ADJUSTED BASED ON
AMSR2 WINDSPEED IMAGE, WHICH SHOWED A COMPACT, SYMMETRIC WINDFIELD.
CURRENT STEERING MECHANISM: NORTHWEST PERIPHERY OF A LOW- TO
MID-LEVEL SUBTROPICAL RIDGE (STR).
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T5.5 - 102 KTS
   RJTD: T4.5 - 77 KTS
   KNES: T4.5 - 77 KTS
   CIMSS SATCON: 59 KTS AT 291703Z
   CIMSS ADT: 74 KTS AT 291740Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 25-30 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: INTRODUCED SUBTROPICAL TRANSITION IN
THE EXTENDED FORECAST PERIOD.
FORECAST DISCUSSION: TY 20W IS FORECAST TO TRACK EAST-NORTHEASTWARD
THROUGH TAU 48 ALONG THE NORTHERN PERIPHERY OF THE SUBTROPICAL
RIDGE (STR) WITHIN THE MIDLATITUDE WESTERLIES. THIS IS EVIDENCED IN
THE 291200Z SOUNDING FROM MINAMIDAITO-JIMA AND THE LATEST CIMSS
UPPER-LEVEL WIND IMAGE, WHICH SHOW STRONG WESTERLIES OVER THE EAST
CHINA SEA AND JAPAN WITH A JET DIPPING DOWN OVER THE NORTHERN
RYUKYU ISLANDS. DUE TO THE SYSTEM'S POSITION UNDER THE SUBTROPICAL
WESTERLIES, TRACK SPEEDS WILL BE FAIRLY QUICK (13-14 KNOTS) THROUGH
TAU 48, FOLLOWED BY A SIGNIFICANT SLOW-DOWN THROUGH TAU 96, WHICH
IS REFLECTED IN THE NUMERICAL MODEL GUIDANCE. THE PRIMARY REASON
FOR THE SLOW-DOWN IS A LOW-LEVEL HIGH THAT BUILDS IN TO THE EAST
AND NORTHWEST THROUGH TAU 96, EFFECTIVELY BLOCKING OR SLOWING
POLEWARD PROGRESSION FOR ABOUT 2 DAYS. AFTER TAU 96, A DEEP,
DYNAMIC SUBTROPICAL SHORTWAVE TROUGH WILL APPROACH AND ACCELERATE
THE SYSTEM NORTHEASTWARD WHILE INITIATING A SUBTROPICAL TRANSITION.
DUE TO THE LOCATION OF THE POLAR FRONT WELL TO THE NORTH OF JAPAN,
THE SYSTEM WILL NOT UNDERGO EXTRA-TROPICAL TRANSITION AND WILL NOT
ACCELERATE LIKE A TYPICAL EXTRA-TROPICAL SYSTEM. TY 20W IS EXPECTED
TO WEAKEN STEADILY THROUGH THE FORECAST PERIOD UNDER THE MARGINAL
CONDITIONS.
MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE HAS BEEN STRUGGLING
RUN-TO-RUN WITH THIS COMPLEX, EVOLVING SYNOPTIC PATTERN. THE
CURRENT RUN IS IN FAIR CROSS-TRACK AGREEMENT THROUGH THE FORECAST
PERIOD. HOWEVER, THERE ARE SOME DIFFERENCES IN ALONG-TRACK SPEEDS.
MOST OF THE MODELS SHOW A SLOW-DOWN (LESS THAN 5 KNOTS) BY TAU 96
WITH A PERIOD OF SHARP ACCELERATION THROUGH TAU 120. THE LATEST
ECMWF TRACKER IS THE FASTEST OUTLIER AND SHOWS 11 KNOT TRACK SPEEDS
THROUGH TAU 96 THEN AN INCREASE TO 18 KNOTS BY TAU 120. THE 291200Z
ECMWF ENSEMBLE (EPS) SHOWS SOME OF THE SOLUTIONS SLOWING BUT HAS
TRENDED TOWARD AN INCREASING NUMBER OF FASTER SOLUTIONS. THE GFS
ENSEMBLE (GEFS) FAVORS SLOWER TRACK SPEEDS IN THE TAU 72 TO TAU 96
PERIOD THEN AN ACCELERATED TRACK AFTER TAU 96. CONSEQUENTLY, THERE
IS LOW CONFIDENCE IN THE JTWC TRACK FORECAST AFTER TAU 72. THERE IS
ALSO LOW CONFIDENCE IN THE JTWC INTENSITY FORECAST AFTER TAU 72
WITH DISSIPATION POSSIBLE.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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