Tropical Storm ROKE Advisory Fr, 30.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (ROKE) WARNING NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 28.2N 135.5E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 477 NM SOUTH-SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 26 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 (20 TO 25 KNOTS). ANIMATED MULTISPECTRAL
SATELLITE IMAGERY DEPICTS AN ASYMMETRIC CENTRAL DENSE OVERCAST WITH
EXPOSED LOW-LEVEL CLOUD LINES OVER THE WESTERN SEMICIRCLE. A 292128Z
SSMIS 37GHZ 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 MEDIUM CONFIDENCE BASED ON THE SSMIS
MICROWAVE IMAGE. THE INITIAL INTENSITY OF 65 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE DVORAK CURRENT INTENSITY ESTIMATES AND
292340Z CIMSS ADT ESTIMATE.
INITIAL WIND RADII BASIS: PERSISTENCE
CURRENT STEERING MECHANISM: NORTHWEST PERIPHERY OF A LOW- TO
MID-LEVEL SUBTROPICAL RIDGE (STR).
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T4.5 - 77 KTS
   RJTD: T4.5 - 77 KTS
   KNES: T4.0 - 65 KTS
   CIMSS ADT: 63 KTS AT 292340Z
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT
ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: 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 LATEST CIMSS UPPER-LEVEL WIND IMAGE, WHICH SHOWS STRONG
WESTERLIES OVER THE EAST CHINA SEA AND JAPAN WITH A JET DIPPING DOWN
SOUTH OF JAPAN. DUE TO THE SYSTEM'S POSITION UNDER THE SUBTROPICAL
WESTERLIES, TRACK SPEEDS WILL BE FAIRLY QUICK (13-16 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 36 HOURS. NEAR TAU 48, A DEEP,
DYNAMIC SUBTROPICAL SHORTWAVE TROUGH WILL ENVELOP THE SYSTEM,
INITIATING SUBTROPICAL TRANSITION, WHICH SHOULD COMPLETE BY TAU 96.
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 7 KNOTS) BY TAU 72
WITH A SLIGHT ACCELERATION THROUGH TAU 96. THE 291800Z 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 MEDIUM
CONFIDENCE IN THE JTWC TRACK FORECAST AFTER TAU 72. THERE IS MEDIUM
CONFIDENCE IN THE JTWC INTENSITY FORECAST AFTER TAU 72 WITH
DISSIPATION POSSIBLE.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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