Tropical Storm CHABA Advisory Sáb, 01-10

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA)
WARNING NR 13//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (CHABA), LOCATED APPROXIMATELY 751 NM
SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWESTWARD AT
10 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) INDICATES A COMPACT, CONSOLIDATING LOW-LEVEL
CIRCULATION CENTER (LLCC) WITH FRAGMENTED CONVECTIVE BANDING. THE
INITIAL POSITION IS BASED ON THE MSI LOOP AND LINES UP WITH A LOW
REFLECTIVITY CIRCULATION FEATURE IN THE 302138Z SSMIS MICROWAVE PASS
WITH GOOD CONFIDENCE. THE INITIAL INTENSITY OF 50 KNOTS IS BASED ON
AN OVERALL ASSESSMENT OF OBJECTIVE AND AGENCY DVORAK ESTIMATES. UPPER
LEVEL ANALYSIS INDICATES THE SYSTEM IS IN AN AREA OF LOW TO MODERATE
(10-15 KNOT) VERTICAL WIND SHEAR (VWS) AND GOOD RADIAL OUTFLOW. TS
21W IS TRACKING ALONG THE SOUTHWEST PERIPHERY OF THE DEEP-LAYERED
SUBTROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. TS CHABA WILL CONTINUE TO TRACK NORTHWESTWARD ALONG THE
SOUTHWEST PERIPHERY OF THE DEEP-LAYERED STR WHILE INTENSIFYING
STEADILY DUE TO OVERALL FAVORABLE ENVIRONMENTAL CONDITIONS. AFTER TAU
48, A MID-LATITUDE TROUGH APPROACHING FROM THE NORTHWEST WILL WEAKEN
THE STR AND CAUSE THE CYCLONE TO TRACK MORE NORTHWARD. TS 21W IS
EXPECTED TO PEAK AT 90 KNOTS NEAR TAU 72.
   C. AFTER TAU 72, TS CHABA WILL ACCELERATE NORTHEASTWARD, MAKE
LANDFALL OVER SOUTHERN KYUSHU, JAPAN, AND DRAG ACROSS SOUTHERN JAPAN.
INCREASING VWS, IN ADDITION TO LAND INTERACTION, WILL RAPIDLY ERODE
THE SYSTEM; DOWN TO 30 KNOTS BY TAU 120. CONCURRENTLY, THE SYSTEM
WILL BEGIN EXTRA-TROPICAL TRANSITION. NUMERIC MODEL GUIDANCE IS IN
TIGHT AGREEMENT UP TO TAU 72 WHERE THE CONSENSUS MEMBERS
SIGNIFICANTLY DIVERGE IN BOTH DIRECTION AND SPEED. THEREFORE, THERE
IS HIGH CONFIDENCE IN THE JTWC TRACK FORECAST UP TO TAU 72, THEN LOW
CONFIDENCE AFTERWARDS.//
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