Tropical Storm CHABA Advisory Tue Oct 04

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 21W (CHABA) WARNING NR
22//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   SUPER TYPHOON (STY) 21W (CHABA), LOCATED APPROXIMATELY 100 NM
SOUTH-SOUTHWEST OF KADENA AB, OKINAWA, JAPAN HAS TRACKED
NORTH-NORTHWESTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. STY CHABA
HAS CONTINUED TO RAPIDLY INTENSIFY, NOW ASSESSED AT 145 KNOTS BASED
ON SUBJECTIVE AND OBJECTIVE DVORAK INTENSITY ESTIMATES. ANIMATED
ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A SHARP 5-NM EYE
FEATURE SURROUNDED BY A VERY INTENSE CONVECTIVE CORE. THE POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON THE EYE FEATURE SEEN IN EIR
AND RADAR IMAGERY. A 030544Z SSMI 85GHZ MICROWAVE IMAGE SHOWS A DEEP
RING OF CONVECTION EXTENDING 25-NM FROM THE CENTER OF THE EYE.
CONVECTIVE BANDING IS PRESENT ALONG THE WESTERN AND SOUTHERN
PERIPHERIES OF THE STORM. THE WIND RADII HAVE BEEN ADJUSTED BASED ON
A RECENT SCATTEROMETER PASS AND OBSERVATIONS FROM KADENA AB, OKINAWA
AND MIYAKO-JIMA. STY 21W HAS STRENGTHENED DUE TO VERY WARM SEA
SURFACE TEMPERATURES (SSTS) AND A ROBUST POLEWARD OUTFLOW CHANNEL
ALONG THE LEADING EDGE OF AN APPROACHING MID-LATITUDE TROUGH
CURRENTLY POSITIONED OVER EASTERN CHINA. HOWEVER, MICROWAVE AND
SATELLITE IMAGERY ARE STARTING TO SHOW A TILT IN THE EYE OF THE
SUPER TYPHOON DUE TO PERSISTENT 15 TO 20 KNOT SOUTHWESTERLY VERTICAL
WIND SHEAR (VWS). STY 21W IS TRACKING ALONG THE SOUTHWESTERN
PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. NO SIGNIFICANT CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING BULLETIN.
   B. STY CHABA WILL CONTINUING MOVING TO THE NORTH-NORTHWEST AS IT
PASSES TO THE WEST OF OKINAWA. AFTERWARDS, STY 21W WILL TURN
NORTHWARD AND THEN NORTHEASTWARD AS IT ROUNDS THE STR AXIS. THE
ROBUST OUTFLOW AND WARM SSTS WILL ALLOW FOR A SLIGHT INTENSIFICATION
BEFORE THE INCREASING VWS FURTHER TILTS THE STORM AND BEGINS THE
WEAKENING PHASE. THE STRONG ZONAL WESTERLY FLOW AND ASSOCIATED HIGH
(30+ KNOT) VWS IS CURRENTLY SITUATED JUST SOUTH OF MAINLAND JAPAN
AND SOUTH KOREA. STY CHABA WILL REACH THIS AREA NEAR TAU 36, WHICH
WILL CAUSE THE SYSTEM TO ELONGATE TO THE NORTHEAST AND RAPIDLY
WEAKEN. THE CYCLONE WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT) AS IT
EMBEDS WITHIN THE MID-LATITUDE WESTERLY FLOW ALONG THE WESTERN EDGE
OF MAINLAND JAPAN. IT WILL CONTINUE TO WEAKEN AS IT TRACKS OVER THE
COOL WATERS IN THE SEA OF JAPAN AND THEN ACROSS THE RUGGED JAPANESE
TERRAIN. STY CHABA WILL COMPLETE ETT AS IT RE-EMERGES OVER THE
WESTERN PACIFIC OCEAN. DYNAMIC MODEL GUIDANCE IS IN TIGHT AGREEMENT
THROUGHOUT THE FORECAST. ADDITIONALLY, GLOBAL MODEL FIELDS SHOW NO
CHANGE TO THE LOCATION OF THE MID-LATITUDE WESTERLIES; THEREFORE,
ONLY MINIMAL CHANGES TO THE FORECAST TRACK ARE EXPECTED. GIVEN THESE
FACTORS, THERE IS HIGH CONFIDENCE IN THE JTWC FORECAST.//
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