Tropical Storm CHABA Advisory Ter, 04-10

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 21W (CHABA) WARNING NR 25//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 21W (CHABA), LOCATED APPROXIMATELY 361 NM SOUTH-
SOUTHWEST OF SASEBO, JAPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 11
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY INDICATES A SLIGHT WEAKENING TREND WITH A 10-NM CLOUD-FILLED
EYE, WHICH PROVIDES GOOD CONFIDENCE IN THE CURRENT POSITION.
HOWEVER, A 032238Z SSMIS 91GHZ IMAGE AND RADAR IMAGERY SHOW A
COMPACT CORE OF DEEP CONVECTION WITH A 70-NM DIAMETER EYEWALL AND
DEEP CONVECTIVE BANDING OVER THE NORTHERN SEMI-CIRCLE. A ROBUST
POLEWARD OUTFLOW AND VERY WARM SEA SURFACE TEMPERATURES (SSTS) ARE
COMBINING TO FUEL THE STORM AND SUSTAIN THE CURRENT INTENSITY. THE
CURRENT INTENSITY IS ASSESSED AT 125 KNOTS BASED ON DVORAK ESTIMATES
RANGING FROM 5.5 (102 KNOTS) TO 7.5 (155 KNOTS). SOUTHWESTERLY
VERTICAL WIND SHEAR (VWS) HAS INCREASED TO 25-35 KNOTS AND IS
RESPONSIBLE FOR THE RECENT WEAKENING OF THE CONVECTIVE STRUCTURE. TY
21W IS TRACKING NORTHWARD ALONG THE WESTERN PERIPHERY OF A DEEP-
LAYERED SUBTROPICAL RIDGE (STR) TO THE EAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY CHABA WILL CONTINUE TRACKING NORTHWARD OVER THE NEXT 12
HOURS AND THEN NORTHEASTWARD AS IT ROUNDS THE STR AXIS. THE SYSTEM
WILL ENCOUNTER INCREASING VWS AS IT MOVES CLOSER TO THE STRONG ZONAL
WESTERLY FLOW SITUATED JUST SOUTH OF THE KOREAN PENINSULA. THIS WILL
FORCE THE SYSTEM TO FURTHER ELONGATE TO THE NORTHEAST AND RAPIDLY
WEAKEN. TY 21W WILL BEGIN EXTRA-TROPICAL TRANSITION (ETT) NEAR TAU
24 AS IT EMBEDS WITHIN THE MID-LATITUDE WESTERLY FLOW ALONG THE
WESTERN EDGE OF MAINLAND JAPAN, AND WILL COMPLETE ETT BY TAU 48.
IT WILL CONTINUE TO WEAKEN AS IT TRACKS OVER THE COOL WATERS IN THE
SEA OF JAPAN AND THEN ACROSS THE RUGGED JAPANESE TERRAIN. 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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