Tropical Storm CHABA Advisory 30 Σεπ.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (CHABA)
WARNING NR 08//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 21W (CHABA), LOCATED APPROXIMATELY 996 NM
SOUTHEAST OF KADENA AB, HAS TRACKED WESTWARD AT 12 KNOTS OVER THE
PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS
AN AREA OF BROAD AND DEEP FLARING CONVECTION OVER THE LAST SIX HOURS
THAT HAS RECENTLY EVOLVED INTO A BANDING FEATURE, DEFINITIVELY
WRAPPING INTO THE LOW LEVEL CIRCULATION CENTER, GIVING HIGH
CONFIDENCE TO THE INITIAL POSITION. THE INITIAL INTENSITY IS
ASSESSED AT 45 KNOTS AND IS BASED ON THE IMPROVING CONVECTIVE
STRUCTURE COMBINED WITH DVORAK ESTIMATES PGTW AND RJTD REPORTING
T3.0 (45 KNOTS). UPPER-LEVEL ANALYSIS SHOWS IMPROVING RADIAL
OUTFLOW, TAPPING INTO A STREAM OF STRONG WESTERLIES TO THE NORTH
FLOWING INTO A FORMING TUTT CELL TO THE EAST. VERTICAL WIND SHEAR IS
VERY LOW (5 TO 10 KNOTS) AND SSTS ARE FAVORABLE NEAR 30 CELSIUS.
CURRENTLY TS CHABA IS TRACKING WEST-SOUTHWEST ALONG THE PERIPHERY OF
A DEEP-LAYERED SUBTROPICAL RIDGE POSITIONED TO THE NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. TS CHABA WILL CONTINUE ALONG THE EDGE OF THE CURRENT STEERING
RIDGE TURNING NORTHWEST IN THE NEXT 12 TO 24 HOURS. ENVIRONMENTAL
CONDITIONS WILL REMAIN FAVORABLE SUPPORTING STEADY INTENSIFICATION
THROUGH TAU 72. DYNAMIC MODEL GUIDANCE IS IN RELATIVELY GOOD
AGREEMENT FOR THIS PERIOD OF THE FORECAST GIVING HIGH CONFIDENCE TO
THE FORECAST TRACK UP TO TAU 72.
   C. IN THE EXTENDED TAUS, A MID-LATITUDE TROUGH WILL ENTER THE
REGION FROM THE WEST NEAR TAU 72 WEAKENING THE STEERING RIDGE. TS
CHABA WILL BEGIN TO DECELERATE BRIEFLY AS IT ROUNDS THE RIDGE AXIS
NEAR TAU 96, SHIFTING TO A NORTHEASTERN TRACK INTO THE WESTERLY FLOW
AND ASSOCIATED TROUGH. HIGH VERTICAL WIND SHEAR AND LOWER SSTS WILL
WEAKEN TS CHABA, AND EXTRATROPICAL TRANSITION WILL BEGIN NEAR TAU
120. MODEL GUIDANCE BECOMES MORE DIVERGENT BEYOND TAU 72 AS THE
SYSTEM ROUNDS THE RIDGE AXIS. NAVGEM AND ECMWF ARE THE FAR WESTWARD
SOLUTIONS SHOWING A WIDER AND SLOWER TURN AS A RESULT OF A WEAKER
TROUGH AND PRONOUNCED STEERING RIDGE. NAVGEM AND ECMWF HAVE SHOWN
INCONSISTENCY IN THEIR SOLUTIONS OVER THE PAST FEW CYCLES, WHILE
GFS, COAMPS-TC, AND THE GFS ENSEMBLE HAVE BEEN FAIRLY CONSISTENT
NEAR THE MULTI-MODEL CONSENSUS. DUE TO THE UNCERTAINTY IN MODEL
TRACKERS BEYOND TAU 72, THERE IS LOW CONFIDENCE IN THE EXTENDED
FORECAST TRACK.//
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