Tropical Storm DUJUAN Advisory 24 Σεπ.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (DUJUAN) WARNING NR
09//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
  TROPICAL STORM (TS) 21W (DUJUAN), LOCATED APPROXIMATELY 577 NM
SOUTHEAST OF KADENA AB, JAPAN HAS TRACKED WEST-NORTHWESTWARD AT 09
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY CONTINUES TO DEPICT A PARTIALLY-EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC) WITH DEEP CONVECTION DISPLACED OVER THE
SOUTHWEST QUADRANT DUE TO PERSISTENT NORTHEASTERLY VERTICAL WIND
SHEAR (VWS). A 231904Z SSMIS 37GHZ IMAGE INDICATES THAT THE SYSTEM
HAS CONSOLIDATED SLIGHTLY WITH DEEP CONVECTION BUILDING OVER THE
LLCC WITH CONVECTIVE BANDING OVER THE SOUTHERN SEMI-CIRCLE WRAPPING
INTO A MORE DEFINED LLCC. OVERALL, THERE IS GOOD CONFIDENCE IN THE
INITIAL POSITION BASED ON THE SSMIS IMAGE. THE INITIAL INTENSITY IS
ASSESSED AT 45 KNOTS BASED ON THE 231150Z ASCAT IMAGE. UPPER LEVEL
ANALYSIS INDICATES A MARGINAL ENVIRONMENT WITH MODERATE (20 KNOTS)
NORTHEASTERLY VWS OFFSET BY DIFFLUENT OUTFLOW. TS DUJUAN IS TRACKING
ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP LAYERED SUB-TROPICAL
RIDGE (STR).
3. FORECAST REASONING.
   A. BASED ON IMPROVED, MORE STABLE MODEL GUIDANCE AND A DETAILED
EXAMINATION OF RECENT 500MB ANALYSES, THE FORECAST PHILOSOPHY NOW
SHOWS A STAIR-STEP TRACK VICE A RE-CURVE SCENARIO.
   B. THE RECENT 500MB ANALYSES DEPICT A SHORTWAVE TROUGH OVER THE
KOREAN PENINSULA EXTENDING SOUTHWARD INTO THE EAST CHINA SEA. DUE TO
THIS TROUGH, TS 21W IS FORECAST TO TURN NORTHWESTWARD AND TRACK
SLOWLY TOWARD THE WEAKNESS IN THE SUBTROPICAL RIDGE (STR) THROUGH
TAU 36. THIS TROUGH IS FORECAST TO WEAKEN AND QUICKLY PROPAGATE
EASTWARD, THEREFORE, THE STR SHOULD GRADUALLY RE-BUILD POLEWARD OF
TS 21W AFTER TAU 36. DESPITE A PERSISTENT BREAK IN THE STR (BETWEEN
THE WESTERN AND EASTERN STR), MODEL GUIDANCE GENERALLY INDICATES
ZONAL WESTERLY FLOW WITH NO DYNAMIC MIDLATITUDE SHORTWAVE TROUGHS
CAPABLE OF INDUCING A RE-CURVE SCENARIO. DYNAMIC GUIDANCE REMAINS
UNCERTAIN, HOWEVER, THE MAJORITY OF THE GUIDANCE IS IN GOOD
AGREEMENT THROUG TAU 72. TS 21W IS FORECAST TO SLOWLY CONSOLIDATE
DUE TO THE BROAD NATURE OF THE LLCC AND MODERATE VWS THROUGH TAU 36,
HOWEVER, 12W SHOULD INTENSIFY AT A GREATER RATE AFTER TAU 36 AS VWS
DECREASES AND POLEWARD OUTFLOW IMPROVES.
   C. IN THE EXTENDED TAUS, TS 21W WILL TURN WESTWARD AS IT BEGINS
TO TRACK UNDER THE STEERING INFLUENCE OF THE WESTERN STR. DYNAMIC
GUIDANCE HAS IMPROVED WITH THE MAJORITY OF MODELS NOW SHOWING A
WESTWARD TRACK NEAR TAIWAN. TS 21W SHOULD CONTINUE TO INTENSIFY
UNTIL IT APPROACHES TAIWAN AT WHICH POINT IT WILL WEAKEN DUE TO LAND
INTERACTION. OVERALL, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST
TRACK DUE TO MODEL UNCERTAINTY IN THE EARLY PHASE OF THE FORECAST.//
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