Tropical Storm DUJUAN Advisory Thu Sep 24

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (DUJUAN) WARNING NR
10//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
  TROPICAL STORM (TS) 21W (DUJUAN), LOCATED APPROXIMATELY 541 NM
SOUTH-SOUTHEAST OF KADENA AB, JAPAN HAS TRACKED WEST-NORTHWESTWARD AT
08 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) DEPICTS A PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC) WITH DEEP CONVECTIVE BANDING OVER THE SOUTHERN SEMI-
CIRCLE. OVERALL, CONVECTIVE ORGANIZATION HAS IMPROVED WITH EVIDENCE
THAT VERTICAL WIND SHEAR (VWS) HAS DECREASED TO LOW TO MODERATE (10
TO 20 KNOTS) LEVELS. CONSEQUENTLY, ANIMATED WATER VAPOR IMAGERY
INDICATES EXPANDING OUTFLOW OVER THE EASTERN SEMI-CIRCLE. THIS
CONSOLIDATION IS EVIDENT IN A 232257Z SSMIS 91GHZ IMAGE, WHICH SHOWS
TIGHTLY-CURVED SHALLOW BANDING WRAPPING INTO A DEFINED CENTER WITH
CURVED DEEP BANDING FORMING OVER THE SOUTH QUADRANT. OVERALL, THERE
IS GOOD CONFIDENCE IN THE INITIAL POSITION BASED ON THE SSMIS IMAGE.
THE INITIAL INTENSITY IS ASSESSED AT 50 KNOTS BASED ON THE 232227Z
RSCAT DATA AND THE IMPROVED CONVECTIVE STRUCTURE AS WELL AS RECENT
SATCON ESTIMATES OF 50 KNOTS. TS DUJUAN IS TRACKING ALONG THE
SOUTHWESTERN PERIPHERY OF A DEEP LAYERED SUB-TROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. NO CHANGE IN THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   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 THROUGH TAU 72. TS 21W IS FORECAST TO SLOWLY CONSOLIDATE
DUE TO THE BROAD NATURE OF THE LLCC AND LOW TO MODERATE VWS THROUGH
TAU 24, HOWEVER, 12W SHOULD INTENSIFY AT A GREATER RATE AFTER TAU 24
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 POSITIONAL AND TRACK UNCERTAINTY IN THE EARLY PHASE OF
THE FORECAST.//
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