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
07//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 21W (DUJUAN), LOCATED APPROXIMATELY 655 NM
SOUTHEAST OF KADENA AB, JAPAN HAS TRACKED WEST-NORTHWESTWARD
AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY DEPICTS SMALLER VORTICES ROTATING AROUND A LARGER
CIRCULATION CENTROID WITH DEEP SYMMETRIC CONVECTION ALONG THE
WESTERN PERIPHERY OF THE LARGER CIRCULATION. ENHANCED INFRARED
SATELLITE IMAGERY SHOWS POSSIBLE MID-LEVEL TURNING WITHIN THE DEEP
CONVECTION. DUE TO THE LARGE CIRCULATION AND MULTIPLE VORTICES
WITHIN THE SYSTEM, THERE IS LOW CONFIDENCE IN THE INITIAL POSITION.
THE HORIZONTAL EXTENT OF THE CIRCULATION IS FURTHER EVIDENT IN A
230136Z ASCAT PASS THAT MISSED THE CENTER OF THE CIRCULATION BUT
SHOWED THE ELONGATED NATURE OF THE PERIPHERY WINDS. A 230234Z AMSR-2
89GHZ MICROWAVE IMAGE SHOWS VERY BROAD SHALLOW BANDING WRAPPING INTO
THE ELONGATED CIRCULATION WITH THE DEEP CONVECTION CONFINED TO THE
WESTERN QUADRANT. THE CURRENT INTENSITY IS HELD AT 40 KNOTS BASED ON
AN OLDER WINDSAT PASS AND SUPPORTS THE UNCHANGED STRUCTURE OF THE
SYSTEM. UPPER LEVEL ANALYSIS INDICATES A MARGINAL ENVIRONMENT WITH
20 TO 25 KNOT EASTERLY VERTICAL WIND SHEAR (VWS) PREVENTING THE
STORM FROM ALIGNING VERTICALLY. HOWEVER, GOOD DIVERGENT OUTFLOW IS
MAINTAINING THE CURRENT CONVECTION. TS DUJUAN IS TRACKING ALONG THE
SOUTHERN PERIPHERY OF A DEEP LAYERED SUB-TROPICAL RIDGE (STR).
3. FORECAST REASONING.
   A. THE EXTENDED FORECAST HAS BEEN ADJUSTED TO SHOW A CONTINUED
NORTHWESTWARD TRACK PASSING JUST NORTH OF OKINAWA. THIS IS FURTHER
EXPLAINED IN PARA 3.C.
   B. TS 21W WILL CONTINUE TO TRACK WEST-NORTHWESTWARD OVER THE NEXT
36 HOURS AS THE STR REMAINS TO THE NORTH OF THE SYSTEM.
CONCURRENTLY, EXPECT MARGINAL INTENSIFICATION DUE TO MODERATE VWS
CONTINUING TO PREVENT THE SYSTEM FROM FULLY CONSOLIDATING. BEYOND
TAU 36, A SHORTWAVE MID-LATITUDE TROUGH WILL CREATE A SMALL BREAK IN
THE STR, ALLOWING TS 21W TO TURN MORE NORTHWESTWARD. DECREASED VWS
AND AN IMPROVED POLEWARD OUTFLOW CHANNEL WILL INCREASE THE RATE OF
INTENSIFICATION.
   C. IN THE EXTENDED FORECAST, TS 21W WILL CONTINUE TO TRACK
TOWARDS THE WEAK BREAK IN THE STEERING STR AND INTENSIFY DUE TO
FAVORABLE ENVIRONMENTAL CONDITIONS. DYNAMIC MODEL GUIDANCE HAS BEEN
INCREASINGLY UNSTABLE AS THEY CONTINUE TO DISPLAY A WIDE RANGE OF
SOLUTIONS; NOW SHOWING A TRIFURCATION. NAVGEM AND THE EUROPEAN
MODELS SHOW A WEAKER SYSTEM PASSING THROUGH THE SOUTHERN RYUKYU
ISLANDS. ENVIRONMENTAL CONDITIONS WILL SUPPORT AN INTENSIFICATION
MAKING THESE SOLUTIONS LESS REALISTIC. HWRF AND THE JAPANESE MODELS
SHOW A SHARP RECURVE BEFORE THE SYSTEM REACHES THE RYUKYU ISLANDS.
TS DUJUAN IS CURRENTLY AND NOT FORECASTED TO BE STRONG ENOUGH TO
BREAK THROUGH THE STEERING STR AND SUPPORT A SOONER RECURVE. GFS,
COAMPS-TC, AND GFDN SHOW A STRAIGHT NORTHWESTWARD TRACK PASSING OVER
OKINAWA WITH THE POSSIBILITY OF A LATER RECURVE. THIS SOLUTION IS
DEEMED MOST FEASIBLE AT THIS TIME BASED ON THE EXPECTED DEVELOPMENT
OF THE SYSTEM AND SUPPORTS THE CURRENT FORECAST TRACK. THE OVERALL
MODEL UNCERTAINTY IS FURTHER EVIDENT IN THE NAVGEM AND ECMWF
ENSEMBLE CLUSTERS WHICH SHOW ALL THREE SCENARIOS AS POSSIBILITIES
AND DO NOT PROVIDE ANY SUPPORT FOR THEIR DETERMINISTIC MODEL TRACKS.
DUE TO THE CONTINUED WIDE SPREAD INSTABILITY IN ALL MODEL GUIDANCE,
THERE IS LOW CONFIDENCE IN THE JTWC FORECAST.//
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