Tropical Storm SEPAT Advisory mar, 01-10

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (SEPAT)
WARNING NR 05//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 21W (SEPAT), LOCATED APPROXIMATELY 391
NM SOUTH-SOUTHEAST OF YOKOSUKA, JAPAN, HAS TRACKED NORTHWESTWARD AT
07 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY SHOWS A FULLY-EXPOSED AND WELL-DEFINED LOW-LEVEL CIRCULATION
CENTER (LLCC) WITH SHALLOW CONVECTION WRAPPING ALONG THE NORTHEAST
QUADRANT AND LONG STREAKS OF COLD AIR STRATOCUMULUS CLOUDS ADVECTING
IN FROM THE NORTHWEST. THESE FEATURES ARE ALSO EVIDENT ON  A 302252Z
SSMI/S MICROWAVE PASS. THERE IS HIGH CONFIDENCE IN THE INITIAL
POSITION IN VIEW OF THE ABOVE IMAGERY. THE INITIAL INTENSITY OF 30
KNOTS IS BASED ON CONGRUENT DVORAK ESTIMATES FROM ALL REPORTING
AGENCIES. UPPER-LEVEL ANALYSIS INDICATES A MARGINAL ENVIRONMENT AS
AN UPPER-LEVEL LOW OVER THE SYSTEM IS CAUSING MODERATE (15 TO 20
KNOT) VERTICAL WIND SHEAR AND IS ALSO HINDERING OUTFLOW. TD 21W IS
TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF THE SUBTROPICAL RIDGE
(STR) TO THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO FORECAST PHILOSOPHY SINCE THE PREVIOUS PROGNOSTIC
REASONING MESSAGE.
   B. TD 21W HAS BEGUN TO ROUND THE WESTERN EDGE OF THE STR. AFTER
TAU 12, THE CYCLONE IS FORECAST TO ACCELERATE AHEAD OF A MIDLATITUDE
SHORTWAVE TROUGH, WHICH IS EXPECTED TO DEEPEN OVER JAPAN. TD 21W IS
FORECAST TO REMAIN IN A MARGINAL ENVIRONMENT THROUGHOUT THE FORECAST
WITH A SLIGHT INCREASE AT TAU 24 AS POLEWARD OUTFLOW IMPROVES,
PEAKING THE INTENSITY TO 35 KNOTS. ADDITIONALLY, ALONG-TRACK SEA
SURFACE TEMPERATURES ARE EXPECTED TO GET COLDER WHICH WILL ALSO
IMPEDE DEVELOPMENT. TD 21W IS FORECAST TO BEGIN EXTRA-TROPICAL
TRANSITION (ETT) BEFORE TAU 36 AS IT BEGINS TO INTERACT WITH THE
MIDLATITUDE WESTERLIES AHEAD OF THE DEEPENING TROUGH. COMPLETE
TRANSITION IS EXPECTED BY TAU 48 AS THE SYSTEM BECOMES EMBEDDED IN
THE COLD BAROCLINIC AIR MASS. THE AVAILABLE DYNAMIC MODELS, AS WELL
AS THE ENSEMBLE MODELS ARE IN EXCELLENT AGREEMENT, LENDING HIGH
CONFIDENCE TO THE JTWC FORECAST TRACK, WHICH IS POSITIONED SLIGHTLY
FASTER AND TO THE RIGHT OF THE MULTI-MODEL CONSENSUS.   //
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