Tropical Storm CHOI-WAN Advisory mar, 01-06

SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (CHOI-WAN) WARNING
NR 010//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 04W (CHOI-WAN), LOCATED APPROXIMATELY 457 NM
SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED WEST-NORTHWESTWARD AT
09 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY DEPICTS A FULLY-EXPOSED LOW-LEVEL CIRCULATION WITH
PERSISTENT DEEP CONVECTION SHEARED OVER THE SOUTHWEST QUADRANT DUE
TO MODERATE TO STRONG (20-25 KNOTS) NORTHEASTERLY VERTICAL WIND
SHEAR (VWS) ASSOCIATED WITH A TUTT CELL NEAR 13N 130E. A 312228Z
SSMIS 91GHZ MICROWAVE IMAGE INDICATES AN INCREASINGLY DISORGANIZED
SYSTEM WITH FRAGMENTED BANDING OVER THE WESTERN SEMICIRCLE, WHICH IS
LOCATED OVER LAND. THE INITIAL INTENSITY IS ASSESSED CONSERVATIVELY
AT 40 KNOTS HEDGED ABOVE PGTW/RJTD DVORAK CURRENT INTENSITY
ESTIMATES OF 2.5 (35 KNOTS) BASED ON RECENT ASCAT DATA. THE SYSTEM
IS LOCATED WITHIN A MARGINALLY-FAVORABLE ENVIRONMENT WITH
NORTHEASTERLY VERTICAL WIND SHEAR (VWS) OFFSET BY GOOD EQUATORWARD
OUTFLOW AND WARM SST VALUES (29-30C). TS 04W HAS DECOUPLED AND IS
TRACKING WEST-NORTHWESTWARD UNDER THE STEERING INFLUENCE OF A DEEP-
LAYERED SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTH AND
NORTHEAST.
3. FORECAST REASONING.
   A. DUE TO THE WESTWARD SHIFT IN THE FORECAST TRACK OVER THE
PHILIPPINES THROUGH TAU 72, THE INTENSITY FORECAST IS SIGNIFICANTLY
LOWER THAN THE PREVIOUS FORECAST.
   B. TS 04W IS FORECAST TO TRACK ALONG THE EASTERN COAST OF THE
PHILIPPINES WHILE MAINTAINING INTENSITY AT 40 KNOTS THROUGH TAU 36.
AFTER TAU 36, FURTHER WEAKENING WILL OCCUR AS THE SYSTEM TRACKS OVER
THE MOUNTAINOUS TERRAIN OF LUZON. WITH THE EXCEPTION OF GFS AND
NAVGEM, WHICH TRACK THE SYSTEM INTO THE SOUTH CHINA SEA, THE BULK OF
THE GUIDANCE SUPPORTS THE JTWC FORECAST WITH MODERATE CONFIDENCE.
ADDITIONALLY, THE 31/18Z ECMWF ENSEMBLE INDICATES INCREASED
POTENTIAL FOR A TRACK OVER LUZON WITH THE BULK OF THE ENSEMBLE
MEMBERS DIRECTLY OVER LUZON.
   C. IN THE EXTENDED PERIOD, THE WESTERN PORTIONS OF THE STR WILL
CONTINUE TO ERODE ALLOWING THE SYSTEM TO RECURVE NORTHEASTWARD. TS
04W WILL WEAKEN AS IT ENCOUNTERS INCREASING VWS ASSOCIATED WITH A
MIDLATITUDE SHORTWAVE TROUGH EXPECTED TO DIG OVER THE EAST CHINA
SEA. TS 04W IS EXPECTED TO BEGIN EXTRA-TROPICAL TRANSITION (ETT)
NEAR TAU 96 BUT MAY DISSIPATE PRIOR TO COMPLETING ETT NEAR TAU 120.
MODEL GUIDANCE DIVERGES WITH A LARGE SPREAD IN SOLUTIONS AT TAU 120.
THE JTWC FORECAST REMAINS POSITIONED CLOSE TO THE MULTI-MODEL
CONSENSUS. BASED ON THE UNCERTAINTY IN THE TIMING AND DEGREE OF
RECURVATURE, THERE IS MODERATE CONFIDENCE IN THE JTWC FORECAST
TRACK.//
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