WDPN32 PGTW 240300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (NANGKA)
WARNING NR 07//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. TROPICAL STORM (TS) 04W (NANGKA) HAS CONTINUED TO TRACK
WEST-NORTHWESTWARD OVER THE PAST 24 HOURS ALONG THE SOUTHWESTERN
PERIPHERY OF THE SUBTROPICAL RIDGE AND HAS MAINTAINED AN
INTENSITY OF 45 KNOTS. INTENSITY IS MAINLY BASED ON CONTINUITY AND
IS SUPPORTED BY A SAB DVORAK ESTIMATE OF 45 KNOTS. INTENSITY HAS
ALSO BEEN MAINTAINED BY STEADILY IMPROVING EQUATORWARD OUTFLOW
AS WELL AS A DEVELOPING POLEWARD OUTFLOW CHANNEL INTO A TROPICAL
UPPER TROPOSPHERIC TROUGH. A 232301Z 91GHZ SSMIS IMAGE SUPPORTS THE
240000Z POSITION AND CURRENT BEST TRACK. THE IMAGE ALSO SHOWS VERY
DEEP, CENTRAL CONVECTION WITH CONVECTIVE BANDING FROM THE SOUTHWEST. 
3. FORECAST REASONING
    A. THE FORECAST PHILOSOPHY HAS NOT CHANGED SIGNIFICANTLY SINCE
THE LAST PROGNOSTIC REASONING BULLETIN.
    B. TS 04W WILL CONTINUE TO TRACK ALONG THE SOUTHWESTERN TO
WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE THROUGHOUT THE FORECAST
PERIOD. NANGKA WILL SLOWLY INCREASE THROUGH TAU 36 AS THE SYSTEM
IS NOT FORECAST TO HAVE A SIGNIFICANTLY ADVERSE INTERACTION WITH
LAND. AN INTENSITY INCREASE IS ALSO SUPPORTED BY THE DEVELOPING
OUTFLOW CHANNELS MENTIONED IN PARA 2.A AND THE SYSTEM'S LOCATION
IN AN AREA OF MINIMAL VERTICAL WIND SHEAR. CONFIDENCE IN THE
FORECAST TRACK THROUGH TAU 48 IS HIGH AS THE MODEL AIDS HAVE COME
INTO CLOSE AGREEMENT. THE WBAR AND GFDN TRACKERS ARE OUTLIERS TO
THE WEST AND EAST, RESPECTFULLY. THE TRACK THROUGH TAU 48 WILL
BEGIN TO MOVE GENERALLY MORE NORTHWESTWARD AS AN UPSTREAM MID-
LATITUDE TROUGH, SUPPORTED BY A DIGGING UPPER LEVEL JET, WILL START
TO WEAKEN THE FINGER OF THE SUBTROPICAL RIDGE THAT IS CURRENTLY
STRETCHING INTO SOUTHERN CHINA.
    C. AFTER TAU 48, TS 04W WILL START TO TAKE A GRADUAL TURN TO
THE NORTHEAST AS THE SYSTEM GAINS LATITUDE AND THE MID-LATITUDE
TROUGH FURTHER WEAKENS THE BRANCH OF THE RIDGE TO THE NORTH.
HOWEVER, IT APPEARS THAT THE TROUGH WILL REMAIN TOO FAR NORTH AND
EAST TO INITIATE AN AGGRESSIVE RECURVE/EXTRATROPICAL SCENARIO.
THE GFS AND ECMWF MODELS ARE IN GOOD AGREEMENT WITH A RELATIVELY
SLOW TRACK (10-15 KNOTS) THROUGH THE TAIWAN STRAIGHT AND A GRADUAL
INTENSITY DECREASE. ADDITIONALLY, AS THE SYSTEM MOVES NORTHEASTWARD,
TS 04W WILL START TO ENCOUNTER MORE UNFAVORABLE UPPER-LEVEL WINDS
FROM THE NORTHEAST (AND THE ASSOCIATED VERTICAL WIND SHEAR),
FURTHER EXPLAINING THE FORECAST DECREASE IN INTENSITY. 
FORECAST TEAM: ECHO//
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