WDPN31 PGTW 101500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (NEPARTAK)/ 
WARNING NR 09//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
    A. RECENT ANIMATED INFRARED IMAGERY INDICATES THAT DEEP CON-
VECTION HAS STARTED TO CONSOLIDATE OVER THE LOW LEVEL CIRCULATION
CENTER (LLCC) AND DVORAK ESTIMATES FROM PGTW HAVE HELD AT A T2.5
OVER THE LAST SIX HOURS WHILE RJTD HAS HELD AT T2.0. OVER THE LAST
12 HOURS, TS NEPARTAK HAS SLOWLY TRACKED ALONG THE WESTERN EDGE OF
THE SUBTROPICAL RIDGE IN A REGION OF MODERATE VERTICAL WIND SHEAR
AND HAS STARTED TO GAIN ENHANCED POLEWARD OUTFLOW INTO THE MID-
LATIDUDES. THE CURRENT POSITION IS BASED ON THE 101130Z PGTW FIX AND
IS SUPPORTED BY THE 100921Z SSMIS MICROWAVE IMAGES. 
3. FORECAST REASONING
    A. THE FORECAST PHILOSOPHY HAS CHANGED SINCE THE PREVIOUS
BULLETIN TO REFLECT A FASTER TRACK INTO THE MID-LATITUDES.
    B.  TS 21W WILL CONTINUE TO TRACK NORTH-NORTHEASTWARD THROUGH
TAU 48 AS THE SYSTEM CRESTS THE SUBTROPICAL RIDGE INTO THE MID-
LATITUDES. THE 101200Z UPPER-AIR SOUNDING OUT OF CHICHIJIMA REPORTS
WESTERLY WINDS THROUGHOUT THE COLUMN, INDICATING THAT NEPARTAK WILL
NOT BE ABLE TO TRAVEL TOO MUCH FURTHER NORTH BEFORE BEING FORCED
EASTWARD. ADDITIONALLY, AS TS 21W CRESTS THE SUBTROPICAL RIDGE AXIS,
POLEWARD OUTFLOW INTO THE MID-LATITUDE WESTERLIES WILL CONTINUE TO
INCREASE AND ENABLE THE SYSTEM TO INTENSIFY TO MODERATE TROPICAL
STORM STRENGTH.
    C.  BY TAU 72, NEPARTAK WILL START TO ACCELERATE INTO THE MID-
LATITUDES AS IT SUCCUMBS TO INCREASING VERTICAL WIND SHEAR ASSOCIATED
WITH THE ENCROACHING BAROCLINIC ZONE. AS SUCH, THE SYSTEM IS EXPECTED
TO DISSIPATE AS A SIGNIFICANT TROPICAL CYCLONE BY TAU 96 AND IS NOT
EXPECTED TO REGENERATE AS AN EXTROPICAL CYCLONE AT THIS TIME. THE
MODEL GUIDANCE IS STILL NOT IN GOOD AGREEMENT AT THIS TIME, THOUGH
THEY DO AGREE ON AN EVENTUAL TRACK NORTHEASTWARD. THE ECMWF AND JMA
SOLUTIONS ARE SLOWER AND MORE POLEWARD THROUGH 48 HOURS WHILE THE
NOGAPS, UKMO, AND GFS TRACKERS TAKE THE SYSTEM MORE AGGRESSIVELY
NORTHWESTWARD. DUE TO THE CURRENT SOUNDING AT CHICHIJIMA AND FASTER
THAN EXPECTED TRACK SPEED OVER THE LAST 12 HOURS, THIS FORECAST
FAVORS THE QUICKER SOLUTIONS.//
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