WDPN31 PGTW 201500
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN)
WARNING NR 01//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL DEPRESSION (TD) 18W (EIGHTEEN), LOCATED APPROXIMATELY
660 NM EAST OF MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 14 KNOTS
OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE IMAGERY SHOWS A
DEEPENED CENTRAL DENSE OVERCAST SLIGHTLY DISPLACED TO THE WEST OF THE
LOW-LEVEL CIRCULATION CENTER (LLCC). IMPROVED FORMATIVE BANDING IN
ALL QUADRANTS IS ALSO EVIDENT. A SERIES OF MICROWAVE IMAGERY
INCLUDING A 201046Z SSMI-S PASS, DEPICTS SLOW CONSOLIDATION AT THE
LOW LEVELS. UPPER-LEVEL ANALYSIS INDICATES THE SYSTEM IS
APPROXIMATELY 5 DEGREES SOUTH OF A RIDGE AXIS IN AN AREA OF LOW
(05-10 KNOT) VERTICAL WIND SHEAR (VWS) WITH STORM MOTION FACTORED IN.
ANIMATED WATER VAPOR IMAGERY SHOWS WEAK POLEWARD OUTFLOW DUE TO
SUBSIDENCE FROM THE RIDGE TO THE NORTHEAST. HOWEVER, EQUATORWARD
OUTFLOW HAS IMPROVED OVER THE LAST SIX HOURS. THE CYCLONE IS TRACKING
ALONG THE SOUTHWEST EXTENSION OF THE SUBTROPICAL RIDGE (STR) ANCHORED
EAST OF JAPAN. 
3. FORECAST REASONING.
   A. THIS IS THE FIRST PROGNOSTIC REASONING BULLETIN AND SETS THE
FORECAST PHILOSOPHY.
   B. AN EASTWARD MOVING MID-LATITUDE TROUGH CURRENTLY OVER THE
YELLOW SEA WILL WEAKEN THE STEERING STR AND CAUSE TD 18W TO SLOW DOWN
AND BECOME QUASI-STATIONARY. WITH LOW VWS, SUSTAINED EQUATORWARD
OUTFLOW, AND PERENNIAL WARM SEA SURFACE TEMPERATURES IN THE
PHILIPPINE SEA (28-30 CELSIUS), THE SYSTEM WILL GRADUALLY INTENSIFY
INTO A TYPHOON BEFORE TAU 72.
   C. AFTER TAU 72, THE AFOREMENTIONED STR IS FORECAST TO REBUILD AND
STEER THE CYCLONE NORTHWESTWARD. THE PASSING OF THE MID-LATITUDE
TROUGH TO THE NORTH WILL ENHANCE POLEWARD OUTFLOW AND PROMOTE FURTHER
INTENSIFICATION OF THE SYSTEM, REACHING 95 KNOTS BY END OF FORECAST.
THE LIMITED NUMERICAL GUIDANCE IS WIDELY SPLIT WITH NOGAPS
UNREALISTICALLY DRIVING THE VORTEX NORTHEASTWARD INTO THE STR AND
GFDN ERRATICALLY LOSING THE VORTEX AT THE MIDDLE TAUS. GFDN, ALONG
WITH JGSM AND WBAR, ALSO KEEPS THE VORTEX TO THE LEFT OF THE MODEL
ENVELOPE, PROPOSING AN ALTERNATE SCENARIO WHERE A SECONDARY STR
BEHIND THE MID-LATITUDE TROUGH BECOMES THE DOMINANT STEERING
MECHANISM AFTER TAU 72. THERE IS LOW CONFIDENCE IN THIS TRACK
FORECAST ESPECIALLY DURING THE EXTENDED TAUS.   //
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