Tropical Storm BANYAN Advisory lun, 31-10

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 27W (TWENTYSEVEN)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 8.2N 135.3E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 76 NM NORTHEAST OF KOROR
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) AND PROXY VISIBLE SATELLITE
IMAGERY DEPICTS A COMPACT, WELL-DEFINED AND VERY TIGHTLY WRAPPING
LOW LEVEL CIRCULATION CENTER (LLCC), WITH FLARING, DISORGANIZED
CONVECTION TO THE SOUTHWEST. THE LLCC HAS ALTERNATIVELY BEEN
EXPOSED COVERED BY CONVECTIVE BLOWOFF OVER THE PAST DAY AS
CONVECTION HAS CONTINUALLY CYCLED, ONLY TO BE SHEARED OFF TO THE
SOUTHWEST IN THE FACE OF PERSISTENT NORTHEASTERLY FLOW ALOFT.
HOWEVER, WHILE THE CONVECTIVE STRUCTURE HAS REMAINED DISORGANIZED,
THE STRUCTURE OF THE LOW-LEVEL CIRCULATION HAS STEADILY IMPROVED,
AND LOW-LEVEL WIND FIELD HAS CONTINUED TO STRENGTHEN. RECENT
SCATTEROMETER DATA INDICATED A COMPACT WIND FIELD WITH WINDS UP TO
30-35 KNOTS IN THE WEST-NORTHWEST SECTOR OF THE CIRCULATION.
ADDITIONALLY, AGENCY FIXES HAVE BEEN IN THE T1.5-T2.0 FOR MORE THAN
24 HOURS, AND KNES HAS BEEN AT T2.5 FOR THE LAST COUPLE OF FIXES.
THE CONGRUENCE OF THE ABOVE DATA LENDS INCREASED CONFIDENCE THAT
WHILE THE SYSTEM DOES NOT LOOK ALL THAT IMPRESSIVE, THE WINDS ARE
ABOVE THE WARNING THRESHOLD. THE ENVIRONMENT IS MARGINAL AT BEST,
WITH WARM SSTS AND GOOD EQUATORWARD OUTFLOW BEING OFFSET BY
PERSISTENT, STRONG NORTHEASTERLY FLOW ALOFT. THE 301200Z SOUNDING
FROM YAP SHOWED 40-50 KNOT NORTHEASTERLIES ABOVE THE 300MB LEVEL,
WHILE THE KOROR SOUNDING SHOWED THE WINDS ARE A BIT DIVERGENT AND
WEAKER THAN AT YAP. CIMSS ANALYSIS REVEALS HIGHLY DIVERGENT FLOW IN
THE IMMEDIATE VICINITY OF TD 27W, WHICH EXPLAINS THE REPEATED
CYCLING OF THE CONVECTIVE FLARE-UPS. THE SYSTEM IS TRACKING
WEST-SOUTHWEST ALONG THE SOUTHERN SIDE OF A DEEP SUBTROPICAL RIDGE
TO THE NORTH.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) ENTRENCHED TO THE NORTH.
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.5 - 35 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.
FORECAST DISCUSSION: TD 27W WILL CONTINUE TRACKING TO THE WEST
THROUGH THE FIRST 24 HOURS, THEN GRADUALLY TURN TO A MORE
NORTHWESTWARD HEADING AFTER TAU 36 AS THE STEERING RIDGE PULLS AWAY
TO THE NORTHEAST AND THE LOWER LEVEL FLOW BECOMES ORIENTED TO A
NORTHWEST TO SOUTHEAST DIRECTION. WHILE THE SYSTEM HAS BEEN ABLE TO
BECOME BETTER SYMMETRIZED AND INTENSIFY OVER THE PAST DAY OR SO,
IT CANNOT CONTINUE THIS TREND IN THE FACE OF THE STRONG AND
PERSISTENT NORTHEASTERLY SHEAR. THE LATEST CIMSS ANALYSIS ALSO
SHOWS AN AREA OF CONVERGENCE UPSTREAM, WHICH IS EXPECTED TO PUSH
DOWN ON TOP OF THE SYSTEM OVER THE NEXT 24 HOURS. THIS, ALONG A
PUSH OF MID-LEVEL DRY AIR, AS REVEALED IN THE YAP SOUNDING, WILL
KILL ANY POSSIBILITY OF INTENSIFICATION. THUS, THE FORECAST CALLS
FOR A SLOW, STEADY DECLINE IN INTENSITY THROUGH THE FORECAST
PERIOD, WITH THE SYSTEM FULLY DISSIPATING NO LATER THAN TAU 48, AND
POTENTIALLY AS EARLY AS TAU 36, TO THE EAST OF THE PHILIPPINES.
MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH THE
FORECAST PERIOD, WITH ALL AVAILABLE CONSENSUS MEMBERS CONFINED TO A
TIGHT, 70NM ENVELOPE, LENDING HIGH CONFIDENCE TO THE JTWC TRACK
FORECAST. INTENSITY GUIDANCE IS SURPRISINGLY MIXED, WITH THE
MESOSCALE HWRF AND COAMPS-TC ALONG WITH THE GFS, INDICATING SLOW,
STEADY WEAKENING WHILE THE DECAY-SHIPS BEING VERY AGGRESSIVE WITH
INTENSIFYING THE SYSTEM UP TO 50 KNOTS IN THE CASE OF THE GFS
VERSION. FOR THE REASONS OUTLINED ABOVE, THE INTENSIFICATION
SCENARIO IS DEEMED HIGHLY UNLIKELY, THUS THE JTWC FORECAST ALIGNS
WITH THE MESOSCALE MODELS, WITH HIGH CONFIDENCE.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   INTENSITY 0 - 72 HR: HIGH//
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