WDPN33 PGTW 131500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (KOGUMA) WARNING NR
035//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 36.8N 155.5E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 704 NM EAST-SOUTHEAST OF MISAWA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 20 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY
EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) UNDERGOING RAPID
SUBTROPICAL TRANSITION (STT). THE MID-LEVEL CIRCULATION AND
ASSOCIATED DEEP CONVECTION SEEN EARLIER HAVE COMPLETELY COLLAPSED AND
ARE NO LONGER DETECTABLE IN ANIMATED EIR IMAGERY. BASED ON THE LATEST
MODEL FIELDS, THERMAL CONVECTION REMAINS MARGINAL, THE SYSTEM HAS
YET TO ACQUIRE DEFINITIVE FRONTAL CHARACTERISTICS, AND IT REMAINS
WELL EQUATORWARD OF THE POLAR FRONT JET AND ASSOCIATED BAROCLINIC
ZONE. DEEP-LAYER VERTICAL WIND SHEAR (VWS) REMAINS EXTREMELY HIGH,
AND DRY MID-LEVEL AIR NOW COMPLETELY OVERLIES THE SHALLOW VORTEX,
WHICH IS CONFINED BELOW 700MB. ACCORDINGLY, THE SYSTEM IS ASSESSED AS
UNDERGOING SUBTROPICAL TRANSITION (STT). THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED LLCC RESOLVED IN
EIR. A 130744Z S1D SYNTHETIC APERTURE RADAR (SAR) PASS REVEALED A
SLIGHTLY ELONGATED LLCC WITH A MEAN VMAX OF 62 KNOTS AND AN OBSERVED
PEAK OF 68 KNOTS. BECAUSE THE RAPID COLLAPSE OF CONVECTION TENDS TO
INTRODUCE A SIGNIFICANT LOW BIAS IN OBJECTIVE SATELLITE ESTIMATES,
CONTINUITY WITH THIS HIGH-RESOLUTION SAR INSTRUMENT PROVIDES STRONG
EMPIRICAL JUSTIFICATION FOR HOLDING INITIAL INTENSITY AT 60 KNOTS,
ASSESSED WITH MEDIUM CONFIDENCE. ENVIRONMENTAL CONDITIONS ARE HIGHLY
UNFAVORABLE FOR TROPICAL DEVELOPMENT, CHARACTERIZED BY STRONG VWS
EXCEEDING 40 KNOTS, COOL SSTS (24-25C), DRY AIR ENTRAINMENT, AND
VORTEX DECOUPLING.

INITIAL WIND RADII BASIS: PERSISTENCE

CURRENT STEERING MECHANISM: NORTHWESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR).

AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.5 - 55 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T3.0 - 45 KTS
   CIMSS ADT: 39 KTS AT 131200Z
   CIMSS AIDT: 45 KTS AT 131200Z
   CIMSS D-MINT: 47 KTS AT 131018Z
   CIMSS D-PRINT: 45 KTS AT 131200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30+ KTS
   SST: 24-25 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: VORTEX DECOUPLING, DRY MID-LEVEL AIR ENTRAINMENT.


ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 27W CONTINUES ITS STEADY PROGRESSION
NORTHEASTWARD ALONG THE NORTHWESTERN SIDE OF THE STR POSITIONED TO
THE SOUTHEAST. NO SIGNIFICANT TRACK CHANGES ARE EXPECTED; THE SYSTEM
WILL GRADUALLY CURVE TOWARD THE EAST AS IT TRANSITS THE POLEWARD
PERIPHERY OF THE STR. TRANSLATION SPEED WILL REMAIN STABLE AT
APPROXIMATELY 20 KNOTS THROUGHOUT THE FORECAST WINDOW, AS LOW- TO
MID-LEVEL RIDGING EXTENDING TO THE EAST BLOCKS THE TYPICAL
ACCELERATION CHARACTERISTIC OF CYCLONES LEAVING THE SUBTROPICS.
REGARDING INTENSITY: 27W WILL CONTINUE TO STEADILY WEAKEN AND
GRADUALLY SPIN DOWN AS IT TRANSITIONS THROUGH STT AND SUBSEQUENT
EXTRATROPICAL TRANSITION (ETT) OVER THE NEXT 48 HOURS. AS OUTLINED
ABOVE, 27W IS CURRENTLY ASSESSED TO BE IN STT MODE, PROJECTED TO
COMPLETE WITHIN THE NEXT 12 HOURS. ALOFT, THE REMNANT VORTEX WILL
PASS BENEATH AN UPPER-LEVEL RIDGE AXIS THAT DISPLACES THE POLAR JET
STREAM FAR TO THE NORTH OVER KAMCHATKA. IN THE LOWER LEVELS,
EXTENSIVE HIGH PRESSURE EXTENDS POLEWARD TO THE EAST OF 27W TOWARD
THE ALEUTIANS. THE LACK OF AN ACTIVE BAROCLINIC ZONE WILL SUSTAIN A
PROLONGED SUBTROPICAL SYSTEM STRUCTURE. BY TAU 36, THE SYNOPTIC
CONFIGURATION IS PROJECTED TO SHIFT AS 27W ENTERS THE NOSE OF AN
APPROACHING POLAR JET MAXIMUM AND PHASES WITH A 500MB SHORTWAVE
TROUGH. THIS INTERACTION WILL TRIGGER EXTRATROPICAL TRANSITION (ETT),
EXPECTED TO BE COMPLETE BY TAU 48.

MODEL DISCUSSION: TRACK GUIDANCE IS IN STRONG AGREEMENT, WITH
MINIMAL ALONG-TRACK AND MODEST (LESS THAN 100NM) CROSS-TRACK SPREAD
THROUGH TAU 48. THE JTWC FORECAST IS POSITIONED NEAR THE
MULTI-MODEL CONSENSUS MEAN WITH HIGH CONFIDENCE. INTENSITY GUIDANCE
IS ALSO IN STRONG AGREEMENT, WITH ALL MEMBERS OF THE CONSENSUS
INDICATING STEADY WEAKENING THROUGH THE FORECAST PERIOD. THE JTWC
FORECAST IS PLACED NEAR THE CONSENSUS MEAN WITH HIGH CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: HIGH//
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