WDXS31 PGTW 241500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27S (TWENTYSEVEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.6S 111.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 384 NM NORTH-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) AND SHORTWAVE INFRARED (SWIR)
SATELLITE IMAGERY DEPICTS A HIGHLY ASYMMETRIC SYSTEM, WITH A
WEDGE-SHAPED AREA OF CONVECTION SHEARED TO THE WEST OF THE LOW
LEVEL CIRCULATION CENTER (LLCC). A 241105 WSF-M 37GHZ MICROWAVE
IMAGE REVEALED A SMALL LLCC WITH SHALLOW BANDING FEATURES EXPOSED
TO THE EAST, AND DEEP CONVECTION BEGINNING TO WRAP UP THE WESTERN
PORTION OF THE CIRCULATION. SUBSEQUENT ANIMATED PROXY VISIBLE
IMAGERY INDICATES THIS CONVECTION HAS FAILED TO PERSIST AND THE
LLCC HAS ONCE AGAIN BECOME EXPOSED. THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE SWIR AND PROXY VISIBLE
IMAGERY AND EXTRAPOLATION OF THE CENTER SEEN IN THE WSF-M MICROWAVE
IMAGERY. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE.
EARLIER ASCAT AND OSCAT DATA INDICATED 35 KNOT WINDS WERE PRESENT
IN POCKETS UNDER THE CONVECTION, ALONG THE WESTERN SIDE OF THE
CIRCULATION. AS THE CONVECTION HAS PERSISTED SINCE THAT TIME, IT IS
LIKELY THE WIND FIELD REMAINS IN A SIMILAR ORIENTATION. THE
ENVIRONMENT IS MARGINALLY FAVORABLE, WITH WARM SSTS, PLENTY OF
MOISTURE AND A STRENGTHENING POLEWARD OUTFLOW CHANNEL. HOWEVER,
EASTERLY SHEAR REMAINS MODERATE, WHICH WILL SERVE TO HINDER RAPID
DEVELOPMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) OVER SOUTHWESTERN AUSTRALIA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   APRF: T2.0 - 30 KTS
   CIMSS D-PRINT: 28 KTS AT 241200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: WEAK POLEWARD OUTFLOW AND STRONG PRESSURE FROM
THE EAST DUE TO EASTERLY SHEAR. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 27S IS FORECAST TO CONTINUE TO TRACK
WEST-SOUTHWESTWARD OVER THE NEXT 36 HOURS ALONG THE NORTHERN
PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) TO THE SOUTH. THE
SYSTEM IS EXPECTED TO SLOW DOWN AFTER TAU 24 AS A PASSING TROUGH
FAR TO THE SOUTH WILL WEAKEN THE STEERING GRADIENT. THE TRACK
FLATTENS OUT TO A DUE WEST DIRECTION AND THE SYSTEM PICKS UP SPEED
AFTER TAU 48, AS A NEW STR MOVES IN BEHIND THE TROUGH AND THE
STRENGTHENS THE STEERING GRADIENT ONCE AGAIN. BY THE END OF THE
FORECAST PERIOD, THE RIDGE SLIDES TO THE SOUTHEAST OF TC 27S, WHICH
WILL RESULT IN A GRADUAL TURN POLEWARD BY TAU 120. IN TERMS OF
INTENSITY, DEVELOPMENT IS GOING TO BE SLOW FOR THE FIRST COUPLE OF
DAYS DUE TO THE HIGHLY TILTED NATURE OF THE VORTEX DUE TO THE
PERSISTENT MODERATE EASTERLY SHEAR. THE HIGHEST WINDS WILL REMAIN
ENSCONCED IN THE WESTERN HALF OF THE SYSTEM, UNDER THE CONVECTIVE
BURSTS. THE RATE OF INTENSIFICATION IS EXPECTED TO INCREASE AFTER
TAU 48, AS MOST OF THE GLOBAL MODELS SUGGEST A REDUCTION IN THE
DEEP-LAYER SHEAR, WHICH WILL ALLOW THE SYSTEM TO SYMMETRIZE AND
THE VORTEX TO BECOME VERTICALLY ALIGNED. PEAK INTENSITY OF 75 KNOTS
IS EXPECTED TO BE REACHED AROUND TAU 96. A GENERALIZED WEAKENING IS
LIKELY TO BEGIN AFTER TAU 96 AS DRY AIR INTRUSION AND COOLING SSTS
WILL OFFSET THE RELATIVELY LOW SHEAR AND DECENT POLEWARD OUTFLOW. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN
GOOD AGREEMENT THROUGH THE FORECAST PERIOD, WITH A TIGHT ENVELOPE
OF JUST 50NM AT TAU 72 AND 150NM AT TAU 120. THE NAVGEM IS THE ONLY
OUTLIER, WHICH IS SIGNIFICANTLY SLOWER THAN EVERY OTHER MODEL AND
DRIVES THE SYSTEM INTO THE RIDGE AFTER TAU 72, WHICH IS HIGHLY
UNLIKELY. HENCE, THE NAVGEM IS DISCOUNTED FROM THE CONSENSUS. THE
JTWC FORECAST IS CONSISTENT WITH THE REMAINDER OF THE GUIDANCE
PACKAGE AND LIES CLOSE TO THE CONSENSUS MEAN, THE ECMWF
DETERMINISTIC AND EC-AIFS TRACKERS WITH HIGH CONFIDENCE. INTENSITY
GUIDANCE IS MIXED, WITH THE COAMPS-TC MARKING THE LOWER END OF THE
ENVELOPE, PEAKING THE SYSTEM AT JUST 60 KNOTS, WHILE THE HAFS-A
PEAKS THE SYSTEM AT NEAR 95 KNOTS AND THE HWRF AT AN UNREALISTIC
120 KNOTS. THE JTWC FORECAST LIES CLOSE TO THE CONSENSUS MEAN, BUT
CONFIDENCE IS MEDIUM DUE TO THE LARGE SPREAD IN THE GUIDANCE. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: HIGH
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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