WDXS31 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 20S (BIANCA)     
WARNING NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.9S 102.1E
   INITIAL INTENSITY: 100 KTS
   GEOGRAPHIC REFERENCE: 667 NM WEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 36 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
DESPITE A STEADILY DEGRADING ENVIRONMENT CHARACTERIZED BY LOW TO
MODERATE VERTICAL WIND SHEAR (VWS) AND COOLING SEA SURFACE 
TEMPERATURES OF 24-25C, TROPICAL CYCLONE (TC) BIANCA HAS MAINTAINED
A 15-20NM RAGGED EYE AND COMPACT, COCOONED CONVECTIVE CORE SUSTAINED 
BY MODERATE POLEWARD OUTFLOW. THE SYSTEM HAS MAINTAINED A DEEP MOIST 
CORE DESPITE EXTENSIVE DRY AIR PRESENT OVER THE WESTERN AND NORTHERN 
PERIPHERIES. A 250243Z GMI 89GHZ MICROWAVE IMAGE REVEALS EXPOSED, 
SHALLOW BANDING ASSOCIATED WITH THE DRY AIR AND INDICATES EROSION OF 
THE CONVECTIVE CORE OVER THE NORTHERN SEMICIRCLE. HOWEVER, DEEP 
CONVECTIVE BANDING REMAINS ENTRENCHED OVER THE SOUTHERN SEMICIRCLE 
WRAPPING INTO A WELL-DEFINED MICROWAVE EYE FEATURE. THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE DEFINED EYE 
EVIDENT IN ANIMATED ENHANCED INFRARED SATELLITE IMAGERY. THE INITIAL 
INTENSITY OF 100 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE 
DVORAK CURRENT INTENSITY ESTIMATES AND ADT, AIDT CIMSS OBJECTIVE 
INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE
POSITIONED TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   KNES: T5.5 - 102 KTS
   DEMS: T5.5 - 102 KTS
   APRF: T5.5 - 102 KTS
   CIMSS SATCON: 104 KTS AT 250203Z
   CIMSS ADT: 109 KTS AT 250530Z
   CIMSS AIDT: 106 KTS AT 250530Z
   CIMSS D-MINT: 64 KTS AT 250243Z
   CIMSS D-PRINT: 94 KTS AT 250630Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 24-25 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: EXTENSIVE DRY AIR OVER THE WESTERN AND NORTHERN
     PERIPHERIES.

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: TC 20S WILL CONTINUE TO TRACK SOUTHWARD THROUGH 
THE FORECAST PERIOD ALONG THE WESTERN PERIPHERY OF THE STEERING RIDGE 
AND WILL BEGIN TO INTERACT WITH A BROAD UPPER-LEVEL SUBTROPICAL TROUGH 
AND SUBTROPICAL WESTERLIES NEAR TAU 24. CONSEQUENTLY, VWS WILL 
INCREASE STEADILY THROUGH THE FORECAST PERIOD, WITH HIGH VWS (40-50 
KNOTS) EXPECTED AFTER TAU 24. ADDITIONALLY, SEA SURFACE TEMPERATURES 
WILL COOL STEADILY TO 22-23C AFTER TAU 24 AND SIGNIFICANT DRY AIR 
ENTRAINMENT WILL OCCUR. THIS WILL LEAD TO SIGNIFICANT WEAKENING AS THE 
SYSTEM PROGRESSES POLEWARD. THE CURRENT FORECAST FAVORS A SUBTROPICAL 
TRANSITION. HOWEVER, THERE IS A POSSIBILITY OF DISSIPATION OCCURRING 
BEFORE THE SYSTEM FULLY TRANSITIONS INTO A SUBTROPICAL LOW NEAR TAU 48 
DUE TO THE DETERIORATING ENVIRONMENTAL CONDITIONS.     

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT 
THROUGH TAU 24, WITH A 67NM CROSS-TRACK SPREAD AT TAU 24. AFTER TAU
24, DETERMINISTIC GUIDANCE DIVERGES WITH INCREASING UNCERTAINTY DUE
TO A SPLIT IN THE GUIDANCE. THE ECMWF AND THE ECMWF ENSEMBLE MEAN 
TRACKERS FAVOR A SLOWING SOUTH-SOUTHEASTWARD TRACK, WITH RAPID
DISSIPATION AFTER TAU 48, WHILE GFS, GFS ENSEMBLE MEAN, NAVGEM,
UKMET AND UKMET ENSEMBLE MEAN INDICATE A STALL AND SHARP WESTWARD
TURN ALONG THE NORTHEASTERN PERIPHERY OF A STRONG LOW-LEVEL HIGH 
BUILDING TO THE SOUTH AFTER TAU 24. RELIABLE INTENSITY GUIDANCE 
SUPPORTS RAPID WEAKENING THROUGH THE FORECAST PERIOD. THE ECMWF 
ENSEMBLE (EPS) ALSO SHOWS RAPID WEAKENING BUT MAINTAINS A MINIMAL 
TROPICAL STORM STRENGTH SYSTEM AFTER TAU 36. THE GFS ENSEMBLE IS MORE 
AGGRESSIVE ON WEAKENING AND DISSIPATING THE SYSTEM DURING THE WESTWARD 
TRACK PHASE AFTER TAU 48.           

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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