WDXS31 PGTW 081500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA)      
WARNING NR 012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.7S 118.6E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 421 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 37 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 21S IS RAPIDLY WEAKENING DUE TO EXTENSIVE DRY
AIR ENTRAINMENT AND STRONG (35-40 KTS) WEST-NORTHWESTERLY VERTICAL
WIND SHEAR (VWS). ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY 
REVEALS AN EXPOSED, TIGHTLY WRAPPED LOW-LEVEL CIRCULATION CENTER 
(LLCC) POSITIONED JUST NORTHWEST OF A SYMMETRIC REGION OF INTENSE 
CONVECTION FUELED BY A STRONG POLEWARD OUTFLOW CHANNEL. A 081025Z 
SSMIS 91 GHZ MICROWAVE IMAGE DEPICTS CONVECTIVE BANDING WRAPPING INTO 
A BROAD BUT DEFINED LLCC, WHICH SUPPORTS THE INITIAL POSITION WITH 
MEDIUM CONFIDENCE. THE INITIAL INTENSITY IS ASSESSED AT 60 KNOTS, 
HEDGED JUST BELOW THE PGTW AND APRF CURRENT INTENSITY ESTIMATES OF 65 
KNOTS CONSISTENT WITH THE BROAD, WEAK CONVECTIVE STRUCTURE.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: LOW- TO MID-LEVEL SUBTROPICAL RIDGE
(STR) POSITIONED TO THE EAST AND SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   APRF: T4.0 - 65 KTS
   KNES: T4.5 - 77 KTS
   CIMSS SATCON: 63 KTS AT 080900Z
   CIMSS ADT: 77 KTS AT 081230Z
   CIMSS AIDT: 66 KTS AT 081230Z
   CIMSS DPRINT: 46 KTS AT 081200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 30+ KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 21S IS EXPECTED TO RAPIDLY WEAKEN THROUGH
THE FORECAST PERIOD DUE TO THE EXTREMELY DRY ENVIRONMENT AND
INCREASING VWS (50 TO 75 KNOTS) BY TAU 36. THE SYSTEM WILL TRACK
SOUTHWESTWARD TO WEST-SOUTHWESTWARD ALONG THE NORTHERN PERIPHERY
OF THE STR, WITH DISSIPATION ANTICIPATED NO LATER THAN TAU 72.
ALTHOUGH TC OLGA WILL APPROACH WITHIN ABOUT 75 NM OF LEARMONTH
SOLAR OBSERVATORY FROM TAU 48 TO TAU 72, THE COMPACT, RAPIDLY
WEAKENING SYSTEM IS NOT EXPECTED TO SIGNIFICANTLY IMPACT THE AREA. 
ADDITIONALLY, A STRONG SUBTROPICAL JET IS FORECAST TO REMAIN OVER 
NORTHWESTERN AUSTRALIA THROUGH TAU 72, WHICH WILL CONTRIBUTE TO THE 
RAPID WEAKENING TREND.     

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
WITH A 100 NM CROSS-TRACK SPREAD AT TAU 48 AND A 110 NM CROSS-TRACK
SPREAD AT TAU 72. THE 080000Z ECMWF ENSEMBLE (EPS) SUPPORTS THE
JTWC FORECAST TRACK SCENARIO WITH ONLY ONE SOLUTION TRACKING THE
SYSTEM NEAR LEARMONTH. THE 080600Z GFS ENSEMBLE (GEFS) DEPICTS A
BIFURCATION IN THE SOLUTIONS WITH ABOUT 15 PERCENT OF THE
SOLUTIONS CURVING THE SYSTEM SOUTHWARD TOWARD LEARMONTH AS A TROPICAL
DEPRESSION STRENGTH SYSTEM. THE BULK OF THE GEFS SOLUTIONS (85 
PERCENT) DISSIPATE THE SYSTEM ON A WESTWARD TRACK AWAY FROM NORTHWEST 
AUSTRALIA. ALL RELIABLE INTENSITY GUIDANCE TO INCLUDE HAFS-A AND 
COAMPS-TC INDICATE RAPID WEAKENING THROUGH THE NEXT 24 HOURS WITH 
DISSIPATION AFTER TAU 48. 

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