WDXS31 PGTW 080900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA)      
WARNING NR 011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.5S 118.6E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 431 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 42 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 21S IS RAPIDLY WEAKENING DUE TO EXTENSIVE DRY
AIR ENTRAINMENT AND STRONG (30-35 KTS) WEST-NORTHWESTERLY VERTICAL
WIND SHEAR (VWS). ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI)
DEPICTS EXPOSED LOW-LEVEL CLOUD LINES OVER THE WESTERN AND NORTHERN
QUADRANTS WRAPPING INTO A BROAD LOW-LEVEL CIRCULATION CENTER
(LLCC), WITH PERSISTENT DEEP CONVECTION OVER THE SOUTHEASTERN
QUADRANT OF THE LLCC. A 080520Z AMSR2 89 GHZ MICROWAVE IMAGE
REVEALS FRAGMENTED DEEP CONVECTIVE BANDING OVER THE SOUTHEAST
QUADRANT CURLING INTO THE SOUTHERN QUADRANT. THE AMSR2 36 GHZ
MICROWAVE IMAGE SUPPORTS THE INITIAL POSITION WITH MEDIUM
CONFIDENCE AND SHOWS CURVED BANDING WRAPPING INTO A DEFINED CENTER.
ROBUST POLEWARD OUTFLOW AND WARM SST VALUES CONTINUE TO FUEL
PERSISTENT, INTENSE CONVECTIVE CLUSTERS; HOWEVER, THE OVERALL
STRUCTURE OF THE SYSTEM IS RAPIDLY ERODING. THEREFORE, SUBJECTIVE
DVORAK CURRENT INTENSITY ESTIMATES HAVE DECREASED QUICKLY AND RANGE
FROM 65 TO 90 KNOTS. THE INITIAL INTENSITY IS ASSESSED WITHIN THE
RANGE OF CURRENT INTENSITY ESTIMATES, AND IS CONSISTENT WITH THE
OVERALL CONVECTIVE STRUCTURE.

INITIAL WIND RADII BASIS: SAR DATA AND SURFACE OBSERVATIONS FROM
ROWLEY SHOALS 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   DEMS: T5.0 - 90 KTS
   APRF: T4.0 - 65 KTS
   CIMSS ADT: 92 KTS AT 080630Z

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 48. 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 100 NM OF LEARMONTH
SOLAR OBSERVATORY FOM TAU 48 TO TAU 72, THE COMPACT, RAPIDLY
WEAKENING SYSTEM IS NOT EXPECTED TO 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: WITH THE EXCEPTION OF NAVGEM, WHICH CONTINUES TO
RECURVE THE SYSTEM SOUTHWARD OVER NORTHWESTERN AUSTRALIA
ERRONEOUSLY, NUMERICAL MODEL GUIDANCE REMAINS IN FAIR AGREEMENT
WITH A 75 NM CROSS-TRACK SPREAD AT TAU 48 AND A 115 NM CROSS-TRACK
SPREAD AT TAU 72. THE 071800Z ECMWF ENSEMBLE (EPS) SUPPORTS THE
JTWC FORECAST TRACK SCENARIO WITH ONLY ONE SOLUTION TRACKING THE
SYSTEM NEAR LEARMONTH. THE 080000Z GFS ENSEMBLE (GEFS) DEPICTS A
BIFURCATION IN THE SOLUTIONS WITH ABOUT 25 PERCENT OF THE SOLUTIONS
CURVING THE SYSTEM SOUTHWARD TOWARD LEARMONTH AS A TROPICAL
DEPRESSION OR WEAK TROPICAL STORM STRENGTH SYSTEM. THE BULK OF THE
GEFS SOLUTIONS (75 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: MEDIUM//
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