WDXS31 PGTW 090300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA) 
WARNING NR 014//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.2S 117.5E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 361 NM NORTH-NORTHEAST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A TIGHTLY 
WRAPPED, BUT SHALLOW EXPOSED LOW-LEVEL CIRCULATION CENTER WITH 
FLARING DEEP CONVECTION OCCURRING ALONG THE CONVERGENT FLOW IN THE 
SOUTHERN SEMICIRCLE. SIGNIFICANT NORTHWESTERLY VERTICAL WIND SHEAR 
(VWS) IS BLOWING OFF ALL CONVECTION TO THE SOUTHEAST, WITH OUTFLOW 
ENHANCED BY A SUBTROPICAL JET POSITIONED TO THE SOUTH. ENVIRONMENTAL 
ANALYSIS INDICATES THAT TC 21S IS LOCATED IN AN UNFAVORABLE 
ENVIRONMENT FOR DEVELOPMENT CHARACTERIZED BY HIGH (35-40 KTS) VWS AND 
THE PRESENCE OF MID-LEVEL DRY AIR IN THE NORTHERN AND WESTERN 
SECTORS. THE STRONG OUTFLOW AND WARM (29-30 DEGREE CELSIUS) SEA 
SURFACE TEMPERATURES PRESENT ARE KEEPING THE SYSTEM FROM MORE RAPID 
WEAKENING. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED 
ON THE EXPOSED LLCC SEEN IN THE ANIMATED MSI. THE INITIAL INTENSITY 
OF 50 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A RANGE OF 
SUBJECTIVE DVORAK ESTIMATES FROM T3.0-3.5, AND FURTHER SUPPORTED BY 
082148Z SAR WIND SPEED PRODUCT SHOWING A BROAD SWATH OF 50 TO 55 KT 
WINDS IN THE SOUTHWESTERN QUADRANT.

INITIAL WIND RADII BASIS: SAR DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   APRF: T3.0 - 45 KTS
   CIMSS AIDT: 48 KTS AT 090000Z
   CIMSS ADT: 51 KTS AT 090000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 30+ KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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, BUT THE TRACK WAS SHIFTED 
SLIGHTLY CLOSER TO THE EXMOUTH PENINSULA, AND THE INITIAL INTENSITY 
WAS INCREASED TO 50 KTS TO REFLECT THE 082148Z SAR DATA PREVIOUSLY 
MENTIONED.

FORECAST DISCUSSION: TC 21S IS FORECAST TO CONTINUE ON A
SOUTHWESTWARD TRACK OVER THE FORECAST PERIOD, ALONG THE WESTERN
PERIPHERY OF A REFLECTION OF A STR POSITIONED TO THE SOUTH. THE
JTWC TRACK HAS BEEN SHIFTED SLIGHTLY TO THE EAST AS MODEL GUIDANCE
HAS SUGGESTED A MORE SOUTHERLY TRACK THAN PRIOR GUIDANCE. THE CPA TO
LEARMONTH IS 73 NM AT 102300Z, WHICH IS A 30 NM DECREASE FROM THE
PREVIOUS FORECAST. VWS VALUES ARE ANTICIPATED TO REACH OVER 50 KTS
BY TAU 24, AIDING IN THE WEAKENING OF THE SYSTEM. ADDITIONALLY, DRY
AIR IS EXPECTED TO PERSIST FOR THE REMAINDER OF THE LIFE OF TC 21S.
AS A RESULT, THE INTENSITY FORECAST CALLS FOR A CONTINUED WEAKENING
TREND, WITH DISSIPATION OCCURRING BY TAU 48. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
WITH A 140 NM CROSS-TRACK SPREAD IN GUIDANCE AT TAU 72. GFS HAS 
SHIFTED TO BECOME THE EASTERNMOST MEMBER OF CONSENSUS, GRAZING THE 
EXMOUTH PENINSULA. THE CONSENSUS MEAN IS APPROXIMATELY 75 NM TO THE 
WEST. INTENSITY GUIDANCE CONTINUES TO AGREE WITH A WEAKENING TREND 
THROUGH THE FORECAST PERIOD. HAFS-A SUGGESTS A SLIGHTLY MORE GRADUAL 
DECREASE, WITH DISSIPATION AFTER TAU 72. OVERALL, THE JTWC INTENSITY 
FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE.

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