WDXS31 PGTW 071500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA)       
WARNING NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.4S 119.2E
   INITIAL INTENSITY: 120 KTS
   GEOGRAPHIC REFERENCE: 505 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 45 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 21S HAS UNDERGONE EXTREME RAPID INTENSIFICATION 
(ERI) OVER THE PAST 24 HOURS, INCREASING 65 KNOTS FROM 55 KNOTS AT 
061200Z TO THE INITIAL INTENSITY OF 120 KNOTS. ANIMATED ENHANCED 
INFRARED SATELLITE IMAGERY DEPICTS A SLIGHTLY OBLONG CONVECTIVE CORE 
SURROUNDING AN 8 NM EYE (FROM THE RCM-3 SAR PASS), WITH TROCHOIDAL 
MOTION EVIDENT OVER THE PAST 12 HOURS. THE INITIAL POSITION IS PLACED 
WITH HIGH CONFIDENCE BASED ON THE EIR IMAGERY. AS A RESULT OF THE 
EXTENSIVE DRY AIR AROUND THE PERIPHERY OF THE SYSTEM, A 071039Z SSMIS 
91 GHZ MICROWAVE IMAGE REVEALS A COMPACT CONVECTIVE CORE, WITH 
LIMITED SPIRAL BANDING OVER THE WESTERN QUADRANT. DUE TO THE SYSTEM'S 
COMPACT SIZE AND ROBUST POLEWARD OUTFLOW, TC OLGA HAS BEEN ABLE TO 
THRIVE AND COCOON ITSELF FROM THE INCREASING (25 TO 30 KNOTS) 
NORTHWESTERLY VERTICAL WIND SHEAR (VWS). HOWEVER, THE HIGH VWS HAS 
STARTED TO IMPINGE ON THE WESTERN EDGE OF THE SYSTEM, WITH A 
PRONOUNCED FLATTENING REVEALED IN THE EIR IMAGERY. ADDITIONALLY, DRY 
AIR IS ENTRAINING INTO THE NORTHEASTERN QUADRANT OF THE SYSTEM. THE 
INITIAL INTENSITY OF 120 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED 
ON A FORTUITOUS 071032Z RCM-3 SAR PASS, INDICATING A MAXIMUM 
INTENSITY OF 122 KNOTS. THE INITIAL INTENSITY IS ASSESSED SLIGHTLY 
HIGHER THAN THE AGENCY DVORAK ESTIMATES OF T6.0 (115 KNOTS) BUT IS 
CLOSE TO THE RECENT ADT AND AIDT ESTIMATES. 

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
POSITIONED TO THE EAST AND SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   KNES: T6.0 - 115 KTS
   APRF: T6.0 - 115 KTS
   CIMSS ADT: 127 KTS AT 071200Z
   CIMSS AIDT: 124 KTS AT 071200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: PEAK INTENSITY HAS BEEN INCREASED
SIGNIFICANTLY FROM THE PREVIOUS FORECAST. 

FORECAST DISCUSSION: TC 21S IS EXPECTED TO TURN GRADUALLY 
SOUTHWESTWARD TO WEST-SOUTHWESTWARD ALONG THE NORTHWESTERN PERIPHERY 
OF THE STR THROUGH TAU 96 WHILE RAPIDLY WEAKENING. THE PEAK INTENSITY 
OCCURRED ABOUT 071140Z, WITH CLEAR SIGNS OF INITIAL WEAKENING 
OCCURRING OVER THE PAST TWO HOURS. ADDITIONALLY, SIGNIFICANT DRY AIR
ENTRAINMENT WILL BEGIN TO ENVELOP THE SYSTEM WITHIN THE NEXT 06-12 
HOURS. AFTER TAU 12, TC OLGA WILL WEAKEN RAPIDLY AS VWS INCREASES TO 
35 TO 45 KNOTS THEN EVEN STRONGER (50 TO 70 KNOTS) VALUES AFTER TAU 
48 AS THE SYSTEM TRACKS UNDER STRONG SUBTROPICAL WESTERLIES. AT THE 
SAME TIME, THE ENVIRONMENT WILL RAPIDLY TRANSITION TO VERY DRY 
CONDITIONS, WHICH WILL LEAD TO DISSIPATION NO LATER THAN TAU 96. 

MODEL DISCUSSION: NAVGEM REMAINS THE SOLE OUTLIER, WITH A RECURVING
TRACK INTO NORTHWESTERN AUSTRALIA. ANALYSIS OF THE NAVGEM FIELDS
INDICATES THAT THIS ERRONEOUS, UNLIKELY SOUTHWARD TRACK IS A
RESULT OF THE VERY LARGE SIZE OF THE SYSTEM (APPROXIMATELY 600 NM
DIAMETER) DEPICTED IN THE MODEL. THE REMAINDER OF THE CONSENSUS
TRACK GUIDANCE INDICATES A SOUTHWESTWARD TO WEST-SOUTHWESTWARD TRACK 
AND IS IN FAIR AGREEMENT, WITH A 130 NM CROSS-TRACK SPREAD FROM
TAU 72 TO TAU 96. THE 070600Z ECMWF ENSEMBLE (EPS) AND 070600Z GFS
ENSEMBLE (GEFS) SUPPORT THE JTWC TRACK FORECAST WITH MEDIUM
CONFIDENCE, WITH ONLY ONE SOLUTION TRACKING A TROPICAL STORM STRENGTH 
SYSTEM SOUTH-SOUTHWESTWARD JUST NORTH AND WEST OF LEARMONTH. RELIABLE 
INTENSITY GUIDANCE SUPPORTS THE RAPID WEAKENING PHASE OF THE JTWC 
FORECAST AFTER TAU 12.     

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