WDXS31 PGTW 070900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (OLGA)       
WARNING NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.8S 119.2E
   INITIAL INTENSITY: 100 KTS
   GEOGRAPHIC REFERENCE: 535 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 40 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 21S HAS UNDERGONE EXTREME RAPID INTENSIFICATION 
(ERI) OVER THE PAST 24 HOURS, FROM 50 KNOTS AT 060600Z TO THE INITIAL 
INTENSITY OF 100 KNOTS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY 
(MSI) DEPICTS ASYMMETRIC CORE CONVECTION SURROUNDING AN 8 NM EYE, 
WITH TROCHOIDAL MOTION EVIDENT OVER THE PAST SIX HOURS. THE INITIAL 
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE MSI. AS A RESULT 
OF THE EXTENSIVE DRY AIR AROUND THE PERIPHERY OF THE SYSTEM, A 
070520Z ATMS 88.2 GHZ MICROWAVE IMAGE REVEALS A COMPACT CONVECTIVE 
CORE, WITH LIMITED BANDING OVER THE SOUTHERN QUADRANT. ENVIRONMENTAL 
CONDITIONS ARE FAVORABLE, WITH LOW VERTICAL WIND SHEAR (VWS), STRONG 
POLEWARD OUTFLOW AND WARM SST VALUES. HOWEVER, AS EXPECTED, 
INCREASING NORTHWESTERLY VWS IS BEGINNING TO IMPINGE ON THE WESTERN 
PERIPHERY OF THE SYSTEM DEGRADING WESTWARD OUTFLOW AS INDICATED IN 
THE ANIMATED WATER VAPOR IMAGERY. ADDITIONALLY, DRY AIR IS ENTRAINING
INTO THE NORTHEASTERN QUADRANT OF THE SYSTEM. THE INITIAL INTENSITY
OF 100 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON CONCURRENT
AGENCY DVORAK ESTIMATES OF T5.5 (102 KNOTS), WITH ADT, AIDT
OBJECTIVE INTENSITY ESTIMATES OF 102 AND 104 KNOTS, RESPECTIVELY.

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: T5.5 - 102 KTS
   KNES: T5.5 - 102 KTS
   APRF: T5.5 - 102 KTS
   CIMSS SATCON: 91 KTS AT 070430Z
   CIMSS ADT: 102 KTS AT 070600Z
   CIMSS AIDT: 104 KTS AT 070600Z
   CIMSS DPRINT: 105 KTS AT 070600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 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 WILL CONTINUE TRACKING SOUTHWESTWARD TO
WEST-SOUTHWESTWARD ALONG THE NORTHWESTERN PERIPHERY OF THE STR
THROUGH TAU 96 WHILE RAPIDLY WEAKENING. THE PEAK INTENSITY IS
LIKELY OCCURRING NOW OR WITHIN THE NEXT SIX HOURS, WITH INCREASING
NORTHWESTERLY VERTICAL WIND SHEAR OF 25 TO 35 KNOTS COMMENCING NEAR
TAU 6 THROUGH TAU 12. ADDITIONALLY, SIGNIFICANT DRY AIR ENTRAINMENT
WILL BEGIN TO ENVELOP THE SYSTEM WITHIN THE NEXT 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 140 NM TO 160 NM CROSS-TRACK SPREAD FROM 
TAU 72 TO TAU 96. THE 061800Z ECMWF ENSEMBLE (EPS) AND 070000Z GFS
ENSEMBLE (GEFS) SUPPORT THE JTWC TRACK FORECAST WITH MEDIUM
CONFIDENCE, WITH NO SOLUTIONS TRACKING THE SYSTEM OVER NORTHWESTERN
AUSTRALIA. 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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