WDXS31 PGTW 131500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (ILSA)       
WARNING NR 020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5S 119.1E
   INITIAL INTENSITY: 130 KTS
   GEOGRAPHIC REFERENCE: 327 NM EAST-NORTHEAST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 50 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 18S PEAKED IN INTENSITY AROUND 130900Z-131200Z 
NEAR 130 KNOTS. SURFACE OBSERVATIONS FROM BEDOUT ISLAND INDICATED
MAXIMUM SUSTAINED SURFACE WINDS (10-MINUTE AVERAGE) OF 121 KNOTS
(129-130 KNOTS CONVERTED TO 1-MINUTE AVERAGE) AT 131320Z WITH
MAXIMUM GUSTS OF 156 KNOTS (167 KNOTS CONVERTED TO 1-MINUTE
AVERAGE). OVER THE PAST TWO HOURS, ANIMATED ENHANCED INFRARED (EIR)
SATELLITE IMAGERY INDICATES A SLIGHT WEAKENING TREND WITH THE EYE
BECOMING MORE RAGGED AND CLOUD FILLED. A 130909Z SSMIS 91GHZ
MICROWAVE IMAGE SHOWS DECAYING SPIRAL BANDING OVER THE SOUTHERN
SEMICIRCLE, HOWEVER, ANIMATED RADAR IMAGERY FROM PORT HEDLAND
CONTINUES TO SHOW AN INTACT, SYMMETRIC EYEWALL. BASED ON THE LATEST 
RADAR IMAGERY, THE CENTER WILL MAKE LANDFALL WITHIN THE NEXT TWO 
HOURS BETWEEN DE GREY AND PARDOO STATION. THE INITIAL POSITION IS 
PLACED WITH HIGH CONFIDENCE BASED ON RADAR IMAGERY. THE INITIAL 
INTENSITY OF 130 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE 
DVORAK INTENSITY ESTIMATES AND THE SURFACE OBSERVATIONS FROM BEDOUT 
ISLAND.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.5 - 127 KTS
   KNES: T6.5 - 127 KTS
   APRF: T6.5 - 127 KTS
   CIMSS SATCON: 117 KTS AT 130909Z
   CIMSS ADT: 107 KTS AT 131130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: FRICTIONAL EFFECTS

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: AFTER PASSING JUST EAST OF BEDOUT ISLAND, TC
18S IS EXPECTED TO MAKE LANDFALL NEAR TAU 03 (131500Z).
ENVIRONMENTAL CONDITIONS WILL CONTINUE TO DEGRADE AS THE SYSTEM
APPROACHES THE COAST AND TRACKS INLAND WITH STEADY WEAKENING
EXPECTED. AFTER TAU 06, TC 18S WILL RECURVE SHARPLY SOUTHEASTWARD
TO EAST-SOUTHEASTWARD AS IT INTERACTS WITH THE MAJOR SHORTWAVE TROUGH 
AND EVENTUALLY ACCELERATES WITHIN THE MIDLATITUDE WESTERLIES. RAPID 
WEAKENING IS EXPECTED WITH DISSIPATION BY TAU 36 OVER CENTRAL 
AUSTRALIA.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN VERY TIGHT AGREEMENT 
THROUGH TAU 24 WITH AN 22NM SPREAD IN SOLUTIONS AT TAU 12 AND A 43NM 
SPREAD IN SOLUTIONS AT TAU 24. AFTER TAU 24, GUIDANCE DIVERGES 
GRADUALLY WITH INCREASING UNCERTAINTY ASSOCIATED WITH THE TRACK 
WITHIN THE MIDLATITUDE WESTERLIES OVER LAND.

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