WDXS31 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (ILSA) WARNING NR 
017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.1S 119.3E
   INITIAL INTENSITY: 105 KTS
   GEOGRAPHIC REFERENCE: 428 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 46 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A LARGE,
HIGHLY SYMMETRICAL SYSTEM WITH A FORMATIVE EYE. CENTRAL CONVECTION
CONTINUED TO DEEPEN AND RAIN BANDS HAVE WRAPPED TIGHTER INTO THE
CENTER. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED
ON THE FORMATIVE EYE, ADJUSTED FOR A SLIGHT WESTWARD TILT. THE
INITIAL INTENSITY OF 105KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE HIGH END OF AUTOMATED INTENSITY ESTIMATES OPEN-AIIR
AND DMN THAT ARE MORE IN LINE WITH THE IMPROVED 6-HR CONVECTIVE
SIGNATURE. THE ENVIRONMENT REMAINS HIGHLY FAVORABLE WITH STRONG
RADIAL OUTFLOW AND VERY WARM SST OFFSETTING THE MEDIUM VWS.

INITIAL WIND RADII BASIS: RECENT SCATTEROMETER DATA.

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.5 - 102 KTS
   CIMSS SATCON: 84 KTS AT 121506Z
   CIMSS ADT: 84 KTS AT 121730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG RADIAL

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.

FORECAST DISCUSSION: A SHORTWAVE TROUGH APPROACHING FROM THE
SOUTHWEST HAS BEGUN TO WEAKEN THE STEERING STR. THIS WILL ALLOW THE
CYCLONE TO TRACK SOUTHWARD THEN SOUTHEASTWARD, MAKING LANDFALL
APPROXIMATELY 102NM ENE OF PORT HEADLAND SHORTLY BEFORE TAU 24,
THEN TRACK RAPIDLY INTO THE CENTRAL AUSTRALIAN OUTBACK. THE HIGHLY
FAVORABLE CONDITIONS, COUPLED WITH INCREASED EASTWARD OUTFLOW
ASSOCIATED WITH THE STRONG WESTERLIES AHEAD OF THE TROUGH, WILL
FUEL A RAPID INTENSIFICATION (RI) TO A PEAK OF 130KTS AT TAU 12
PRIOR TO LANDFALL. AFTERWARD, INCREASING VWS AND LAND INTERACTION
WILL RAPIDLY ERODE THE SYSTEM TO DISSIPATION BY TAU 48. 

MODEL DISCUSSION: NUMERICAL MODELS ARE IN VERY TIGHT AGREEMENT WITH
AN EVEN SPREAD TO 115NM BY TAU 48 WITH UEMN THE SOLE OUTLIER OPTING
FOR A CLOCKWISE U-TURN AFTER TAU 36, LENDING HIGH CONFIDENCE IN THE
JTWC TRACK FORECAST. THERE IS ONLY MEDIUM CONFIDENCE IN THE
INTENSITY FORECAST DUE TO THE VARIABLES ASSOCIATED WITH RI AND LAND
PASSAGE.

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