WDXS31 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 11.3S 128.2E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 174 NM WEST-NORTHWEST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A
CONSOLIDATING SYSTEM WITH POORLY ORGANIZED BUT DEEP CONVECTION THAT 
IS MOSTLY SHEARED SOUTHWESTWARD OF AN OBSCURED LOW LEVEL CIRCULATION
(LLC). A SECONDARY BALL OF CONVECTION HAS DEVELOPED EAST OF THE LLC. 
LOW LEVEL CLOUD LINES NORTH OF AND BETWEEN THESE AREAS OF CONVECTION 
ARE FEEDING INTO THE LLC. THE INITIAL POSITION IS PLACED WITH MEDIUM 
CONFIDENCE BASED ON EXTRAPOLATION FROM THE AFOREMENTIONED LOW CLOUD 
LINES. THE INITIAL INTENSITY IS BASED WITH HIGH CONFIDENCE FROM 
RECENT SCATTEROMETER DATA, SUPPORTED BY AGENCY DVORAK ESTIMATES, AND 
CONSISTENT WITH THE IMPROVEMENT IN THE EIR CONVECTIVE SIGNATURE OVER 
THE PAST SIX HOURS. TC 18S IS IN A MARGINALLY FAVORABLE ENVIRONMENT 
WITH MODERATE VWS OFFSET BY STRONG POLEWARD OUTFLOW AND VERY WARM SST 
IN THE TIMOR SEA.

INITIAL WIND RADII BASIS: RECENT SCATTEROMETER DATA.  

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE
SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 18S WILL CONTINUE TO TRACK SOUTHWESTWARD
UNDER THE STEERING STR TOWARD THE SOUTH INDIAN OCEAN. AFTER TAU 72, A 
TRANSITORY SHORTWAVE TROUGH APPROACHING FROM THE SOUTHWEST WILL ERODE 
THE STR AND ALLOW THE CYCLONE TO RECURVE SOUTHEASTWARD, MAKE LANDFALL 
JUST EAST OF PORT HEADLAND AROUND TAU 108, AND TRACK INLAND OVER 
WESTERN AUSTRALIA. THE MARGINALLY FAVORABLE ENVIRONMENT WILL 
INITIALLY PROMOTE SLOW INTENSIFICATION TO 65KTS BY TAU 48. AFTERWARD, 
LOW VWS AND VERY WARM SST IN THE INDIAN OCEAN WILL FUEL A RAPID 
INTENSIFICATION (RI) TO A PEAK OF 140KTS BY TAU 96, RIGHT BEFORE 
LANDFALL. AFTER LANDFALL, INTERACTION WITH THE TERRAIN WILL REDUCE 
THE INTENSITY TO 70KTS BY TAU 120.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN OVERALL GOOD AGREEMENT
WITH THE LAID-OUT FORECAST TRACK, GRADUALLY SPREADING OUT TO 210NM
AT TAU 96, WITH NAVGEM THE NOTABLE LEFT-OF-TRACK OUTLIER. HOWEVER,
GIVEN THE FORMATIVE STAGE OF THE SYSTEM, THERE IS ONLY MEDIUM
CONFIDENCE IN THE JTWC TRACK FORECAST. THERE IS ALSO MEDIUM
CONFIDENCE IN THE INTENSITY FORECAST UP TO TAU 72 THEN LOW
CONFIDENCE AFTERWARD ON ACCOUNT OF THE HIGH VARIABILITY IN THE
INTENSITY GUIDANCE ASSOCIATED WITH RI. 

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