WDXS31 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (ILSA) WARNING NR 
016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.5S 119.5E
   INITIAL INTENSITY: 100 KTS
   GEOGRAPHIC REFERENCE: 176 NM WEST-NORTHWEST OF BROOME, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 39 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT SYMMETRIC CIRCULATION WITH VERTICAL HOT TOWERS (VHT)
EVIDENT IN THE CORE OF THE SYSTEM. AN ANALYSIS OF BRIGHTNESS
TEMPERATURE INDICATES THE SYSTEM IS STILL STRUGGLING TO FORM AN EYE
AS EVIDENCED BY THE SHORT LIVED BUT FREQUENT WARM SPOTS FORMING
OVER THE SYSTEMS CENTER. A 121021Z SSMIS 91GHZ MICROWAVE IMAGE
REVEALS A SMALL MICROWAVE EYE AND MUCH IMPROVED EYE WALL STRUCTURE.
A COMPARISON OF THE LAST SEVERAL HOURS OF MICROWAVE IMAGERY REVEALS
A NOTABLE TROCHOIDAL WOBBLE OF THE MICROWAVE EYE MAKING EXACT
POSITIONING MORE DIFFICULT. THE INITIAL POSITION IS THEREFOR PLACED
WITH MEDIUM CONFIDENCE BASED ON EIR AND SSMIS IMAGERY. THE INITIAL
INTENSITY OF 100 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON
THE ANTICIPATED LAG OF AUTOMATED INTENSITY ESTIMATES AS AN EYE
FEATURE STRUGGLES TO FORM. AGENCY DVORAK INTENSITY ESTIMATES ARE
ALSO SLOW TO CLIMB, INDICATING T4.5-5.0 WHILE CIMSS ADT AND DEEP
MICRONET (DMN) INDICATE 82-88KTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   APRF: T4.5 - 77 KTS
   CIMSS SATCON: 73 KTS AT 120602Z
   CIMSS ADT: 82 KTS AT 120830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: RAPID INTENSIFICATION (RI) 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 18S (ILSA) IS FORECAST TO CONTINUE ROUNDING
THE STR AXIS OVER THE NEXT 24 HOURS. DURING THAT TIME THE SYSTEM
WILL CONTINUE TO TRACK SOUTH-SOUTHWESTWARD THEN SOUTHWARD AND
EVENTUALLY SOUTH-SOUTHEASTWARD ALL THE WHILE CONTINUING TO
INTENSIFY. BY TAU 24, WARM SSTS (30-31C), HIGH OHC VALUES, LOW
VERTICAL WIND SHEAR AND STRONG OUTFLOW ALOFT WILL MORE THAN
COMPENSATE FOR THE DRY AIR FIGHTING TO ENTRAIN THE SYSTEM. AS A
RESULT TC ILSA IS FORECAST TO REACH ITS PEAK INTENSITY NEAR 130KTS
AND POSSIBLY A BIT HIGHER. NEAR TAU 30 THE SYSTEM WILL MAKE
LANDFALL OVER EIGHTY MILE BEACH, PROCEED INLAND AND QUICKLY WEAKEN,
EVENTUALLY DISSIPATING BY TAU 72.

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, NUMERICAL MODELS
ARE IN EXCELLENT AGREEMENT THAT 18S WILL ROUND THE STR AXIS AND
MAKE LANDFALL OVER THE RELATIVELY FLAT TERRAIN OF EIGHTY MILE BEACH
AND PROCEED INLAND UNTIL DISSIPATION. NAVGEM TAKES A SLIGHTLY MORE
AGGRESSIVE TURN TOWARD LANDFALL BUT EVEN SO, THE SPREAD IN MODEL
OUTPUT REMAINS LESS THAN 60NM THROUGH TAU 36. FOR THIS REASON THE
JTWC FORECAST TRACK IS PLACED WITH HIGH CONFIDENCE. RELIABLE MODEL
INTENSITY GUIDANCE IS IN GOOD AGREEMENT WITH ALL MEMBERS INCLUDING
SEVERAL RAPID INTENSIFICATION (RI) AIDS INTENSIFYING THE SYSTEM
THROUGH TAU 24 AFTER WHICH ALL MEMBERS SHARPLY WEAKEN UNTIL
DISSIPATION. THE SPREAD IN PEAK INTENSITY VALUE AMONGST THE VARIOUS
MODELS IS MUCH IMPROVED BUT STILL REFLECTS A DELTA OF 20KTS AND AS
A RESULT, THE JTWC INTENSITY FORECAST IS PLACED WITH MEDIUM
CONFIDENCE.

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