WDXS32 PGTW 170900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) 
WARNING NR 025//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.1S 113.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 318 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 18S (EIGHTEEN) WITH DISORGANIZED FLARING CONVECTION IN
THE WESTERN PERIPHERY OF A PARTIALLY EXPOSED AND BROAD LOW-LEVEL 
CIRCULATION CENTER (LLCC). THE CURRENT ENVIRONMENT IS
ASSESSED AS MARGINAL BASED ON THE WEAK OUTFLOW ALOFT AND DRY AIR
BEING ENTRAINED INTO THE SYSTEM FROM THE NORTH, WHICH IS OFFSET BY
WARM (29-30C) SEA SURFACE TEMPERATURES AND LOW (10-15 KTS) VERTICAL 
WIND SHEAR. A 170518Z GMI 37 GHZ SATELLITE MICROWAVE IMAGE, ALONG 
WITH ANIMATED MSI, HELPED PLACE THE INITIAL POSITION WITH HIGH
CONFIDENCE. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH LOW
CONFIDENCE BASED ON OBJECTIVE AND SUBJECTIVE DVORAK ESTIMATES
LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: COMPETING STEERING PATTERN.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS
   CIMSS AIDT: 39 KTS AT 170600Z
   CIMSS DMINT: 32 KTS AT 170557Z
   CIMSS MW SOUNDERS: 38KTS AT 170626Z


FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: LOW
   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 IS CURRENTLY TRACKING WESTWARD WHILE
LOCATED IN A WEAK STEERING ENVIRONMENT. TC 18S IS FORECAST TO TRACK
SLOWLY WESTWARD UNTIL TAU 12. AFTER TAU 12, THE SYSTEM IS FORECAST
TO TURN SOUTHWESTWARD AND CONTINUE ON A VERY SLOW SOUTHWESTWARD
TRACK UNTIL TAU 36. AFTER TAU 36, THE STEERING ENVIRONMENT BECOMES
MORE DEFINED WITH A SUBTROPICAL RIDGE (STR) POSITIONED SOUTH OF
THE SYSTEM, CAUSING TRACK SPEEDS TO INCREASE. AS A RESULT, THE JTWC
FORECAST TRACKS TC 18S WESTWARD, ALONG THE NORTHERN PERIPHERY OF
THE STR THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD. INTENSITY IS 
FORECAST TO VERY GRADUALLY RISE OVER THE FORECAST INTERVAL GIVEN A 
MARGINAL TO MARGINALLY FAVORABLE ENVIRONMENT ALONG TRACK AND REACH 45 
KTS BY TAU 96, SUSTAINING THROUGH TAU 120.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN BETTER AGREEMENT
CONCERNING TRACK GUIDANCE. WITH THE EXCEPTION OF NAVGEM, WHICH
TRACKS THE SYSTEM SOUTHWESTWARD AFTER TAU 72 VICE WESTWARD, THERE
IS A 120 NM CROSS-TRACK SPREAD AT TAU 72 AND A 225 NM CROSS-TRACK
SPREAD AT TAU 120. EVEN WITH THE JTWC CONSENSUS MEMBERS BECOMING
MORE ALIGNED, THE JTWC TRACK FORECAST IS PLACED WITH OVERALL LOW
CONFIDENCE PRIMARILY DUE TO THE WEAK STEERING ENVIRONMENT THAT THE
SYSTEM IS LOCATED IN. THE JTWC INTENSITY FORECAST IS ASSESSED WITH
OVERALL LOW CONFIDENCE AS WELL. THIS IS MAINLY DUE TO GFS HAVING
THE SYSTEM ATTAIN AROUND 65 KTS AT TAU 96, WHILE COAMPS-TC, HAFS-A,
AND HWRF ALL KEEP THE SYSTEM AROUND 35-45 KTS THROUGHOUT THE
FORECAST PERIOD.

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