WDXS32 PGTW 160900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN)
WARNING NR 021//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.3S 114.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 300 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 03 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 A WEAKENING CONVECTIVE STRUCTURE
THAT IS OFFSET TO THE WEST OF A POORLY ORGANIZED LOW-LEVEL
CIRCULATION CENTER (LLCC). A 160210Z ASCAT-B IMAGE REVEALS WINDS OF
25-30 KTS IN THE NORTHERN SEMICIRCLE. ENVIRONMENTAL ANALYSIS
INDICATES THAT 18S IS IN A MARGINAL ENVIRONMENT FOR INTENSIFICATION
WITH MODERATE WESTWARD OUTFLOW ALOFT, LOW TO MODERATE (15-20 KTS)
EASTERLY VERTICAL WIND SHEAR, AND WARM (28-29 C) SEA SURFACE
TEMPERATURES. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON THE DISORGANIZED PARTIALLY EXPOSED LLCC OBSERVABLE IN 
ANIMATED MSI. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM 
CONFIDENCE BASED ON THE OBJECTIVE AND SUBJECTIVE INTENSITY ESTIMATES 
LISTED BELOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: COMPETING STEERING ENVIRONMENT.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   DEMS: T2.5 - 35 KTS
   CIMSS AIDT: 42 KTS AT 160600Z
   CIMSS DPRINT: 32 KTS AT 160600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TC 18S IS CURRENTLY TRACKING SLOWLY
WEST-NORTHWESTWARD WHILE LOCATED WITHIN A COMPETING STEERING
ENVIRONMENT. A NEAR EQUATORIAL RIDGE TO THE NORTH AND A SUBTROPICAL
RIDGE TO THE SOUTH ARE CAUSING THE SYSTEM TO DRIFT SLOWLY IN A QUASI-
STATIONARY  FASHION. TC 18S IS FORECAST TO MAINTAIN A WEAK STEERING 
MECHANISM OVER THE NEXT 60-72 HOURS. DURING THIS TIME THERE IS A 
VARIETY OF DIFFERENT TRACKS THE SYSTEM COULD TAKE, AND THE MODEL 
GUIDANCE SUGGESTS THAT. TC 18S IS FORECAST TO TRACK SLOWLY WESTWARD 
UNTIL TAU 24. AFTER TAU 24, TC 18S IS FORECAST TO TURN SOUTH-
SOUTHWEST, STILL VERY SLOWLY UNTIL TAU 48. AFTER TAU 60, THE STEERING 
MECHANISM WILL BE MORE DEFINED, AND THE SYSTEM IS EXPECTED TO TRACK 
WESTWARD ALONG THE NORTHERN PERIPHERY OF A SUBTROPICAL RIDGE (STR) 
FOR THE REMAINDER OF THE FORECAST PERIOD. REGARDING INTENSITY, THE 
JTWC INTENSITY FORECASTS HAS TC 18S SLOWLY INTENSIFYING TO 45 KTS AT 
TAU 48, MAINLY DUE TO IMPROVING DIVERGENCE ALOFT AROUND THAT TIME. TC
18S IS FORECAST TO MAINTAIN 45 KTS THROUGH TAU 96 AS THE SYSTEM
CONTINUES TO EXPERIENCE 15-20 KT VERTICAL WIND SHEAR AND LOW OCEAN
HEAT CONTENT, INCREASING TO NEAR 50 KTS BY TAU 120.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN RELATIVELY POOR
AGREEMENT, AS IS SOMEWHAT EXPECTED DUE TO THE COMPETING STEERING
MECHANISM, AND AS SUCH IS ASSESSED WITH OVERALL LOW CONFIDENCE. GFS
AND NAVGEM DIFFER FROM THE REST OF THE GUIDANCE CONSIDERABLY. THEY
TRACK THE SYSTEM SOUTHWARD OVER THE FIRST 48 HOURS, AND GFS
TRACKS IT BACK NORTHWARD UNTIL TAU 96 BEFORE MAKING THE WESTWARD
TURN. THE JTWC TRACK FORECAST FOLLOWS MORE CLOSELY WITH THE REST OF
THE GUIDANCE, THAT HAS THE SYSTEM FOLLOWING A TRACK IN A SIMILAR
WAY AS DESCRIBED ABOVE IN THE FORECAST DISCUSSION. THE JTWC
INTENSITY FORECAST IS ALSO ASSESSED WITH LOW CONFIDENCE BASED ON THE 
SPREAD IN GUIDANCE. COAMPS-TC HAS THE SYSTEM REACHING A PEAK OF 45 
KTS AT TAU 48, THEN WEAKENING AFTERWARD. HAFS-A AND HWRF BOTH HAVE 
THE SYSTEM QUICKLY INTENSIFYING AFTER TAU 60 WITH HWRF ATTAINING 90
KTS AT TAU 108 AND HAFS-A ATTAINING 95 KTS AT TAU 120. THE JTWC
INTENSITY FORECAST MOST CLOSELY FOLLOWS THE CONSENSUS MEAN AND IS 
ASSESSED WITH OVERALL LOW CONFIDENCE DUE TO THE VARIABILITY IN MODEL 
GUIDANCE.

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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