WDXS31 PGTW 112100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (ZELIA) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.5S 118.9E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 353 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS DEEP
CONVECTION BUILDING OVER THE OBSCURED LOW-LEVEL CIRCULATION CENTER
(LLCC). THE APPEARANCE OF LOW-LEVEL CLOUD BANDS WRAPPING AROUND THE
NORTHEASTERN QUADRANT OF THE SYSTEM HIGHLIGHTS THE TILTED NATURE OF
THE SYSTEM DUE TO NORTHERLY SHEAR. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON RADAR AND A 111608Z GPM GMI 37GHZ
MICROWAVE IMAGE REVEALING THE ASYMMETRIC CLOUD BANDS WRAPPING
AROUND THE WESTERN PERIPHERY OF THE LLCC. THE INITIAL INTENSITY OF
50 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON OBSERVATIONS
REPORTED FROM ROWLEY SHOALS REFLECTING 42KT WINDS AT 10 MINUTE
INTERVALS ACCOMPANIED BY THE AGENCY AND OBJECTIVE FIXES LISTED
BELOW.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: A COMPETING STEERING ENVIRONMENT
BETWEEN TWO SUBTROPICAL RIDGES

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 52 KTS AT 111729Z
   CIMSS ADT: 43 KTS AT 111730Z
   CIMSS AIDT: 37 KTS AT 111930Z
   CIMSS D-PRINT: 41 KTS AT 111930Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 17S WILL SLOWLY CRAWL
SOUTHWARD DURING THE FIRST 48 HOURS OF THE FORECAST WHILE IN A
COMPETING STEERING ENVIRONMENT BETWEEN TWO SUBTROPICAL RIDGES.
AFTER THE SYSTEM MAKES LANDFALL, THE TRACK SPEED WILL INCREASE AS
THE SYSTEM SHALLOWS AND IS STEERED BY LOW-LEVEL FLOW. IN THE
NEAR-TERM FORECAST, A DROP IN VERTICAL WIND SHEAR COMBINED WITH
VERY HIGH SEA SURFACE TEMPERATURES WILL ALLOW FOR STRONG
INTENSIFICATION TO A PEAK OF 95KTS BY TAU 48. ON THE UPPER-LEVEL
ENVIRONMENT, A JET MAXIMUM TO THE SOUTH WILL ALLOW FOR STRONG
UPPER-LEVEL DIVERGENCE. FOLLOWING LANDFALL, THE SYSTEM WILL
DISSIPATE OVER LAND. 

MODEL DISCUSSION: THERE IS UNCERTAINTY IN THE FORECAST DUE TO
VARIATION WITHIN THE ENSEMBLE SOLUTIONS. THE ECMWF ENSEMBLE
SOLUTIONS LEAD TO A STRONG POTENTIAL FOR RAPID INTENSIFICATION
WITHIN THE FIRST 48 HOURS OF THE FORECAST DUE TO A LONGER
QUASI-STATIONARY PERIOD OVER WATER. ALTERNATIVELY, THE GFS ENSEMBLE
MOVES THE SYSTEM QUICKLY OVER LAND AND DOES NOT ALLOW FOR A LONG
PERIOD OF INTENSIFICATION. THE JTWC SHIPS RAPID INTENSIFICATION
AIDS, HAFS-A, AND HWRF ALL INDICATE POSSIBLE INTENSITIES ABOVE
100KTS WITHIN THE FIRST 48 HOURS. THE JTWC FORECAST LIES SLIGHTLY
ABOVE CONSENSUS DUE TO THE STRONG POTENTIAL FOR RAPID
INTENSIFICATION.

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