WDXS31 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (ZELIA) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.0S 118.7E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 78 NM NORTH OF PORT HEDLAND, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 17S (ZELIA) WITH A COMPACT CORE OF DEEP CONVECTION WITH A
WARM SPOT DEVELOPING OVER THE CENTER, HINTING AT AN EYE BEGINNING
TO FORM. POLEWARD OUTFLOW IS ROBUST, DENOTED BY THE DEFINED CIRRUS
FILAMENTS EXPANDING OUTWARD FROM THE SYSTEM. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON ANIMATED RADAR IMAGERY FROM
PORT HEDLAND. THE INITIAL INTENSITY OF 65 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE HIGH SIDE OF AGENCY DVORAK AND
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW DUE TO THE DEVELOPING
EYE. ENVIRONMENTAL ANALYSIS INDICATES THAT 17S IS IN A HIGHLY
FAVORABLE ENVIRONMENT CHARACTERIZED BY THE STRONG POLEWARD OUTFLOW,
LOW (10-15 KTS) VERTICAL WIND SHEAR, AND VERY WARM (30-31 C) SEA
SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: 120038Z SCATTEROMETER DATA

CURRENT STEERING MECHANISM: A COMPETING STEERING ENVIRONMENT
BETWEEN TWO SUBTROPICAL RIDGES (STR) TO THE EAST AND WEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   KNES: T3.5 - 55 KTS
   APRF: T3.5 - 55 KTS
   CIMSS ADT: 57 KTS AT 120600Z
   CIMSS DPRINT: 56 KTS AT 120600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE PEAK INTENSITY IS HIGHER THAN THE
PREVIOUS FORECAST. 

FORECAST DISCUSSION: TC 17S IS FORECAST TO TRACK SLOWLY
WEST-SOUTHWESTWARD THROUGH TAU 36 AS RIDGING FROM THE WEST EXTENDS
OVER TO THE SOUTH OF THE SYSTEM. HOWEVER, THE STEERING PATTERN
REMAINS WEAK, SO THERE IS VERY HIGH UNCERTAINTY REGARDING THE
TRACK. AFTER TAU 36, THE RIDGE TO THE WEST WILL PROPOGATE WESTWARD
AND RIDGING OVER AUSTRALIA WILL BECOME THE PRIMARY STEERING
MECHANISM THROUGHOUT THE REMAINDER OF THE FORECAST PERIOD. THIS
WILL CAUSE THE SYSTEM TO TRACK GENERALLY SOUTHWARD AS IT MAKES
LANDFALL AND TRACKS FURTHER INLAND. LANDFALL IS CURRENTLY FORECAST
TO OCCUR NEAR TAU 60, EAST OF DAMPIER, BUT FLUCTUATIONS IN THE
LANDFALL LOCATION ARE POSSIBLE. REGARDING INTENSITY, 17S IS
FORECAST TO RAPIDLY INTENSIFY OVER THE NEXT 48 HOURS TO A PEAK OF
120 KTS IN RESPONSE TO THE CONTINUED HIGHLY FAVORABLE ENVIRONMENT.
AFTER MAKING LANDFALL, TERRAIN INTERACTION WILL QUICKLY ERODE THE
VORTEX, WITH DISSIPATION FORECAST TO OCCUR NEAR TAU 96. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE CONTINUES TO BE IN VERY
POOR AGREEMENT. GFS AND ITS ENSEMBLE MEAN ARE PRIMARY OUTLIERS,
TRACKING THE SYSTEM MUCH FURTHER WEST, NEARLY TO LEARMONTH. ECMWF
ENSEMBLE HAS A VERY LARGE SPREAD IN POSSIBLE TRACKS, FANNING OUT
FROM THE INITIAL POSITION. THE JTWC TRACK FORECAST IS PLACED
SLIGHTLY WEST OF CONSENSUS, WITH OVERALL LOW CONFIDENCE. INTENSITY
GUIDANCE IS ALSO IN POOR AGREEMENT WITH PEAK INTENSITIES RANGING
FROM 105 (COAMPS-TC) TO 145 KTS (HAFS-A). THE AMOUNT OF TIME OVER
WATER WILL HAVE A LARGE INFLUENCE ON THE PEAK INTENSITY OF THE
SYSTEM. THE JTWC INTENSITY FORECAST IS PLACED NEAR CONSENSUS,
CALLING FOR A PEAK OF 120 KTS, WITH OVERALL LOW CONFIDENCE AS WELL.

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