WDXS31 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (ZELIA) WARNING NR 
007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.2S 118.3E
   INITIAL INTENSITY: 85 KTS
   GEOGRAPHIC REFERENCE: 68 NM NORTH-NORTHWEST OF PORT HEDLAND,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE (TC) 17 (ZELIA) WITH A COMPACT AND INCREASINGLY SYMMETRICAL
CENTRAL DENSE OVERCAST (CDO) FEATURE. TC ZELIA JUST RECENTLY
DEVELOPED A PINHOLE EYE FEATURE, WHICH IS RAPIDLY EXPANDING AND
BECOMING MORE SYMMETRICAL. THE SYSTEM IS CURRENTLY IN A HIGHLY
FAVORABLE ENVIRONMENT WITH VERY WARM (30-31C) SEA SURFACE
TEMPERATURE (SST), LOW (5-10KTS) VERTICAL WIND SHEAR AND STRONG
OUTFLOW. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED
ON THE INFRARED EYE. THE SYSTEM IS CURRENTLY RAPIDLY INTENSIFYING
THEREFORE, THE INITIAL INTENSITY OF 85 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON THE AGENCY DVORAK AND OBJECTIVE INTENSITY
ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE 

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


AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T6.0 - 115 KTS
   DEMS: T4.0 - 65 KTS
   APRF: T5.0 - 90 KTS
   CIMSS ADT: 77 KTS AT 121730Z
   CIMSS AIDT: 91 KTS AT 121800Z
   CIMSS D-PRINT: 85 KTS AT 121800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

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: TC 17S IS FORECAST TO CONTINUE SLOWLY
TRACKING WEST-SOUTHWESTWARD OVER THE NEXT 24 HOURS, STEERED BY
COMPETING, YET WEAK INFLUENCES OF TWO SUBTROPICAL RIDGES TO ITS
WEST AND EAST-SOUTHEAST. AROUND TAU 24, TC 17S IS FORECAST TO BEGIN
TURNING SOUTHWARD, AS THE WESTERN RIDGE BEGINS TO MOVE WESTWARD,
AND THE STR TO THE EAST WILL DOMINATE THE STEERING, ULTIMATELY
LEADING TC ZELIA ON A SOUTH-SOUTHEASTWARD PATH INLAND, EAST OF
DAMPIER, AUSTRALIA. WHILE THE SYSTEM CONTINUES ITS VERY SLOW
PROGRESSION OVER WATER, IT IS FORECAST TO CONTINUE RAPIDLY
INTENSIFYING, WITH A PEAK BETWEEN TAUS 24 AND 36. CURRENT INTENSITY
MAXIMUM IS FORECAST TO BE 125 KTS, THE SYSTEM MAY REACH SLIGHTLY
HIGHER MAXIMUM WIND SPEEDS, IF IT SLOWS DOWN EVEN MORE AND SPENDS
MORE TIME WITHIN THE HIGHLY FAVORABLE ENVIRONMENT. AFTER TAU 36, TC
ZELIA WILL MAKE LANDFALL AND BEGIN TO STEADILY WEAKEN AS ITS WIND
FIELD INTERACTS WITH THE TERRAIN AND THE CORE IS CUT OFF THE
FAVORABLE OCEAN ENVIRONMENT. AFTER LANDFALL, TC 17S IS FORECAST TO
TRACK SOUTHWARD, WITH A SLIGHT CURVE AROUND THE HAMERSLEY
MOUNTAIN RANGE TO THE WEST. LAND INTERACTION WILL CONTINUE TO ERODE
THE VORTEX AND THE DISSIPATION IS CURRENTLY FORECAST TO BE
COMPLETED BEFORE TAU 96.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN POOR AGREEMENT IN
REGARDS TO THE TRACK, AS SLOW TRANSLATION SPEED OVER WATER,
COMBINED WITH THE WEAK STEERING PATTERN CONTRIBUTE TOWARD INCREASED
UNCERTAINTY. DETERMINISTIC MODELS GENERALLY AGREE THAT THE SYSTEM
WILL BEGIN TURNING SOUTHWARD AROUND TAU 24, HOWEVER THE SPEED OF
THE SYSTEM AND THEREFORE THE LOCATION OF THE VORTEX IS HIGHLY
UNCERTAIN, AS WITNESSED BY THE 70NM CROSS-TRACK SPREAD AT TAU 24,
WHICH INCREASES TO A MAXIMUM OF 175NM PAST LANDFALL. GFS
DETERMINISTIC AND ECMWF ENSEMBLE MEAN ARE THE TWO MOST WESTWARD
TRACKERS, WHILE GALWEM AND ECMWF DETERMINISTIC ARE MOST EASTWARD.  
JTWC TRACK FORECAST IS LAID WITH LOW CONFIDENCE, JUST WEST OF THE
MULTI-MODEL CONSENSUS. THE TRACK POSITION TREND HAS BEEN EASTWARD
SINCE THE LAST CYCLE, HOWEVER MANY MEMBERS, INCLUDING THE
INDIVIDUAL ECMWF ENSEMBLE MEMBERS, STILL ALLOW FOR A TRACK WEST OF
DAMPIER, EVEN AS FAR WEST AS AROUND THE NORTHWESTERN COAST OF
AUSTRALIA. INTENSITY GUIDANCE IS MORE CONSISTENT, WITH NEARLY ALL
RAPID INTENSIFICATION AIDS SUGGESTING A 35-45 KTS INTENSIFICATION
OVER THE NEXT 24-36 HOURS. AFTER TAU 36 ALL INTENSITY GUIDANCE
PREDICTS RAPID WEAKENING, AS THE VORTEX IS BEING TRACKED OVER LAND.
THE TIMELINE OF THE PEAK AND ASSOCIATED CHANGE IN DEVELOPMENT
TRENDS IS HIGHLY UNCERTAIN AT THE MOMENT AND THEREFORE THE JTWC
INTENSITY FORECAST IS ASSESSED WITH LOW CONFIDENCE. 

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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