WDXS31 PGTW 132100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (ZELIA) WARNING NR 
011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.3S 118.3E
   INITIAL INTENSITY: 115 KTS
   GEOGRAPHIC REFERENCE: 62 NM NORTH-NORTHWEST OF PORT HEDLAND,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: EASTWARD AT 01 KTS
   SIGNIFICANT WAVE HEIGHT: 36 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
FILLING EYE AND INCREASING IRREGULARITY IN THE SHAPE OF THE CENTRAL
CONVECTIVE CORE. RADAR IMAGERY FROM PORT HEDLAND, AUSTRALIA
SUGGESTS THAT AN EYEWALL REPLACEMENT CYCLE PROCESS MAY BE IN ITS
INITIAL STAGES AS A NEARLY COMPLETE 38 NM DIAMETER CONCENTRIC RING
OF CONVECTION NOW SURROUNDS THE 15 NM DIAMETER INNER EYEWALL.
ADDITIONALLY, 20 TO 25 KT OF EASTERLY VERTICAL SHEAR IS NOW ACTING
ON THE VORTEX AND IS PERIODICALLY DISRUPTING THE INNER CORE
STRUCTURE, EVIDENT VIA ABUNDANT CIRRUS CLOUD ELEMENTS MOVING
WESTWARD IN EIR IMAGERY TOWARDS THE LLCC. OCEAN ANALYSIS FROM THE
HAFS-A MODEL ALSO SHOWS THAT WATERS BENEATH THE QUASI-STATIONARY
CYCLONE ARE NOW COOLING SIGNIFICANTLY UNDER THE NORTHERN HALF OF
THE INNER CORE, WHICH MAY BE CONTRIBUTING TO THE SHORT-TERM
WEAKENING TREND. UPPER-LEVEL OUTFLOW IS EXPANSIVE BOTH POLEWARD AND
EQUATORWARD, THOUGH MUCH STRONGER POLEWARD INTO THE EASTERN FLANK
OF A LONGWAVE TROUGH OVER SOUTHWESTERN AUSTRALIA. THE INITIAL
INTENSITY IS SET AT 115 KT BASED ON A BLEND OF FALLING AGENCY
DVORAK ESTIMATES, OBJECTIVE SATELLITE-BASED INTENSITY ESTIMATES
FROM THE UNIVERSITY OF WISCONSIN, AND EARLIER 131031Z RCM-3 SAR
DATA THAT HELPED TO NARROW THE RANGE OF INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: DEEP-LAYER STEERING RIDGE OVER NORTHERN
AUSTRALIA AND TIMOR SEA

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.5 - 127 KTS
   KNES: T7.0 - 140 KTS 
   CIMSS SATCON: 121 KTS AT 131412Z
   CIMSS ADT: 124 KTS AT 131730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-25 KTS
   SST: 27-30 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TROPICAL CYCLONE 17S (ZELIA) CONTINUES TO MOVE
LETHARGICALLY BETWEEN TWO DEEP-LAYER RIDGES TO ITS WEST AND EAST, NOW 
DRIFTING SLOWLY EASTWARD ABOUT 62 NM OFFSHORE OF PORT HEDLAND, 
AUSTRALIA. THIS SLOW MOVEMENT HAS LIKELY RESULTED IN DYNAMIC OCEAN 
COOLING BENEATH A PORTION OF THE INNER CORE, WHICH COMBINED WITH 
MODERATE VERTICAL SHEAR AND THE APPARENT INITIATION OF AN EYEWALL 
REPLACEMENT CYCLE (ERC) IS CAUSING WEAKENING. THE SHORT-TERM 
INTENSITY FORECAST PRIOR TO LANDFALL IS CHALLENGING, AS IT IS UNCLEAR 
WHETHER THIS ERC WILL BE ABLE TO COMPLETE BEFORE THEN. IF IT DOES, 
THE INNER CORE COULD BRIEFLY STRENGTHEN AGAIN, AIDED BY WARMER 
COASTAL WATERS AS THE CYCLONE MOVES SOUTHWARD BEYOND ITS CURRENT COLD 
WAKE. HOWEVER, IT IS ALSO POSSIBLE THAT ZELIA COULD FAIL TO 
REINTENSIFY OR EVEN WEAKEN FURTHER. THE JTWC INTENSITY FORECAST SHOWS 
MILD REINTENSIFICATION TO 125 KT IN 12 HOURS JUST PRIOR TO LANDFALL, 
BUT LOWER LANDFALL INTENSITY THAN THE PREVIOUS FORECAST. THE TRACK 
FORECAST BRINGS ZELIA JUST EAST OF DUE SOUTHWARD INTO THE COASTLINE 
JUST WEST OF PORT HEDLAND, AUSTRALIA. THIS SHORT-TERM TRACK IS 
SLIGHTLY EAST OF THE PREVIOUS FORECAST BUT SLIGHTLY WEST OF THE 
DYNAMICAL MODEL CONSENSUS, WHICH HAS SHIFTED EASTWARD OVER THE PAST 
12 HOURS. IT SHOULD BE NOTED THAT TROPICAL CYCLONES UNDERGOING 
EYEWALL REPLACEMENT CYCLES TEND TO WOBBLE UNPREDICTABLY, AND THE 
EXACT LANDFALL LOCATION MAY DEPEND ON SUCH WOBBLES OVER THE NEXT 12 
HOURS. AFTER LANDFALL, ZELIA IS FORECAST TO TRACK GENERALLY SOUTHWARD 
OVER WESTERN AUSTRALIA, GUIDED BY A DEEP-LAYER RIDGE OVER NORTHERN 
AUSTRALIA, EVENTUALLY DECAYING BY 72 HOURS.

MODEL DISCUSSION: DYNAMICAL MODEL TRACK GUIDANCE CONTINUES TO COME
INTO BETTER AGREEMENT ON THE SHORT-TERM TRACK FORECAST. THE JTWC
TRACK FORECAST IS JUST WEST OF THE MULTI-MODEL CONSENSUS. THE JTWC
INTENSITY FORECAST IS A LITTLE ABOVE THE MULTI-MODEL INTENSITY
FORECAST PRIOR TO LANDFALL, BUT CLOSE TO THE CONSENSUS THEREAFTER.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   INTENSITY 0 - 72 HR: MEDIUM//
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