WDXS31 PGTW 091500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN)   
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.8S 125.1E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 848 NM NORTHEAST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 18S HAS STRUGGLED TO CONSOLIDATE OVER THE
PAST SIX HOURS DUE TO A RENEWED BURST OF EASTERLY VERTICAL WIND SHEAR 
(VWS) EVIDENT IN ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY. 
EIR DEPICTS A NEAR-SYMMETRIC CLUSTER OF DEEP CONVECTION SHEARING TO 
THE SOUTHWEST WITH VORTICAL HOT TOWERS (CLOUD TOP TEMPERATURES AS COLD 
AS -91C) ACCOMPANIED BY INTENSE CLOUD TO GROUND LIGHTNING STRIKES. DUE 
TO THE LACK OF RECENT ASCAT IMAGERY AND HIGH-RESOLUTION MICROWAVE 
IMAGERY, THERE IS LOW CONFIDENCE IN THE INITIAL POSITION WITH A LARGE 
100NM SPREAD IN POSITION FIXES FROM KNES, PGTW AND APRF. A 090804Z 
SSMIS 37GHZ MICROWAVE IMAGE SHOWS CURVED BANDING OVER THE WESTERN 
SEMICIRCLE WITH WEAK LOW-LEVEL STRUCTURE OVER THE EASTERN SEMICIRCLE 
BUT GENERALLY SUPPORTS, ALONG WITH THE EASTERLY VWS, AN INITIAL 
POSITION DISPLACED ALONG THE NORTHEASTERN EDGE OF THE DEEP CONVECTIVE 
MASS. THE INITIAL INTENSITY IS HELD AT 45 KNOTS BASED ON THE PGTW AND 
APRF CURRENT INTENSITY ESTIMATES OF 3.0 AS WELL AS THE 091300Z ADT 
ESTIMATE.    

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR)
POSITIONED TO THE SOUTH AND SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   APRF: T3.0 - 45 KTS
   CIMSS ADT: 47 KTS AT 091300Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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 EXPECTED TO TRACK WEST-SOUTHWESTWARD
TO SOUTHWESTWARD THROUGH TAU 48 ALONG THE NORTHWEST PERIPHERY OF
THE STR. AFTER TAU 48, THE SYSTEM WILL RECURVE SOUTHWARD AS A
MIDLATITUDE SHORTWAVE TROUGH DIGS INTO WESTERN AUSTRALIA WEAKENING
THE STR. THE SYSTEM WILL ACCELERATE SOUTHEASTWARD AFTER TAU 96
WHEN ANOTHER SHORTWAVE TROUGH PROPAGATES OVER WESTERN AUSTRALIA. 
ENVIRONMENTAL CONDITIONS SHOULD IMPROVE WITHIN THE NEXT 12 HOURS 
ALLOWING TC 18S TO RAPIDLY INTENSIFY AIDED BY ENHANCED POLEWARD 
OUTFLOW, WARM SST VALUES AND HIGH OCEAN HEAT CONTENT, ESPECIALLY OFF 
THE COAST OF NORTHWEST AUSTRALIA. THE PEAK INTENSITY OF 130 KNOTS 
SHOULD OCCUR JUST PRIOR TO LANDFALL.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
WITH A 140NM CROSS-TRACK SPREAD IN SOLUTIONS AT TAU 72 REFLECTING 
UNCERTAINTY IN THE RECURVE POINT. AFTER TAU 72, CROSS-TRACK SPREAD 
DECREASES (WITH THE EXCEPTION OF NAVGEM), HOWEVER, ALONG TRACK 
UNCERTAINTY INCREASES AS THE SYSTEM INTERACTS WITH THE AFOREMENTIONED 
SHORTWAVE TROUGH AND ACCELERATES SOUTHEASTWARD. THE RAPID 
INTENSIFICATION CONSENSUS (RICN) GUIDANCE HAS TRIGGERED SUPPORTING 
RAPID INTENSIFICATION (RI) OVER THE NEXT TWO DAYS. ADDITIONALLY, THE 
090600Z COAMPS TC ENSEMBLE (CTCXEPS) CONTINUES TO INDICATE A 95 TO 100 
PERCENT PROBABILITY OF RI FROM TAU 48 TO TAU 78 SUPPORTING THE RI 
FORECAST IN THE DAY 2 TO DAY 3 PERIOD. ADDITIONAL INTENSIFICATION 
ABOVE 130 KNOTS IS POSSIBLE PRIOR TO LANDFALL NEAR TAU 78.  

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