WDXS32 PGTW 150900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN) WARNING 
NR 017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8S 114.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 273 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 18S HAS CONTINUED TO SHOW SIGNS OF SLOW
CONSOLIDATION WITH SUSTAINED CONVECTION IN CLOSE PROXIMITY TO THE
LOW-LEVEL CIRCULATION CENTER (LLCC). HOWEVER, THE SURFACE CENTER
REMAINS PARTIALLY EXPOSED BETWEEN TWO CONVECTIVE STRUCTURES ON THE
EASTERN AND WESTERN FLANKS OF THE LOW-LEVEL CURVED CLOUD LINES
WRAPPING INTO THE TIGHTEST TURNING, AS REVEALED IN THE ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI). WEAK EQUATORWARD OUTFLOW IS
EVIDENT OVER THE LLCC, AS UPPER-LEVEL OUTFLOW CONTINUES TO MAINTAIN
THE VERY SLOW INTENSIFICATION TREND. THE INITIAL POSITION IS PLACED 
WITH MEDIUM CONFIDENCE BASED ON A 150609Z 89 GHZ AMSR2 MICROWAVE 
IMAGE. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED 
ON UW-CIMSS OBJECTIVE INTENSITY ESTIMATES RANGING BETWEEN 32-33 KTS, 
WHILE SUBJECTIVE DVORAK ESTIMATES ASSESS TC 18S BETWEEN 30-35 KNOTS.

INITIAL WIND RADII BASIS: OBTK

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE SOUTH,
LOCATED OVER CENTRAL AUSTRALIA AND EXTENDING WESTWARD INTO THE
SOUTHERN INDIAN OCEAN.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS
   CIMSS ADT: 33 KTS AT 150610Z
   CIMSS AIDT: 32 KTS AT 150610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 18S IS CURRENTLY TRACKING SOUTHWESTWARD
ALONG THE WESTERN PERIPHERY OF THE STR LOCATED OVER CENTRAL
AUSTRALIA AND EXTENDING WESTWARD INTO THE SOUTHERN INDIAN OCEAN
(IO). AS THE AFOREMENTIONED DEEP-LAYER STR CONTINUES TO BUILD
WESTWARD, TC 18S IS EXPECTED TO SLOW AND MOVE GENERALLY WEST-
SOUTHWESTWARD THROUGH TAU 72. AFTER TAU 72, AN EQUATORWARD-EXTENDING 
DEEP LONGWAVE TROUGH WILL WEAKEN THE STR, HOWEVER, A STRONG LOW-LEVEL 
HIGH PRESSURE BEGINS BUILDING INTO THE SOUTHERN IO. AS THE HIGH BUILDS 
OVER THE SOUTHERN IO, TC 18S WILL BEGIN A WEST-NORTHWESTWARD TRACK 
ALONG THE NORTHERN PERIPHERY OF THE SOUTHEAST TRADE FLOW THROUGH TAU 
120. THE CYCLONE IS EXPECTED TO ENTER A MORE CONDUCIVE ENVIRONMENT AS 
VERTICAL WIND SHEAR (VWS) DECREASES (10-15 KNOTS), AND SEA SURFACE 
TEMPERATURES SUSTAIN BETWEEN 29-31C, RESULTING IN A STEADY INCREASE IN 
INTENSITY THROUGH TAU 72 UP TO 45 KNOTS, AND A PEAK UPWARDS OF 60 
KNOTS INTO TAU 96. AFTER TAU 96, A GRADUAL WEAKENING TREND IS EXPECTED 
AS DRY AIR BEGINS SLOWLY WRAPPING AROUND THE NORTHERN QUADRANT, 
LIMITING FURTHER TC DEVELOPMENT.   

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, THE DETERMINISTIC
MODEL TRACK GUIDANCE CONTINUES TO IMPROVE IN AGREEMENT THROUGH TAU
72 WITH A CROSS-TRACK SPREAD OF 70 NM, INCREASING SIGNIFICANTLY
THROUGH TAU 120 WITH A CROSS-TRACK SPREAD OF 215 NM. THE 150000Z
ECMWF ENSEMBLE (EPS) AND GFS ENSEMBLE (GEFS) ALSO INDICATE CONTINUED
UNCERTAINTY AFTER TAU 72, WITH A DIVERGENT SPREAD IN SOLUTIONS. THE 
150000Z COAMPS-TC INTENSITY ENSEMBLE (CTCXEPS) SHOWS A STEADY 
INTENSIFICATION TREND, WITH PEAK PROBABILITIES REACHING MODERATE
INTENSIFICATION OF 30 TO 40 PERCENT BETWEEN TAU 48 AND TAU 60.

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