WDXS32 PGTW 110900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.2S 101.3E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 950 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: EASTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 18S HAS CONTINUED TO STRUGGLE TO DEVELOP DUE
TO HIGH (30-40 KNOTS) EASTERLY VERTICAL WIND SHEAR AS EVIDENCED IN
THE ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI), WHICH DEPICTS A
FULLY-EXPOSED LOW-LEVEL CIRCULATION (LLC) WITH PERSISTENT AND DEEP
CONVECTION SHEARED OVER THE WESTERN QUADRANT. ALTHOUGH THE DEEP
CONVECTIVE STRUCTURE IS WEAK, THE LOW-LEVEL BANDING IS DEFINED AND
WRAPS NEATLY INTO THE CENTER, AS EVIDENT IN THE 110209Z MHS 89 GHZ
MICROWAVE IMAGE. HOWEVER, AN 110304Z ASCAT-C IMAGE HAS CONTINUED TO
CONFIRM A BROAD CENTER WITH EXTENSIVE GALE-FORCE WINDS (35-45
KNOTS) DISPLACED ALONG THE PERIPHERY. THIS EXTENSIVE WIND FIELD IS
REFLECTED IN THE WIND RADII ANALYSIS. THE INITIAL POSITION IS
PLACED WITH HIGH CONFIDENCE BASED ON MSI, AND THE AFOREMENTIONED
ASCAT IMAGE. THE INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE ASCAT DATA.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE (NER) POSITIONED TO
THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 30-40 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TC 18S IS FORECAST TO TRACK EAST-SOUTHEASTWARD
THROUGH TAU 72 UNDER THE STEERING INFLUENCE OF THE NER. AFTER TAU 72,
THE SYSTEM WILL SLOW, POSSIBLY BECOMING QUASI-STATIONARY, AS THE
STEERING INFLUENCE GRADUALLY TRANSITIONS TO A BUILDING STR OVER
AUSTRALIA. THE SYSTEM SHOULD TURN SOUTH-SOUTHWESTWARD BY TAU 120
ALONG THE NORTHWESTERN PERIPHERY OF THE AFOREMENTIONED STR. THE
SYSTEM WILL INTENSIFY SLOWLY THROUGH TAU 48, WITH A SLIGHTLY FASTER
INTENSIFICATION RATE THROUGH TAU 96 DUE TO IMPROVING POLEWARD OUTFLOW.
AFTER TAU 96, THE SYSTEM WILL BEGIN TO INTERACT WITH A MAJOR SHORTWAVE
TROUGH, WHICH SHOULD LEAD TO A SLIGHT WEAKENING TREND.   

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
THROUGH TAU 72 WITH A 200 NM CROSS-TRACK SPREAD AT TAU 72. AFTER
TAU 72, THE NUMERICAL MODEL GUIDANCE DIVERGES SIGNIFICANTLY DUE TO
COMPETING STEERING MECHANISMS, LENDING LOW CONFIDENCE TO THE JTWC
EXTENDED FORECAST. THE 110000Z ECMWF ENSEMBLE (EPS) INDICATES A
VERY LARGE SPREAD IN SOLUTIONS AND HIGH UNCERTAINTY IN THE TAU 72
TO TAU 120 PERIOD. RELIABLE INTENSITY GUIDANCE INDICATES FAIR
AGREEMENT THROUGH THE FORECAST PERIOD WITH A STEADY INTENSIFICATION
TREND. 

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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