WDXS32 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (EIGHTEEN)
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.1S 107.7E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 662 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: EASTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 18S WITH PERSISTENT DEEP CONVECTION COMING INTO BETTER 
VERTICAL ALIGNMENT WITH A PARTIALLY EXPOSED AND ELONGATED LOW-LEVEL 
CIRCULATION CENTER (LLCC) OVER THE PAST SIX HOURS. ENVIRONMENTAL 
ANALYSIS INDICATES THAT 18S IS IN A MARGINAL ENVIRONMENT WITH LOW TO 
MODERATE OUTFLOW ALOFT AND HIGH (25-30 KTS) VERTICAL WIND SHEAR 
(VWS), OFFSET BY WARM (29-30 C) SEA SURFACE TEMPERATURES (SST). THE 
INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON A 121135Z 
SSMIS 91 GHZ SATELLITE MICROWAVE IMAGE. THE INITIAL INTENSITY OF 40 
KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON PERSISTENCE OF 
SCATTEROMETRY DATA AND CIMSS SATELLITE CONSENSUS.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A NEAR EQUATORIAL
RIDGE (NER) TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS
   APRF: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 120900Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 25-30 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 CURRENTLY TRACKING EASTWARD ALONG
THE SOUTHERN PERIPHERY OF A NER TO THE NORTH. THROUGH TAU 48, TC 18S
IS FORECAST TO TAKE AN EAST-SOUTHEAST TRACK ALONG THE NER. BEGINNING 
NEAR TAU 48, TC 18S IS FORECAST TO START A POLEWARD TURN TO A 
SOUTHWESTWARD TRACK BY TAU 120 AS A RIDGE OVER WESTERN AUSTRALIA 
INFLUENCES THE STEERING OF THE SYSTEM. TC 18S IS FORECAST TO MAINTAIN 
AN INTENSITY OF NEAR 40 KTS UNTIL TAU 24 DUE TO HIGH (25-30 KTS) VWS 
AND WEAK DIVERGENCE ALOFT. NEAR TAU 24, THE SYSTEM WILL LIKELY BEGIN 
A GRADUAL INTENSIFICATION TREND CONTINUING THROUGH THE REMAINDER OF 
THE FORECAST PERIOD TO AN INTENSITY OF NEAR 70KTS BY TAU 120, 
PRIMARILY DUE TO DECREASING VWS VALUES AND IMPROVING UPPER-LEVEL 
DIVERGENCE.

MODEL DISCUSSION: GLOBAL NUMERICAL MODEL GUIDANCE IS IN OVERALL FAIR
AGREEMENT REGARDING TRACK WITH A 70 NM SPREAD AT TAU 24, WHICH
RAPIDLY INCREASES TO 450 NM BY TAU 72. DUE TO THE LARGE UNCERTAINTY 
PRESENTED IN JTWC CONSENSUS MEMBERS BY TAU 72, THE OVERALL CONFIDENCE 
OF THE FORECAST TRACK IS LOW. INTENSITY GUIDANCE IS IN BETTER
AGREEMENT, WITH NUMERICAL MODEL GUIDANCE SUGGESTING A GRADUAL RISE IN
INTENSITY AFTER TAU 24, EXCLUDING COAMPS-TC. THE OVERALL JTWC 
INTENSITY FORECAST IS THEREFORE ASSESSED WITH MEDIUM CONFIDENCE.

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