WDXS31 PGTW 020300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (HERMAN) WARNING NR 
016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.8S 106.7E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 441 NM WEST-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 02 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE 17S CONTINUES TO SLOW ITS PROGRESSION TO THE
SOUTH-SOUTHWEST, WHILE ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) SHOWS A SHEARED SYSTEM WITH A BULK OF THE CONVECTION
DISPLACED TO THE EAST. THE INITIAL POSITION IS PLACED WITH LOW
CONFIDENCE USING LOW CLOUD TRACING IN THE MSI LOOP DUE TO THE LACK
OF HIGH-RESOLUTION MICROWAVE IMAGERY OR RECENT SCATTEROMETRY DATA.
THE INITIAL INTENSITY OF 65 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE AGENCY DVORAK ESTIMATES WITH PGTW T4.0, KNES T4.5,
APRF T3.5, AND A COMBINATION OF OBJECTIVE DVORAK ESTIMATES.
ENVIRONMENTAL CONDITIONS REMAIN MARGINALLY UNFAVORABLE WITH HIGH
(20-25 KNOTS) VERTICAL WIND SHEAR (VWS), MODERATE EASTWARD OUTFLOW,
WARM (26-27C) SEA SURFACE TEMPERATURES (SSTS) AND DRY AIR ENTRAINMENT.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: THE SYSTEM IS LOCATED WITHIN A WEAK
STEERING ENVIRONMENT SOUTHWEST OF A DEEP LAYER SUBTROPICAL RIDGE
(STR) TO THE EAST AND A LOW- TO MID-LEVEL EXTENSION OF A STR TO THE
SOUTH.  

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   KNES: T4.5 - 77 KTS
   APRF: T3.5 - 55 KTS
   CIMSS SATCON: 66 KTS AT 012000Z
   CIMSS ADT: 69 KTS AT 012330Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 20-25 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: WEAK EASTWARD

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 17S (HERMAN) IS FORECAST TO CONTINUE SLOWLY
TRACKING SOUTHWESTWARD AS THE SYSTEM APPROACHES A BREAK IN THE
STEERING OF A STR TO THE EAST WHILE A LOW- TO MID-LEVEL EXTENSION OF A 
STR TO THE SOUTH BECOMES THE MAIN STEERING FEATURE. AS THIS OCCURS, 
THE SYSTEM IS EXPECTED TO SHARPLY TURN AND TRACK TOWARD THE NORTHWEST
THROUGHOUT THE FORECAST PERIOD. TC 17S SHOULD RAPIDLY WEAKEN OVER
THE NEXT 24-36 HOURS, AS CONDITIONS WILL DEGRADE STEADILY WITH
SIGNIFICANT DRY AIR ENTRAINMENT, INCREASING VWS AND SUSTAINED
MARGINALLY-FAVORABLE SST VALUES OF 25-26C, LEADING TO DISSIPATION
BY 48.   

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH TAKES AN
UNREALISTIC ABRUPT TURN SOUTHEAST, THE BULK OF THE GUIDANCE
CONTINUES THE SYSTEM SOUTHWESTWARD THROUGH TAU 24 AND THEN
NORTHWESTWARD THROUGH TAU 48 WITH A 180 NM SPREAD IN SOLUTIONS. FOR
THIS REASON, THE JTWC TRACK FOLLOWS CLOSE TO THE MULTI-MODEL
CONSENSUS THROUGH 48 HOURS AND SLIGHTLY RIGHT OF CONSENSUS
FOLLOWING CLOSER TO AVNI. RELIABLE INTENSITY GUIDANCE CONTINUES TO
SUPPORT A RAPID WEAKENING TREND DUE TO IMPEDING ENVIRONMENTAL
FACTORS LEADING MEDIUM CONFIDENCE TO THE INTENSITY
FORECAST. 

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