WDXS31 PGTW 201500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (TWENTYONE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.1S 113.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 552 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 11 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
RAPIDLY CONSOLIDATING CENTRAL DENSE OVERCAST STRUCTURE WITH SPIRAL
BANDS OF DEEP CONVECTION AND STRONG RADIAL OUTFLOW. A 200935Z SSMIS
91GHZ MICROWAVE IMAGE DEPICTS CURVED BANDING OVER THE EASTERN
SEMICIRCLE WRAPPING INTO A SMALL CORE OF INTENSE CONVECTION WITH
FRAGMENTED CONVECTIVE BANDING OVER THE WESTERN SEMICIRCLE. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON EIR AND
AGENCY SATELLITE FIX POSITIONS. THE INITIAL INTENSITY OF 35 KTS IS
ASSESSED WITH HIGH CONFIDENCE BASED ON CONCURRENT DVORAK INTENSITY
ESTIMATES.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: DEEP LAYERED SUBTROPICAL RIDGE (STR) TO
THE SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   APRF: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 21S IS EXPECTED TO TO TRACK SOUTHWESTWARD
TO WEST-SOUTHWESTWARD ALONG THE NORTHWESTERN PERIPHERY OF THE STR.
THE SYSTEM IS CURRENTLY LOCATED IN A VERY FAVORABLE ENVIRONMENT,
CHARACTERIZED BY WARM SEA SURFACE TEMPERATURES (30-31C), LOW
VERTICAL WIND SHEAR (VWS) AND STRONG RADIAL OUTFLOW, RESULTING IN A
HIGH LIKELIHOOD OF RAPID INTENSIFICATION THROUGH TAU 24. THE SYSTEM
WILL REACH A PEAK INTENSITY OF 75 KNOTS BY TAU 36. AFTER REACHING
ITS PEAK INTENSITY, THE STEERING ENVIRONMENT SHIFTS AS A SHORTWAVE
TROUGH BREAKS DOWN THE STEERING STR RESULTING IN SLOWER TRACK
SPEEDS AS THE SYSTEM TRACKS SOUTHWESTWARD. MEANWHILE, ENVIRONMENTAL 
CONDITIONS WILL BEGIN TO RAPIDLY DETERIORATE DUE TO SIGNIFICANT DRY 
AIR ENTRAINMENT, WHICH WILL CONTINUE THROUGH THE REMAINDER OF THE 
FORECAST RESULTING IN A RAPID WEAKENING TREND. BETWEEN TAU 72 AND TAU 
96, TC 21S WILL BEGIN INTERACTING WITH A NEW SHORTWAVE TROUGH AND 
WILL BEGIN TRACKING SOUTH-SOUTHEASTWARD AS IT CONTINUES WEAKENING DUE 
TO INCREASED VWS AND DRY AIR ENTRAINMENT, EVENTUALLY DISSIPATING BY 
TAU 120.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
SUPPORTING THE JTWC FORECAST TRACK WITH MEDIUM CONFIDENCE. THE JTWC
FORECAST TRACK IS POSITIONED CLOSE TO THE MULTI-MODEL CONSENSUS
(CONW). RELIABLE INTENSITY GUIDANCE IS IN FAIR AGREEMENT WITH THE
BULK OF THE MODELS SHOWING A RAPID INTENSIFICATION PHASE THROUGH
TAU 24 TO TAU 36 THEN A RAPID WEAKENING TREND. THE JTWC INTENSITY
FORECAST IS PLACED HIGHER THAN THE INTENSITY CONSENSUS, WHICH IS
SKEWED TOO LOW DUE TO COAMPS-TC (NAVGEM VERSION) AND DECAY SHIPS.
HWRF PEAKS THE INTENSITY NEAR 70 KNOTS WHILE AHNI PEAKS AT 78
KNOTS.  

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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