WDXS31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21S (TWENTYONE) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.5S 112.6E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 535 NM NORTH OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT AREA OF DEEP, FLARING CONVECTION WHICH HAS CONTINUED TO
STEADILY CONSOLIDATE OVER THE PAST SIX HOURS. A 201738Z AMSR2 36GHZ
MICROWAVE IMAGE SHOWED STRONG CONVECTION IN A SMALL REGION TO THE
SOUTH OF THE LOW LEVEL CIRCULATION CENTER (LLCC), AND A TAIL OF
WEAKER CONVECTION EXTENDING OUTWARD WELL UP TO THE NORTH. THE AMSR2
IMAGERY PROVIDED HIGH CONFIDENCE TO THE INITIAL POSITION, WHICH IS
PLACED JUST NORTH OF THE DEEPEST CONVECTION SEEN IN THE EIR. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE AT 40 KNOTS,
HEDGED HIGHER THAN THE PGTW AND APRF T2.5 (35 KNOTS) IN LIGHT OF A
KNES T3.0 (45 KNOTS) DVORAK CURRENT INTENSITY ESTIMATE. OTHER THAN
THE NOAA ADT AT T3.2, THERE ARE NO OTHER OBJECTIVE INTENSITY
ESTIMATES AVAILABLE AT THIS TIME. EARLIER SCATTEROMETER DATA
INDICATED THE PRESENCE OF 35-40 KNOT WINDS ACROSS A BROAD ARC IN
THE SOUTHERN HEMISPHERE OF THE CIRCULATION, WHILE WINDS WERE MUCH
LIGHTER ACROSS THE NORTHWESTERN SECTORS AND THE WIND RADII HAVE
BEEN UPDATED TO REFLECT THIS WIND FIELD STRUCTURE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED OVER CENTRAL AUSTRALIA AND WITH AN
EXTENSION RIDGING WEST TO NEAR 20S 100E. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   APRF: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG RADIAL
   OTHER FACTORS: THE SYSTEM IS BEGINNING TO TAP INTO SOME POLEWARD
OUTFLOW INTO THE SUBTROPICAL WESTERLIES AHEAD OF A BROAD LONGWAVE
TROUGH TO THE SOUTHWEST. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 21S IS CURRENTLY TRACKING AROUND 10 KNOTS
WEST-SOUTHWESTWARD ALONG THE NORTHWEST PERIPHERY OF THE STRONG STR
OVER AUSTRALIA. THIS GENERAL MOTION IS FORECAST TO CONTINUE THROUGH
TAU 72, HOWEVER THE SYSTEM IS EXPECTED TO SLOW DOWN OVER THIS
PERIOD, TO LESS THAN 04 KNOTS BY TAU 72, AS AN EXTENSION OF THE STR
IN THE MID AND UPPER-LEVELS BUILDS WESTWARD TO THE SOUTH OF THE
SYSTEM. BY AROUND TAU 72, THE RIDGE WILL RAPIDLY ERODE AS A STRONG
SUBTROPICAL LOW AND TROUGH MOVE IN FROM THE WEST, AND BY TAU 96, TC
21S WILL TURN SHARPLY SOUTH-SOUTHEASTWARD WHILE QUICKLY PICKING UP
SPEED AS IT HEADS TOWARDS THE WEST COAST OF AUSTRALIA.
ENVIRONMENTAL CONDITIONS ARE VERY FAVORABLE FOR RAPID
INTENSIFICATION IN THE NEAR-TERM, WITH VERY WARM SSTS, LOW VWS AND
ROBUST RADIAL OUTFLOW, WITH AN ANTICIPATED DEVELOPMENT OF A
MODERATELY STRONG POLEWARD OUTFLOW CHANNEL AND TC 21S IS FORECAST
TO RAPIDLY INTENSIFY TO A PEAK OF AT LEAST 80 KNOTS BY TAU 36.
ENVIRONMENTAL CONDITIONS WILL RAPIDLY DETERIORATE AFTER TAU 36
HOWEVER, WITH MID-LEVEL DRY AIR ENTRAINMENT, AND INCREASING SHEAR
LEADING TO SLOW WEAKENING THROUGH TAU 48, AND MORE RAPID WEAKENING
THEREAFTER. BY TAU 72, TC 21S WILL BEGIN TO INTERACT WITH
STRONG UPPER-LEVEL WESTERLY FLOW, INCREASED SHEAR AND WILL BE
ENVELOPED IN A DRY ATMOSPHERE, LEADING TO DISSIPATION BY TAU 120.
THERE IS POSSIBILITY THAT THE SYSTEM WILL NOT FULLY DISSIPATE BUT
RATHER REMAIN ABOVE WARNING CRITERIA (35 KNOTS) AND UNDERGO
SUBTROPICAL TRANSITION (STT) AS INDICATED BY SOME OF THE GLOBAL
MODELS. AT THIS POINT HOWEVER, IT IS ANTICIPATED THAT THE SYSTEM
WILL IN FACT DISSIPATE FIRST BEFORE TRANSITIONING TO A SUBTROPICAL
LOW. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN
OVERALL AGREEMENT ON THE GENERAL SCENARIO, BUT DISPLAY SOME
SIGNIFICANT DIFFERENCES IN THE SPECIFICS. THE ECMWF AND ENSEMBLE
KEEP A FLATTER, MORE WESTWARD TRACK THROUGH TAU 72 AND MARK THE
WESTERN EDGE OF THE ENVELOPE. MEANWHILE THE UKMET AND NAVGEM TO A
LARGER DEGREE, HAVE A MORE POLEWARD TRACK WITH A SHARPER TURN
TOWARDS AUSTRALIA, AND THE REMAINDER OF THE GUIDANCE IS FAIRLY
TIGHTLY PACKED ABOUT THE CONSENSUS MEAN. THE JTWC TRACK IS HEDGED
TOWARDS THE ECMWF TRACKER AND LIES ON THE NORTHERN SIDE OF THE
TIGHTEST MODEL GROUPING THROUGH TAU 72, THEN IS SLOWER AND SLIGHTLY
INSIDE THE CONSENSUS THROUGH TAU 120. INTENSITY GUIDANCE IS IN
FAIRLY GOOD AGREEMENT WITH THE GFS, COAMPS-TC, COAMPS-TC ENSEMBLE,
HWRF AND RIPA ALL INDICATING RAPID INTENSIFICATION THROUGH AT LEAST
TAU 24. THE JTWC FORECAST FOLLOWS RIPA THROUGH TAU 24, BUT REMAINS
SLIGHTLY LOWER THAN HWRF WHICH PEAKS NEAR 85 KNOTS AT TAU 36.  

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