WDPS31 PGTW 172100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 19P (MEGAN) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.4S 137.2E
   INITIAL INTENSITY: 90 KTS
   GEOGRAPHIC REFERENCE: 406 NM EAST-SOUTHEAST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 27 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 19P (MEGAN) HAS CONTINUED TO INTENSIFY
OVERNIGHT, AS IT APPROACHES THE SOUTHWESTERN COAST OF THE GULF OF
CARPENTARIA. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY, A
SERIES OF MICROWAVE IMAGES AND A 170919Z RCM-3 SAR PASS SHOWED THAT
TC 19P HAS REMAINED QUASI-STATIONARY OVER THE PAST 12 HOURS,
PERFORMING A CLOCKWISE PIROUETTE, AS IT SLOWLY MEANDERED IN A WEAK
STEERING PATTERN. A 171642Z ATMS 88.2GHZ MICROWAVE IMAGE REVEALED
THAT THE SYSTEM HAS NOW STARTED TO MOVE SOUTH-SOUTHWEST AT ABOUT 5
KNOTS AS THE NER TO THE NORTHEAST HAS STRENGTHENED A BIT.
ADDITIONALLY, THE ATMS IMAGERY SHOWED A WEAK MICROWAVE EYE, WITH
DEEP CONVECTION PRIMARILY LIMITED TO THE WESTERN HEMISPHERE OF THE
CORE. THE MICROWAVE EYE FEATURE LINED UP WELL WITH BOTH THE AGENCY
FIX POSITIONS AND THE EYE FEATURE SEEN IN ANIMATED RADAR DATA FROM
MORNINGTON ISLAND, PROVIDING HIGH CONFIDENCE TO THE INITIAL
POSITION. IN-DEPTH ANALYSIS OF THE EARLIER SAR PASS, SUPPORTS THE
1200Z INTENSITY ESTIMATE OF 95 KNOTS, EVEN THOUGH THE SAR INDICATED
WINDS AS HIGH AS 123 KNOTS IN THE NORTHERN PORTION OF THE
CIRCULATION. THESE HIGHER WINDS ARE DEEMED AS NOT REPRESENTATIVE
AND LIKELY DUE TO TRANSIENT MESOSCALE EFFECTS. REGARDLESS, THE
INITIAL INTENSITY AT 1800Z IS ASSESSED WITH HIGH CONFIDENCE BASED
ON THE CONGRUENCE OF THE BULK OF OBJECTIVE AND SUBJECTIVE FIX
ESTIMATES BELOW. WHILE THE OVERALL ENVIRONMENT REMAINS FAVORABLE,
DEEP-LAYER AND MID-LEVEL SHEAR HAVE STARTED TO PICK UP SHARPLY OVER
THE PAST 6 TO 12 HOURS AS DEPICTED IN THE MODEL SOUNDING DATA.
ANIMATED WATER VAPOR IMAGERY ALSO SUPPORTS AN INCREASED
NORTHEASTERLY SHEAR VECTOR, WITH THE PULSING CONVECTION FAILING TO
PUSH OUTFLOW AS FAR EASTWARD AS WAS SEEN YESTERDAY. OTHER
ENVIRONMENTAL PARAMETERS REMAIN FAVORABLE.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A MID-LEVEL
NER TO THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.0 - 90 KTS
   ADRM: T4.5 - 77 KTS
   CIMSS SATCON: 91 KTS AT 171800Z
   CIMSS ADT: 82 KTS AT 171800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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 19P IS EXPECTED TO CONTINUE TRACK
SOUTH-SOUTHWEST ALONG THE WESTERN SID OF THE NER CENTERED TO THE
NORTHEAST. TRACK SPEEDS ARE EXPECTED TO ROUGHLY MAINTAIN THE PACE
OF ADVANCE, THOUGH SOME SLOWING CANNOT BE RULED OUT AS THE LLCC
TRACKS OVER THE SIR EDWARD PELLEW GROUP. LANDFALL ON THE MAINLAND
COAST IS EXPECTED NEAR TAU 12. AFTER LANDFALL, THE SYSTEM WILL
SLOWLY TRACK OVER TO A SOUTHWESTWARD THEN WESTWARD TRACK AS IT
COMES UNDER THE STEERING INFLUENCE OF A BUILDING RIDGE TO THE
SOUTH. TC 19P LIKELY PEAKED OUT AT 95-100 KNOTS AROUND 0900Z, PER
THE SAR IMAGERY, AND IS NOW STARTING TO WEAKEN SLOWLY, AS IT STARTS
TO FEEL THE EFFECTS OF THE STEADILY INCREASING NORTHEASTERLY SHEAR,
PARTICULARLY IN THE SUB-OUTFLOW LEVELS. HOWEVER, THE WIND FIELD
WILL NOT REACT IMMEDIATELY TO THE DESTRUCTION OF THE VORTEX, THUS
THE SYSTEM IS FORECAST TO MAINTAIN 90 KNOTS INTENSITY UP THROUGH
LANDFALL. ONCE ASHORE HOWEVER, FRICTIONAL EFFECTS, DRY AIR
ENTRAINMENT AND A FURTHER INCREASE IN SHEAR WILL COMBINE TO RAPIDLY
WEAKEN TC 19P. FULL DISSIPATION IS ANTICIPATED NO LATER THAN TAU 72
AND POTENTIALLY AS EARLY AS TAU 48.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN VERY GOOD
AGREEMENT THROUGH THE DURATION OF THE FORECAST, WITH MINIMAL SPREAD
IN THE CONSENSUS MEMBERS. THE JTWC FORECAST IS PLACED ALONG THE
NORTHERN EDGE OF THE GUIDANCE PACKAGE WITH HIGH CONFIDENCE.
INTENSITY GUIDANCE IS ALSO IN GOOD AGREEMENT, WITH NEARLY ALL OF
THE CONSENSUS MEMBERS SHOWING RAPID WEAKENING FROM THE TAU 00,
REACHING 30 KNOTS BETWEEN TAU 48 AND TAU 72. THE ONLY EXCEPTION IS
THE GFS VERSION OF SHIPS, WHICH SHOWS THE SYSTEM MAINTAINING
CURRENT INTENSITY THROUGH TAU 12, THEN FOLLOWING THE REST OF THE
PACK THROUGH TAU 72. THE JTWC FORECAST FOLLOWS THE SHIPS GUIDANCE
WITH MEDIUM CONFIDENCE. 

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