WDPS31 PGTW 180300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 19P (MEGAN) WARNING NR 
011//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.6S 137.0E
   INITIAL INTENSITY: 85 KTS
   GEOGRAPHIC REFERENCE: 401 NM EAST-SOUTHEAST OF DARWIN, AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 24 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 19P (MEGAN) IS IN THE PROCESS OF MAKING
LANDFALL OVER NORTH ISLAND AND MOVING DOWN OVER CENTRE ISLAND AT
ANALYSIS TIME. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI)
DEPICTS A WEAKENING CENTRAL DENSE OVERCAST (CDO) FEATURE, WITH
WARMING CLOUD TOPS, WHICH IS BEING ERODED ON THE EAST SIDE DUE TO
PERSISTENT AND STRENGTHENING EASTERLY SHEAR. THE INITIAL POSITION
WAS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY FIX
POSITIONS, ANIMATED RADAR DATA FROM THE MORNINGTON ISLAND RADAR,
AND EXTRAPOLATION OF WELL-DEFINED EYE IN A 172027Z RCM-2 SAR
NORMALIZED RADAR CROSS SECTION (NRCS) IMAGE. THE INITIAL INTENSITY
IS SET AT 85 KNOTS WITH MEDIUM CONFIDENCE, BASED ON AN AVERAGE OF
THE SUBJECTIVE AGENCY FIXES, AND IN LIGHT OF THE RAPIDLY DECREASING
ADT, AIDT AND DPRINT ESTIMATES AS WELL AS THE OVERALL DEGRADATION
IN THE STRUCTURE. SURFACE OBSERVATIONS FROM CENTRE ISLAND SHOW A
PEAK SUSTAINED 10MIN WIND OF 52 KTS, WITH GUSTS AS HIGH AS 68 KTS,
WITH THE LOWEST PRESSURE READING THUS FAR OF 981MB. THE SAR IMAGERY
SHOWED THE HIGHEST WINDS WERE LOCATED TO THE NORTH-NORTHEAST OF THE
LOW LEVEL CIRCULATION CENTER (LLCC), AND THUS WINDS AT CENTRE
ISLAND LIKELY WILL BE AT THEIR HIGHEST AFTER THE STORM PASSES.
ANALYSIS REVEALS A MARGINALLY FAVORABLE NOW THAT THE SYSTEM IS
MOVING INLAND. EASTERLY SHEAR IS QUICKLY INCREASING ALONG WITH THE
ENTRAINMENT OF DRY AIR OFF THE CONTINENT, LEADING TO A RAPIDLY
DETERIORATING ENVIRONMENT.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A MID-LEVEL NEAR
EQUATORIAL RIDGE (NER) TO THE NORTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   ADRM: T4.5 - 77 KTS
   CIMSS ADT: 67 KTS AT 172330Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 19P WILL MAKE LANDFALL ON THE MAINLAND
COAST OF AUSTRALIA WITHIN THE NEXT COUPLE OF HOURS, AFTER PASSING
JUST EAST OR DIRECTLY OVER CENTRE ISLAND THEN MOVING ASHORE IN THE
MCARTHUR RIVER DELTA REGION. THE SYSTEM WILL CONTINUE INLAND,
PASSING CLOSE ABOARD TO BORROLOOLA, BEFORE QUICKLY MOVING FURTHER
INLAND ALONG THE NORTHERN SIDE OF A BUILDING RIDGE TO THE SOUTH.
ADDITIONAL ANALYSIS OF THE 172027Z SAR PASS SUGGESTS THAT TC 19P
REACHED 100 KNOTS INTENSITY AS EARLY AS 1200Z AND THEN MAINTAINED
THAT INTENSITY FOR AT LEAST EIGHT HOURS AND THE 1200Z AND 1800Z
INTENSITY ESTIMATES HAVE BEEN UPDATED ACCORDINGLY. HOWEVER, THE
RAPID INCREASE IN NORTHEASTERLY SHEAR, AND INGESTION OF RELATIVELY
DRY LOW-LEVEL AIR FROM THE CONTINENT INTO THE CORE OF THE SYSTEM
LED TO A FAIRLY QUICK DROP-OFF IN INTENSITY UP TO THE CURRENT
ANALYSIS TIME. SHEAR AND DRY AIR WILL ONLY GET WORSE MOVING FORWARD
AND THE SYSTEM IS FORECAST TO UNDERGO RAPID WEAKENING AS IT MOVES
FURTHER INLAND. DISSIPATION BELOW THE 35 KNOT WARNING THRESHOLD IS
EXPECTED NO LATER THAN TAU 48. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN VERY GOOD
AGREEMENT ON THE TRACK SCENARIO, WITH LITTLE IN THE WAY OF
CROSS-TRACK SPREAD THROUGH THE FORECAST PERIOD. ALONG-TRACK SPREAD
HAS ALSO LESSENED SINCE THE PREVIOUS RUN. THE JTWC FORECAST LIES
CLOSE TO THE MULTI-MODEL CONSENSUS WITH HIGH CONFIDENCE. INTENSITY
GUIDANCE IS ALSO IN STRONG AGREEMENT, WITH ALL GUIDANCE SHOWING
RAPID WEAKENING AND DISSIPATION BY TAU 48. 

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