WDXS31 PGTW 222100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27P (NARELLE) WARNING 
NR 020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.3S 130.1E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 118 NM SOUTH-SOUTHWEST OF DARWIN,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: OVER LAND

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
CONVECTIVE BANDS FLARING OVER THE NORTHERN FLANK OF A LLCC THAT HAS
BECOME BROADER AND SHALLOWER AS IT TRAVERSES OVER THE TOP END AND
APPROACHES THE JOSEPH BONAPARTE GULF. AN 1847Z GMI 89 GHZ IMAGE
CONFIRMS THE ENLARGED CENTER SURROUNDED BY A FRAGMENTED INNER CORE.
THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE
SHORTWAVE IR, MICROWAVE, AND RADAR IMAGERY, SUPPORTED BY SURFACE
WIND OBSERVATIONS. THE INITIAL INTENSITY IS SET AT 35 KTS WITH
MEDIUM CONFIDENCE WITH DARWIN AIRPORT REPORTING SUSTAINED WINDS OF
20-25 KTS AND A 1251Z ASCAT-B IMAGE REVEALING NEAR-GALES JUST
OFFSHORE. THOUGH CONDITIONS ARE UNFAVORABLE DUE THE FRICTIONAL AND
DRYING EFFECTS OF LAND, THE SYSTEM MAINTAINS ROBUST EQUATORWARD
OUTFLOW IN THE PRESENCE OF MODERATE EASTERLY VERTICAL WIND SHEAR
(VWS).

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A DEEP
SUBTROPICAL RIDGE OVER CENTRAL AUSTRALIA

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   CIMSS D-MINT: 43 KTS AT 1640Z
   CIMSS D-PRINT: 36 KTS AT 1830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 15-20 KTS
   SST: OVER LAND
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: TERRAIN INTERACTION

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: A STRONG, DEEP-LAYER RIDGE OVER CENTRAL
AUSTRALIA WILL STEER TC 27P IN A WEST-SOUTHWESTWARD DIRECTION OVER
THE TOP END. THE SYSTEM WILL MAINTAIN THIS MOTION AS IT FULLY
EMERGES OUT OVER THE SOUTH INDIAN OCEAN AFTER TAU 36, DRIVEN BY AN
EXTENSION OF THE RIDGE ORIENTED TOWARDS THE NORTHWEST. ONCE TC 27P
NEARS THE NORTHWESTERN TIP OF AUSTRALIA, THE EXTENSION OF THE RIDGE
WILL WEAKEN, WHICH WILL ENABLE A MORE POLEWARD MOTION TOWARDS
LEARMONTH. BY TAU 120, THE SYSTEM WILL BEGIN TO ROUND THE WESTERN
PERIPHERY OF THE RIDGE AND ACCELERATE POLEWARD. WHILE THE FORECAST
DEPICTS A STEADY 35 KT INTENSITY THROUGH TAU 24, A BRIEF PERIOD OF
SLIGHT INTENSIFICATION IS POSSIBLE WHILE TC 27P TRAVERSES OVER THE
WARM WATERS OF THE JOSEPH BONAPARTE GULF. ALL BETS ARE OFF ONCE IT
TRACKS OVER THE 30 DEGREE CELSIUS WATERS OF THE SOUTH INDIAN OCEAN.
SLOW TO STEADY INTENSIFICATION WILL COMMENCE AFTER EMERGENCE OVER
THE INDIAN OCEAN, BUT THE BROADER NATURE OF THE CIRCULATION SHOULD
INITIALLY LIMIT THE RATE OF INTENSIFICATION AT TAU 36 AND 48. THE
JTWC FORECAST REFLECTS A SHARPER RATE BY TAU 72 AS THE INNER CORE
BECOMES ESTABLISHED WITHIN A HIGHLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY LOW VWS, MODERATE MID-LEVEL MOISTURE, AND VERY
WARM SEA SURFACE TEMPERATURES. THE SYSTEM WILL UNDERGO RAPID
INTENSIFICATION (RI), PEAKING AT 105 KTS AS IT MAKES A VERY CLOSE
APPROACH TO LEARMONTH AT TAU 96. THE RATE OF INTENSIFICATION WILL
LARGELY DEPEND ON THE STRUCTURE AND SIZE OF THE LOW-LEVEL
CIRCULATION WHEN IT ENCOUNTERS THE INDIAN OCEAN. AFTER IT ROUNDS
THE COAST OF EXMOUTH TOWARDS TAU 120, IT WILL FEEL THE EFFECTS OF
COOLER WATERS AND BEGIN TO RAPIDLY WEAKEN.

MODEL DISCUSSION: MODELS REMAIN IN TIGHT AGREEMENT ON THE TRACK OF
TC 27P FOR THE FIRST 36 HOURS. MODEL SPREAD INCREASES THEREAFTER
DUE TO DIFFERENCES IN THE STRENGTH OF THE SUBTROPICAL RIDGE
EXTENSION AND IN THE PLACEMENT OF THE SUBTROPICAL RIDGE ITSELF. THE
DETERMINISTIC AND MESOSCALE MODELS REMAIN POSITIONED ON THE
NORTHERN AND WESTERN SIDE OF THE ENVELOPE, WHILE THE AI GUIDANCE
DRAGS THE SYSTEM INLAND AS FAR EASTWARD AS DAMPIER. THE INITIAL
TRACK IS PLACED WITH HIGH CONFIDENCE ON THE MULTI-MODEL CONSENSUS
THROUGH TAU 72. THE TRACK IS THEN HEDGED SOUTH OF THE CONSENSUS AND
CLOSER TO THE AI MODELS WITH LOWER CONFIDENCE AFTER TAU 72, WITH
THE CROSS-TRACK SPREAD INCREASING TO 300 NM BY TAU 96. MODELS AGREE
IN MAINTAINING THE MINIMAL TROPICAL STORM INTENSITY THROUGH TAU 24.
MODELS DEPICT A SHARP INCREASE IN INTENSITY, WITH THE COAMPS-TC
PEAKING AT 90 KTS AND THE HAFS-A PARENT TRACKER REACHING 100 KTS,
WHILE THE GOOGLE DEEPMIND PEAKS AT 105 KTS. THE INTENSIFY FORECAST
IS HEDGED ABOVE MOST OF THE GUIDANCE, OWING TO THE STRONG POTENTIAL
FOR RI WITHIN A HIGHLY FAVORABLE ENVIRONMENT. FOLLOWING THE PEAK,
ALL MODELS SHOW A STEADY WEAKENING TREND AT THE END OF THE FORECAST
PERIOD.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//

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