WDPS31 PGTW 181500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27P (NARELLE) WARNING 
NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 13.0S 150.6E
   INITIAL INTENSITY: 90 KTS
   GEOGRAPHIC REFERENCE: 364 NM NORTHEAST OF CAIRNS, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TC NARELLE IS ANALYZED TO HAVE RAPIDLY INTENSIFIED OVER THE PAST 24
HOURS FROM AN INTENSITY OF 60 KTS TO THE CURRENT INTENSITY OF 90
KTS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
HIGHLY COMPACT CORE OF DEEP CONVECTION WITH CLOUD TOPS MEASURING
COLDER THAN -80 C ENTIRELY SURROUNDING THE LOW-LEVEL CIRCULATION
CENTER (LLCC). A PINHOLE EYE FEATURE HAS BEEN FLUCTUATING SINCE
181000Z. A 180621Z F18 SSMIS 91 GHZ MICROWAVE IMAGE SHOWS THAT THE
SYSTEM HAS BECOME VERTICALLY STACKED AND AXISYMMETRIC. HOWEVER,
THERE HAS BEEN A LACK OF MICROWAVE IMAGERY OVER THE PAST 6 HOURS
LENDING UNCERTAINTY IN THE CURRENT INNER STRUCTURE OF THE VORTEX.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
SHALLOW PINHOLE EYE FEATURE IN ANIMATED EIR IMAGERY. THE INITIAL
INTENSITY OF 90 KTS IS ASSESSED WITH LOW CONFIDENCE BASED PRIMARILY
ON THE AGENCY DVORAK FIXES RANGING FROM T4.5-T5.5. CIMSS INTENSITY
ESTIMATES COULD BE UNDERESTIMATING THE CURRENT INTENSITY DUE TO THE
COMPACT NATURE OF THE SYSTEM. THE ENVIRONMENT IS HIGHLY FAVORABLE,
CHARACTERIZED BY MODERATE RADIAL OUTFLOW ALOFT, LOW (10-15 KTS)
VERTICAL WIND SHEAR (VWS), AND WARM (29-30 C) SEA SURFACE
TEMPERATURES.

INITIAL WIND RADII BASIS: PERSISTENCE WITH SLIGHT ADJUSTMENTS BASED
ON THE OBJECTIVE BEST TRACK.

CURRENT STEERING MECHANISM: DEEP-LAYER SUBTROPICAL RIDGE (STR)
POSITIONED OVER THE NORTHEASTERN COAST OF AUSTRALIA.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   KNES: T5.5 - 102 KTS
   ABRF: T5.0 - 90 KTS
   CIMSS ADT: 77 KTS AT 181200Z
   CIMSS AIDT: 79 KTS AT 181200Z
   CIMSS D-PRINT: 83 KTS AT 181230Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 27P (NARELLE) HAS
TRACKED WEST-SOUTHWESTWARD UNDER THE INFLUENCE OF THE STR TO THE
SOUTH OVER THE PAST 12 HOURS. A SHORTWAVE TROUGH TRANSITING
EASTWARD TO THE SOUTH OF THE STR CONTINUES TO WEAKEN THE CURRENT
STEERING RIDGE. THE SYSTEM IS EXPECTED TO CONTINUE TRACKING
WEST-SOUTHWESTWARD OVER THE NEXT 12 HOURS. AFTER TAU 12 A STR
CENTERED OVER WESTERN AUSTRALIA WILL BECOME THE PRIMARY STEERING
MECHANISM SHIFTING THE TRAJECTORY TO A NEARLY STRAIGHT WESTWARD
TRACK. INITIAL LANDFALL IS FORECAST TO OCCUR JUST BEFORE TAU 36
ALONG THE CAPE YORK PENINSULA. TC NARELLE WILL THEN EMERGE OVER THE
GULF OF CARPENTARIA AROUND TAU 48. THE SYSTEM WILL CONTINUE ON A
WESTWARD TRACK WITH A SECOND LANDFALL FORECAST TO OCCUR NEAR TAU 72
IN THE VICINITY OF GROOTE EYLANDT. THE SYSTEM WILL THEN TRACK
ACROSS THE TOP END, SOUTH OF DARWIN, THROUGH THE REMAINDER OF THE
FORECAST PERIOD. REGARDING INTENSITY, TC NARELLE IS FORECAST TO
CONTINUE RAPIDLY INTENSIFYING OVER THE NEXT 24 HOURS TO A PEAK
INTENSITY AROUND 115-120 KTS. EAST-NORTHEASTERLY VWS IS EXPECTED TO
SLIGHTLY INCREASE TO 15-20 KTS AS THE SYSTEM APPROACHES THE CAPE
YORK PENINSULA PUTTING A CAP ON THE INTENSIFICATION TREND. TERRAIN
INTERACTION ALONG THE PENINSULA WILL CAUSE TC NARELLE TO RAPIDLY
WEAKEN AFTER LANDFALL. THE SYSTEM WILL RECOVER WITHIN THE GULF OF
CARPENTARIA OVER WARM SEA SURFACE TEMPERATURES, WITH A SECOND PEAK
INTENSITY EXPECTED TO OCCUR AROUND TAU 60 THAT IS NOT DEPICTED IN
THE WARNING DUE TO TEMPORAL RESTRICTIONS. ONCE THE SYSTEM BEGINS TO
INTERACT WITH THE EASTERN COAST OF THE TOP END, ANOTHER WEAKENING
TREND WILL ENSUE. TERRAIN EFFECTS WILL RESULT IN THE SYSTEM
WEAKENING TO 35 KTS BY THE END OF THE FORECAST PERIOD.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE REMAINS IN VERY
GOOD AGREEMENT, WITH THE EXCEPTION OF NAVGEM, WHICH CONTINUES TO
TRACK THE VORTEX MUCH FARTHER SOUTH THAN THE REMAINDER OF GUIDANCE.
DISCOUNTING NAVGEM, THERE IS A MERE 30 NM CROSS-TRACK SPREAD AT TAU
72, WITH AN ALONG-TRACK SPREAD OF 60 NM. THE CROSS-TRACK SPREAD
ONLY SLIGHTLY INCREASES TO 75 NM AT TAU 120, HOWEVER, ALONG-TRACK
SPREAD INCREASES TO 170 NM WITH GALWEM AS THE SLOWEST MODEL AND
ECMWF AS THE FASTEST. THE JTWC TRACK IS PLACED CLOSE TO THE
MODIFIED CONSENSUS WITH HIGH CONFIDENCE THROUGH TAU 72 AND MEDIUM
CONFIDENCE THEREAFTER. INTENSITY GUIDANCE CONTINUES TO DEPICT
FURTHER DEVELOPMENT OVER THE NEXT 24 HOURS WITH PEAK INTENSITIES
RANGING FROM 100-130 KTS AND NEARLY EVERY AVAILABLE RAPID
INTENSIFICATION (RI) AID HAS TRIGGERED. HAFS-A AND HWRF BOTH DEPICT
A PEAK INTENSITY OF 105 KTS WHICH IS SLIGHTLY UNDER THE RI AID
CONSENSUS (120 KTS). AFTER TAU 24, MODELS AGREE ON WEAKENING
THROUGH TAU 48 FOLLOWED BY STEADY INTENSITY THROUGH TAUY 72 BEFORE
FURTHER WEAKENING. OF NOTE, COAMPS-TC (GFS BASED) HAS A
SIGNIFICANTLY SLOWER TRACK SPEED THROUGH THE GULF OF CARPENTARIA
AND IS THEREFORE SUGGESTING A MUCH HIGHER REINTENSIFICATION EVENT.
THE JTWC INTENSITY FORECAST IS PLACED CLOSER TO THE RI AID
CONSENSUS THROUGH TAU 24, THEN CLOSE TO HAFS-A THROUGH THE
REMAINDER OF THE FORECAST PERIOD WITH OVERALL MEDIUM CONFIDENCE.

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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