WDXS31 PGTW 232100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (LUANA)
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.0S 121.4E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 123 NM NORTH-NORTHWEST OF BROOME,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: SOUTHEASTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 17S (LUANA) IS CURRENTLY UNDERGOING RAPID
INTENSIFICATION (RI), JUST OFFSHORE OF THE DAMPIER PENINSULA.
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS VORTICAL
HOT TOWERS (VHT) ORGANIZING AROUND A COMPACT INNER CORE. A 231707Z
AMSR2 36GHZ MICROWAVE IMAGE DEPICTED A WELL-DEFINED MICROWAVE EYE
FEATURE, ALTHOUGH CORRELATION WITH COINCIDENT RADAR IMAGERY FROM 
BROOME SUGGESTS A POSSIBLE TILTED OR MULTI-VORTEX CONFIGURATION, WITH 
THE MID-LEVEL ROTATION DISPLACED WEST-SOUTHWEST OF THE LOW-LEVEL 
CENTER. HIGH RESOLUTION MESOSCALE MODEL FIELDS FROM HAFS-A SUPPORT 
SUCH A STRUCTURE. THE INITIAL POSITION IS ASSESSED WITH HIGH 
CONFIDENCE BASED ON ANALYSIS OF THE MICROWAVE AND RADAR IMAGERY. THE 
INITIAL INTENSITY OF 45 KNOTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED 
ON THE SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES NOTED BELOW, WHICH 
LIKELY UNDERESTIMATE THE CURRENT MAGNITUDE DUE TO THE ONGOING RI 
PHASE. ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR DEVELOPMENT, 
CHARACTERIZED BY LOW VERTICAL WIND SHEAR (VWS), VERY WARM SSTS AND 
MODERATE POLEWARD OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA

CURRENT STEERING MECHANISM: SOUTHERN EXTENSION OF A NEAR
EQUATORIAL RIDGE (NER).

AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T3.0 - 45 KTS
   APRF: T3.5 - 55 KTS
   CIMSS SATCON: 54 KTS AT 231717Z
   CIMSS ADT: 49 KTS AT 231730Z
   CIMSS AIDT: 43 KTS AT 231730Z
   CIMSS D-MINT: 34 KTS AT 231717Z
   CIMSS D-PRINT: 33 KTS AT 231800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: PROXIMITY TO LAND.

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 17S (LUANA) IS FORECAST TO TRACK
SOUTHEASTWARD ALONG THE SOUTHWESTERN PERIPHERY OF A DEEP-LAYER NER
POSITIONED FAR TO THE NORTHEAST OVER THE NEXT 12 TO 24 HOURS.
LANDFALL ALONG THE DAMPIER PENINSULA IS EXPECTED TO OCCUR WITHIN
THE NEXT 12 HOURS, AFTER WHICH THE SYSTEM WILL TRACK INLAND TO THE
EAST OF BROOME. AFTER TAU 24, STEERING INFLUENCE TRANSITIONS TO A 
SUBTROPICAL RIDGE BUILDING OVER CENTRAL AUSTRALIA, RECURVING THE 
REMNANTS OF TC 17S SOUTHWARD TOWARDS THE GREAT SANDY DESERT. DURING 
THE LIMITING DURATION REMAINING OVER WATER, TC 17S WILL TRAVERSE VERY 
WARM (30-31C) WATERS OFFSHORE THE DAMPIER PENINSULA, SUSTAINING 
INTENSIFICATION UNTIL COASTAL INTERFACE. THE DETERMINISTIC FORECAST 
CALLS FOR A PEAK INTENSITY OF 55 KNOTS COINCIDENT WITH LANDFALL; 
HOWEVER, STATISTICAL PROBABILITY SUGGESTS A PEAK CLOSER TO 60-65 
KNOTS, SHOULD THE TRACK DEFLECT FURTHER SOUTH THAN ANTICIPATED—A TREND 
INDICATED BY RECENT RADAR IMAGERY. FOLLOWING LANDFALL, THE SYSTEM WILL 
DECAY RAPIDLY DUE TO FRICTIONAL EFFECTS AND REDUCED MOISTURE FLUX, 
DISSIPATING AS A SIGNIFICANT TROPICAL CYCLONE BY TAU 36.

MODEL DISCUSSION: NUMERICAL WEATHER PREDICTION (NWP) TRACK GUIDANCE IS 
IN STRONG AGREEMENT, VALIDATING A SOUTHEASTWARD TRAJECTORY WITH 
LANDFALL WITHIN 12 HOURS, FOLLOWED BY A POLEWARD TURN BY THE END OF 
THE FORECAST PERIOD. THE JTWC FORECAST TRACK IS ALIGNED WITH THE 
MULTI-MODEL AND AI CONSENSUS MEANS, WITH HIGH CONFIDENCE. INDICATE A 
SOUTHERN DEVIATION OVER THE PAST TWO HOURS, WHICH MAY REPRESENT EITHER 
A TROCHOIDAL OSCILLATION OR A TRACK SHIFT. IF THE MORE SOUTHWARD 
TRAJECTORY PERSISTS, THE SYSTEM WILL REMAIN OVER WATER LONGER AND THE 
LANDFALL POSITION WILL SHIFT CLOSER TO BROOME, THOUGH AT THIS POINT IN 
TIME THIS IS A REMOTE POSSIBILITY. INTENSITY GUIDANCE IS ALSO IN GOOD 
AGREEMENT, WITH THE MAJORITY OF MODELS INDICATING INTENSIFICATION 
THROUGH TAU 12, UP TO A PEAK OF 55 KNOTS, FOLLOWED BY RAPID WEAKENING 
THROUGH THE END OF THE FORECAST PERIOD. THE POTENTIAL FOR A HIGHER 
PEAK INTENSITY IS DERIVED FROM ENVIRONMENTAL ANALYSIS AND IS NOT 
EXPLICITLY CAPTURED IN THE MODEL DATA. ADDITIONALLY, IF THE SYSTEM 
TRACKS MORE SOUTHWARD VICE SOUTHEASTWARD AND REMAINS OVER WATER
LONGER, THE PEAK INTENSITY WILL BE HIGHER AS A RESULT. THE JTWC
FORECAST IS PLACED SLIGHTLY ABOVE THE CONSENSUS MEAN WITH MEDIUM
CONFIDENCE.

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