WDXS31 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 27S (TWENTYSEVEN) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.8S 109.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 434 NM NORTHWEST OF LEARMONTH, AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
CYCLONE 27S WITH A WELL-DEFINED, PARTIALLY EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC) AND DEEP CONVECTIVE BURSTS BEGINNING TO
FORM OVER THE CENTER. A 250616Z AMSR2 89 GHZ MICROWAVE IMAGE SHOWS
THE DEEP CONVECTION OFFSET SLIGHTLY TO THE WEST OF THE LLCC.
ENVIRONMENTAL ANALYSIS INDICATES THAT 27S IS IN A MARGINALLY
FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE POLEWARD OUTFLOW
ALOFT, MODERATE (15-20 KTS) VERTICAL WIND SHEAR, DRY AIR WITHIN THE
NORTHERN PERIPHERY, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE
EXPOSED LOW-LEVEL CLOUDS IN THE ANIMATED MSI. THE INITIAL INTENSITY
OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A 250225Z
METOP-C ASCAT IMAGE SHOWING A SWATH OF 35 KNOT WINDS WITHIN THE
SOUTHERN SEMICIRCLE. THE AGENCY DVORAK AND OBJECTIVE INTENSITY
ESTIMATES FURTHER SUPPORTED THE INITIAL INTENSITY ASSESSMENT.

INITIAL WIND RADII BASIS: 250225Z METOP-C SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   APRF: T2.5 - 35 KTS
   CIMSS SATCON: 40 KTS AT 250328Z
   CIMSS ADT: 35 KTS AT 250610Z
   CIMSS AIDT: 35 KTS AT 250610Z
   CIMSS DMINT: 35 KTS AT 250616Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY AIR WITHIN THE NORTHERN PERIPHERY

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: TC 27S IS FORECAST TO CONTINUE TRACKING
WESTWARD, ALONG THE NORTHERN PERIPHERY OF THE STR, THROUGH TAU 48.
NEAR TAU 48, A LONGWAVE TROUGH IS EXPECTED TO BREAK DOWN THE STR,
BUT A SECOND STR WILL FOLLOW AND BUILD TO THE SOUTH OF THE SYSTEM.
THIS WILL ALLOW 27S TO TRACK FURTHER WESTWARD. THE SYSTEM WILL THEN
BEGIN TO ROUND THE NORTHWESTERN PERIPHERY OF THE STR NEAR TAU 120.
REGARDING INTENSITY, TC 27S IS FORECAST TO GENERALLY MAINTAIN
INTENSITY THROUGH TAU 48 DUE TO THE PERSISTENT NORTHEASTERLY
VERTICAL WIND SHEAR. AFTER TAU 48, SHEAR IS EXPECTED TO DROP TO
BELOW 10 KTS, ALLOWING FOR THE SYSTEM TO INTENSIFY TO AROUND 65 KTS
AT TAU 120. HOWEVER, DRY AIR AND SLIGHTLY COOLING SEA SURFACE
TEMPERATURES ARE FORECAST TO HINDER SIGNIFICANT DEVELOPMENT.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN MODERATE AGREEMENT
REGARDING THE TRACK OF 27S. CROSS-TRACK SPREAD AT TAU 72 IS 95 NM,
BUT ALONG-TRACK SPREAD IS HIGHER, NEAR 225 NM. GFS IS THE SLOWEST
MODEL, WHILE ECMWF IS THE FASTEST. THE HIGH ALONG-TRACK SPREAD
PERSISTS THROUGHOUT THE FORECAST PERIOD, LENDING OVERALL MEDIUM
CONFIDENCE TO THE JTWC TRACK FORECAST. INTENSITY GUIDANCE
DISAGREES, PARTICULARLY AFTER TAU 72, WITH A 70 KNOT SPREAD IN
GUIDANCE AT TAU 120. GFS KEEPS THE VORTEX MUCH WEAKER, NEVER
REACHING ABOVE 40 KTS, WHILE HAFS-A SPIKES THE INTENSITY AFTER TAU
84, TO AROUND 100 KTS AT TAU 120. THE ECMWF ENSEMBLE HAS MANY
MEMBERS REACHING OVER 60 KTS. THE JTWC INTENSITY FORECAST IS PLACED
CLOSE TO THE MULTI-MODEL CONSENSUS THROUGH TAU 72 AND SLIGHTLY
HIGHER NEAR THE END OF THE FORECAST WITHOUT BITING OFF ON THE
HAFS-A SUGGESTION DUE TO THE EXPECTED DRY AIR ACTING ON THE VORTEX
AND LACK OF A STRONG OUTFLOW CHANNEL.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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