WDPS31 PGTW 311500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04P (SETH) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.8S 155.3E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 427 NM NORTH-NORTHEAST OF BRISBANE,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 17 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TC 04P (SETH) HAS RAPIDLY TRANSITIONED FROM A HYBRID-TYPE SYSTEM
WITH PRIMARILY SUBTROPICAL CHARACTERISTICS, TO A WELL-DEFINED
TROPICAL CYCLONE. ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY DEPICTED THE RAPID DEVELOPMENT OF A SYMMETRIC BALL OF DEEP
CONVECTION NEAR THE ASSESSED LOW LEVEL CIRCULATION CENTER (LLCC),
WHICH DETACHED FROM THE COMMA-SHAPED BAND OF CONVECTIVE ACTIVITY
BETWEEN THE 0600Z AND 1000Z HOURS. A 310910Z GPM COLOR 89GHZ
COMPOSITE MICROWAVE IMAGE INDICATED THE LLCC HAD TUCKED UNDER THE
AREA OF SOMEWHAT DISORGANIZED CONVECTION. AT THAT TIME,
NORTHWESTERLY SHEAR WAS BEARING DOWN ON THE SYSTEM AND ELONGATING
THE CONVECTIVE AREA, BUT SUBSEQUENT IMAGERY SUGGESTS THE SHEAR HAS
RELAXED, AND ALLOWED THE CONVECTIVE AREA TO CONSOLIDATE. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON A
311146Z ASCAT-C BULLSEYE PASS. THE BULLSEYE PASS ALSO PROVIDED HIGH
CONFIDENCE TO THE INITIAL INTENSITY, WHICH IS SET SLIGHTLY HIGHER
THAN THE AGENCY DVORAK INTENSITY ESTIMATES. SURFACE WIND
OBSERVATIONS FROM FREDERICK REEF WERE AS HIGH AS 63 KNOTS (10-MIN)
AT 0900Z AS THE SYSTEM PASSED THE STATION, BUT THE HEIGHT OF THE
OBSERVATION WAS 30-METERS, WHILE THE SURFACE PRESSURE READING OF
989MB WOULD SUGGEST A 45-50 KNOT INTENSITY. ENVIRONMENTAL
CONDITIONS HAVE IMPROVED OVER THE PAST SIX TO TWELVE HOURS, BUT
REMAIN ONLY MARGINALLY FAVORABLE. SSTS ARE WARM AND POLEWARD
OUTFLOW IS GOOD, AND FOR THE TIME BEING SHEAR IS IS RELATIVELY LOW
IN THE IMMEDIATE VICINITY OF THE SYSTEM. BUT DRY AIR IS FIRMLY
ENTRENCHED TO THE WEST AND NORTH AND IS OFFSETTING THE OTHERWISE
FAVORABLE CONDITIONS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T2.5 - 35 KTS
   ABRF: T3.5 - 55 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: MID-LEVEL DRY AIR TO THE WEST AND NORTH. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: AS MENTIONED EARLIER, TC 04P (SETH) HAS
RAPIDLY TRANSITIONED FROM A HYBRID SUBTROPICAL LOW TO A
FULL-FLEDGED TROPICAL CYCLONE OVER THE PAST SIX TO TWELVE HOURS.
THE FORECAST HOWEVER CALLS FOR A TRANSITION BACK TO A SUBTROPICAL
CYCLONE WITHIN THE NEXT 48 HOURS. THE SYSTEM IS FORECAST TO SLOW
DOWN AS IT TRACKS SOUTHEAST, THEN TURNS SOUTH BY TAU 36 AS THE
STEERING MECHANISM SHIFTS TO A STRONG SUBTROPICAL RIDGE (STR) TO
THE SOUTHEAST. THE SYSTEM IS EXPECTED TO MAINTAIN ITS CURRENT
INTENSITY THROUGH TAU 24, THEN WEAKEN STEADILY THROUGH THE
REMAINDER OF THE FORECAST PERIOD UNDER INCREASING SHEAR, INCREASING
MID-LEVEL DRYNESS AND DECREASING SSTS. TC 04P WILL MOVE UNDER A
500MB TROUGH BEGINNING AROUND TAU 12, THEN TRANSITION TO A
SUBTROPICAL STORM NO LATER THAN TAU 48, AS IT BECOMES VERTICALLY
STACKED UNDER A 500MB COLD-CORE LOW AND CROSSES INTO WATERS COOLER
THAN 26C. WHILE THE FORECAST ONLY COVERS THE 48 HOUR PERIOD THROUGH
SUBTROPICAL TRANSITION, THE REMNANT GALE-FORCE SUBTROPICAL LOW WILL
LIKELY MEANDER SLOWLY WESTWARD THROUGH TAU 72 WHILE CONTINUING TO
STEADILY WEAKEN. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN POOR AGREEMENT, WITH
SIGNIFICANT SPREAD AFTER TAU 36. THERE ARE TWO DISTINCT GROUPINGS
IN THE TRACK GUIDANCE, WITH THE NORTHERN GROUP OF UKMET
ENSEMBLE, GALWEM, ECMWF AND THE ECMWF ENSEMBLE TURNING THE
CIRCULATION SHARPLY WEST TOWARDS THE COASTLINE AFTER TAU 36. THE
SOUTHERN GROUPING CONSISTING OF GFS, GFS ENSEMBLE AND HWRF TRACK
THE SYSTEM STEADILY SOUTHWARD PARALLEL TO THE COAST THROUGH THE
FORECAST PERIOD AND BEYOND. THE JTWC FORECAST LIES ON THE EAST SIDE
OF THE ENVELOPE THROUGH TAU 36, THEN TURNS WEST ON THE SOUTHERN
SIDE OF THE NORTHERN GROUPING THROUGH THE END OF THE FORECAST
PERIOD. CONSIDERING THAT THE MAJORITY OF THE MODEL SPREAD OCCURS
AFTER TAU 36, THERE IS MEDIUM CONFIDENCE IN THE JTWC FORECAST.
INTENSITY GUIDANCE IS SIMILARLY MIXED, WITH THE SHIPS GUIDANCE
BEING THE MOST AGGRESSIVE, PEAKING BETWEEN 65 AND 75 KNOTS, WHILE
THE HWRF IS THE LEAST AGGRESSIVE, PEAKING AT THE ANALYSIS OF 50
KNOTS AND DECREASING STEADILY THEREAFTER. THE JTWC FORECAST MOST
CLOSELY FOLLOWS THE HWRF SOLUTION, WITH MEDIUM CONFIDENCE. 

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