WDPS31 PGTW 312100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04P (SETH) WARNING NR 
002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.4S 156.6E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 421 NM NORTH-NORTHEAST OF BRISBANE,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS THE
CENTRAL CONVECTION HAS BEGUN TO WARM UP AND ELONGATE SOUTHEASTWARD;
HOWEVER, FEEDER BAND WRAP IMPROVED SLIGHTLY RESULTING IN SLIGHT
INTENSIFICATION. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE ON THE EXTRAPOLATED INTERSECTION OF THE FEEDER BANDS AND
COLOCATED WITH THE WARMEST CENTRAL PIXELS IN THE 311800Z IRBD
IMAGE. THE INITIAL INTENSITY OF 55KTS IS BASED ON THE PGTW DVORAK
ESTIMATE OF T3.5 AND REFLECTS THE THE SLIGHT INTENSIFICATION.
ANALYSIS INDICATES A MARGINALLY FAVORABLE ENVIRONMENT WITH WARM
SSTS AND STRONG POLEWARD OUTFLOW OFFSET BY MODERATE TO STRONG
VERTICAL WIND SHEAR AND DRY AIR INTRUSION IN THE MID LEVELS. THE
CYCLONE IS TRACKING ALONG THE SOUTHWEST PERIPHERY OF THE NER TO THE
NORTHEAST.

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: T3.5 - 55 KTS
   CIMSS ADT: 51 KTS AT 311430Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: MID-LEVEL DRY AIR INTRUSION FROM THE WEST.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 SETH WILL CONTINUE ON ITS CURRENT TRACK
UNDER THE NER. AFTER TAU 12, A SUBTROPICAL RIDGE TO THE SOUTHEAST
WILL ASSUME STEERING AND DRIVE THE SYSTEM MORE SOUTHWARD. THE
ENVIRONMENT WILL BEGIN TO DETERIORATE AS VWS INCREASES, SSTS
DECREASE, AND THE COLD DRY AIR INTRUSION PERSISTS RESULTING IN A
GRADUAL WEAKENING. CONCURRENTLY, BY TAU 24, THE CYCLONE WILL BEGIN
TO TRANSITION BACK TO A SUBTROPICAL SYSTEM AND BY TAU 36, WILL
BECOME FULLY SUBTROPICAL WITH AN EXPANSIVE WIND FIELD.  

MODEL DISCUSSION: NUMERICAL MODELS ARE IN FAIRLY GOOD AGREEMENT,
LENDING MEDIUM CONFIDENCE TO THE JTWC TACK AND INTENSITY FORECAST. 

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