WDPS33 PGTW 261500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21P (SERU)       
WARNING NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.8S 170.6E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 227 NM SOUTHEAST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 26 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
AS INDICATED IN ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY,
TROPICAL CYCLONE (TC) 21P CONTINUES TO STRUGGLE UNDER PERSISTENT
HIGH VERTICAL WIND SHEAR (VWS), WHILE MAINTAINING AN ASYMMETRIC
CORE OF FLARING DEEP CONVECTION OVER THE LOW-LEVEL CIRCULATION
CENTER (LLCC). THIS HAS MADE IT DIFFICULT TO ACCURATELY TRACK THE LLCC 
OVER THE PAST 12 HOURS ESPECIALLY CONSIDERING THE LACK OF DECENT 
MICROWAVE IMAGERY. HOWEVER, A FORTUITOUS 261050Z ASCAT-C BULLSEYE 
IMAGE REVEALED A WELL-DEFINED CENTER, WHICH SUPPORTS THE INITIAL 
POSITION WITH HIGH CONFIDENCE, AND MAXIMUM WINDS OF ABOUT 45 KNOTS 
OVER THE SOUTHWEST QUADRANT. A DETAILED ANALYSIS OF THIS ASCAT-C IMAGE 
SUPPORTS THE INITIAL WIND RADII WITH HIGH CONFIDENCE. THE INITIAL 
INTENSITY IS HELD SLIGHTLY HIGHER THAN THE ASCAT DATA INDICATED BASED 
ON THE AGENCY SUBJECTIVE DVORAK ESTIMATES AND THE CIMSS ADT, AIDT AND 
SATCON OBJECTIVE INTENSITY ESTIMATES RANGING FROM 47 TO 57 KNOTS. 
INITIAL CONDITIONS ARE MARGINALLY FAVORABLE, WITH THE HIGH VWS AND DRY 
AIR ENTRAINMENT OFFSET BY WARM (27-28C) SST VALUES AND MODERATE 
OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: MID-LEVEL SUBTROPICAL RIDGE POSITIONED
TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   NFFN: T3.0 - 45 KTS
   CIMSS SATCON: 52 KTS AT 260726Z
   CIMSS ADT: 57 KTS AT 261130Z
   CIMSS AIDT: 47 KTS AT 261130Z
   CIMSS D-PRINT: 60 KTS AT 261230Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 25-30 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: MID-LEVEL DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   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 21P IS FORECAST TO TRACK SOUTHEASTWARD TO
EAST-SOUTHEASTWARD ALONG THE SOUTHWESTERN PERIPHERY OF THE LOW- TO
MID-LEVEL SUBTROPICAL RIDGE THROUGH TAU 48, WITH GRADUAL WEAKENING
UNDER INCREASING VWS AND INCREASING DRY AIR ENTRAINMENT. AFTER TAU
48, THE SYSTEM WILL TRACK UNDER THE SUBTROPICAL WESTERLIES AND
TRACK EAST-NORTHEASTWARD TO NORTHEASTWARD AS A STRONG LOW-LEVEL
HIGH BUILDS TO THE SOUTH AND EAST BLOCKING ANY POLEWARD PROGRESS.
TC 21P WILL DISSIPATE BY TAU 72 AS IT LINGERS WITHIN THE
INCREASINGLY HOSTILE ENVIRONMENT. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIR AGREEMENT
THROUGH THE FORECAST PERIOD, WITH A LARGE SPREAD IN SOLUTIONS
OCCURRING AFTER TAU 48 AS THE SYSTEM WEAKENS AND THE LOW-LEVEL HIGH
BUILDS QUICKLY. THE EPS AND GEFS ENSEMBLES INDICATE A SIMILAR
SPREAD, SUGGESTING INCREASING UNCERTAINTY AFTER TAU 48. RELIABLE
INTENSITY GUIDANCE SUPPORTS THE STEADY WEAKENING TREND, WITH
DISSIPATION OCCURRING NEAR TAU 72. COAMPS-TC (GFS) IS THE LONE 
EXCEPTION WITH HIGHER INTENSITIES MAINTAINED THROUGH TAU 108. HOWEVER, 
THE TRACKER IS ALSO THE SOLE OUTLIER, WELL OUTSIDE THE ENVELOPE 
ACCELERATING THE SYSTEM RAPIDLY EASTWARD. 

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