WDPS33 PGTW 251500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 21P (SERU) WARNING NR 
003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8S 171.2E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 166 NM EAST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
CONSOLIDATING TROPICAL CYCLONE (TC) 21P (SERU). WHILE THE SYSTEM
BENEFITS FROM MODERATE POLEWARD AND EQUATORWARD OUTFLOW, THE SPIRAL
BANDS OF DEEP CONVECTION ARE WRAPPING INTO THE LOW-LEVEL
CIRCULATION (LLCC), WITH PERSISTENT CONVECTION ON TOP. ENVIRONMENT
REMAINS FAVORABLE FOR TROPICAL CYCLONE INTENSIFICATION, WITH
AFOREMENTIONED GOOD OUTFLOW, WARM (28-29 C) SEA SURFACE
TEMPERATURES (SST), AND MODERATE (10-15 KTS) VERTICAL WIND SHEAR
(VWS). THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED
ON THE EXTRAPOLATION OF A 251018Z ASCAT PASS, WHILE THE INITIAL
INTENSITY OF 65 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
BLEND OF AGENCY AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.
ADDITIONALLY, A 250232Z AMSR2 PASS SHOWED 80-85 KTS MAXIMUM WIND
SPEEDS, WITH THE PATCH OF STRONGEST WINDS LOCATED DIRECTLY
UNDERNEATH STRONGEST CONVECTION, THEREFORE ASSESSED AS SLIGHTLY
OVERESTIMATING ACTUAL WIND FIELD STRENGTH, BUT INDICATING INTENSITY
HIGHER THAN PREVIOUSLY ASSESSED.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO
THE EAST-SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T3.0 - 45 KTS
   PHFO: T3.5 - 55 KTS
   CIMSS SATCON: 53 KTS AT 250959Z
   CIMSS ADT: 59 KTS AT 251130Z
   CIMSS AIDT: 66 KTS AT 251130Z
   CIMSS D-MINT: 66 KTS AT 250651Z
   CIMSS D-PRINT: 60 KTS AT 251210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: INITIAL INTENSITY IS HIGHER THAN
ASSESSED WITH THE PREVIOUS FORECAST, BASED ON 250232Z AMSR2 PASS

FORECAST DISCUSSION: TC 21P IS FORECAST TO TRACK SOUTHEASTWARD,
UNDER THE INFLUENCE OF A STR TO THE EAST-SOUTHEAST. BETWEEN TAUS 72
AND 120, ANOTHER RIDGE WILL BEGIN BUILDING TO THE NORTHWEST OF THE
SYSTEM. THE COMPETING STEERING WILL LEAD TO THE SYSTEM TO SLOWING 
DOWN. AT THE SAME TIME, ENVIRONMENTAL CONDITIONS WILL BEGIN TO
DETERIORATE AROUND TAU 72, WITH RAPIDLY INCREASING VWS AND COOLING
SST. THEREFORE TC 21P IS FORECAST TO BEGIN DISSIPATING TOWARDS THE
END OF THE CURRENT FORECAST PERIOD. IN REGARDS TO THE TRACK BETWEEN 
TAU 96 AND 120, THE SYSTEM IS FORECAST TO MAKE A SLIGHT TURN TOWARDS 
NORTHEAST, ASSUMING THE VORTEX WILL SURVIVE THE INCREASINGLY LESS 
FAVORABLE ENVIRONMENT. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN FAIR
AGREEMENT FOR THE INITIAL 72 HOURS, WITH A CROSS-TRACK SPREAD OF 80
NM, BUT A MUCH HIGHER ALONG-TRACK SPREAD OF 290 NM. ALL MODELS
AGREE ON A GENERALLY SOUTH-SOUTHEASTWARD TRACK, FOLLOWED BY AN
EASTWARD TURN. PAST 72 HOWEVER, THE TRACK GUIDANCE BECOMES MORE
ERRATIC, WITH SOME GUIDANCE (GALWEM, UKMET DETERMINISTIC) TAKING AN
EARLY AND SHARP NORTHEASTWARD TURN, AND OTHER (NAVGEM, ECMWF
ENSEMBLE) TAKING THE SYSTEM EASTWARD THROUGHOUT AND PAST THE
FORECAST PERIOD. THEREFORE, PAST TAU 72, THE JTWC TRACK IS LAID
WITH LOW CONFIDENCE. THE INTENSITY FORECAST IS ASSESSED WITH MEDIUM
CONFIDENCE AND CLOSE TO THE MULTI-MODEL CONSENSUS INDICATING A
STEADY INTENSIFICATION THROUGH TAU 24, FOLLOWED BY SLOW AND STEADY
WEAKENING. THERE ARE RAPID INTENSIFICATION (RI) AIDS INDICATING A 
POTENTIAL FOR RI OVER THE NEXT 24 (CHIPS, AI-RI), BUT THE PROBABILITY 
FOR RI REMAINS LOW. ON THE OTHER END OF THE SPECTRUM, GFS 
DETERMINISTIC MODEL IS SUGGESTING IMMEDIATE BEGINNING OF WEAKENING 
TREND. CONSIDERING MOSTLY FAVORABLE ENVIRONMENTAL CONDITIONS, JTWC
INTENSITY FORECAST IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS WITH 
MEDIUM CONFIDENCE THROUGHOUT THE FORECAST PERIOD.

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