WDPS33 PGTW 092100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12P (TWELVE)    
WARNING NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.3S 167.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 62 NM WEST-NORTHWEST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE (TC) 12P UNDERGOING RAPID EXPANSION OF CORE CONVECTION 
TYPICAL OF CENTRAL COLD COVER (CCC) DEVELOPMENT, WITH A SUDDEN 
SPREADING OF COLD CONVECTIVE TOPS OBSCURING THE LOW-LEVEL
CIRCULATION CENTER OVER THE LAST FEW HOURS. PERSISTENT NORTH-
NORTHWESTERLY VERTICAL WIND SHEAR (VWS) IS CLEARLY EVIDENT IN
THE EIR IMAGERY WITH A SHARP THERMAL GRADIENT ALONG THE WESTERN
EDGE OF THE CCC. A 091404Z ATMS 88.2 GHZ MICROWAVE IMAGE REVEALS A
FAIRLY UNIMPRESSIVE LOW-LEVEL STRUCTURE AS SHALLOW AND FRAGMENTED
BANDING WRAPS INTO THE BROAD CENTER OF CIRCULATION. SURFACE 
OBSERVATIONS FROM PORT VILA ARE SOUTHEASTERLY AT LESS THAN 10 KNOTS 
WITH A MINIMUM SEA LEVEL PRESSURE OF 997 MB. THE UPPER-LEVEL 
ENVIRONMENT REMAINS MARGINAL WITH MODERATE YET BROAD OUTFLOW, BOTH 
EQUATORWARD AND POLEWARD, OFFSET BY THE VWS (15-20 KNOTS). DUE TO THE 
BROAD NATURE OF THE LLC, THERE IS MEDIUM CONFIDENCE IN THE INITIAL
POSITION. DUE TO LAND INTERACTION WITH THE ISLANDS OF VANUATU, 
PERSISTENT VWS, AND UPPER-LEVEL DRY-AIR ENTRAINMENT, THE INITIAL 
INTENSITY REMAINS ASSESSED AT 35 KNOTS WITH HIGH CONFIDENCE. THE 
INITIAL WIND RADII WERE ADJUSTED BASED OFF THE AVAILABLE SURFACE 
OBSERVATIONS ON VANUATU.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK AND SURFACE
OBSERVATIONS.

CURRENT STEERING MECHANISM: LOW TO MID-LEVEL NEAR-EQUATORIAL RIDGE 
(NER) ENTRENCHED TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   PHFO: T2.5 - 35 KTS
   CIMSS ADT: 35 KTS AT 091730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: FRICTIONAL IMPACTS INFLUENCED BY VANUATU AND
                  WEAK UPPER-LEVEL DRY-AIR ENTRAINMENT.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   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 12P IS FORECASTED TO CONTINUE TRACKING
EAST-SOUTHEASTWARD ALONG THE SOUTHERN PERIPHERY OF THE NER THROUGH 
TAU 72. AS A WEAK STEERING RIDGE BEGINS TO BUILD TO THE EAST NEAR TAU 
72, THE GENERAL EASTWARD PROGRESSION BEGINS TO STEADILY SLOW DOWN. 
AFTER TC 12P TRACKS PAST THE VANUATU ARCHIPELAGO, A SLIGHT INCREASE 
TO A PEAK INTENSITY OF 40 KNOTS IN THE NEXT 12 TO 24 HOURS IS 
EXPECTED. SIGNIFICANT INTENSIFICATION IS NOT EXPECTED THROUGH THE 
REMAINDER OF THE FORECAST PERIOD AS THE SYSTEM WILL CONTINUE TO 
INTERACT WITH MODERATE TO HIGH NORTH-NORTHWESTERLY VWS AT 20-25 KNOTS 
(AFTER TAU 24) AND INCREASING DRY-AIR ENTRAINMENT (AFTER TAU 48). THE 
SYSTEM IS EXPECTED TO DISSIPATE BY TAU 72.

MODEL DISCUSSION: WITH THE EXCEPTION OF THE NAVGEM AND GALWEM
OBJECTIVE AIDS, WHICH TRACK THE SYSTEM DIRECTLY INTO THE 
AFOREMENTIONED BUILDING RIDGE, THE NUMERICAL MODEL GUIDANCE IS IN 
GOOD AGREEMENT WITH A CROSS-TRACK SPREAD OF 90 NM AT TAU 48, 
DIVERGING SIGNIFICANTLY BY TAU 72 DUE TO MODEL DIFFERENCES IN HOW THE 
STEERING RIDGE BUILDS EAST OF TC 12P. INTENSITY GUIDANCE IS IN
FAIR AGREEMENT WITH THE WEAKENING TREND OBSERVED IN THE JTWC 
INTENSITY FORECAST. HAFS-A IS EXPECTING THE SYSTEM TO WEAKEN TO 30 
KTS BY TAU 36 THUS THE SYSTEM MAY DISSIPATE EARLIER.

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