WDPS31 PGTW 040900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 23P (TWENTYTHREE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.8S 162.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 331 NM WEST OF PORT VILA, VANUATU
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC) WITH IMPROVED
DEEP CONVECTIVE BANDING WRAPPING INTO A WELL-DEFINED CENTER, WHICH
SUPPORTS THE INITIAL POSITION WITH HIGH CONFIDENCE. A 040618Z SSMIS
91GHZ MICROWAVE IMAGE REVEALS SPIRAL BANDING PRIMARILY OVER THE
NORTHERN AND EASTERN SEMICIRCLES CONVERGING INTO THE EASTERN
QUADRANT OF THE LLCC WITH EXPOSED, SHALLOW BANDING OVER THE
SOUTHERN SEMICIRCLE. LOW TO MODERATE (15-20 KNOTS) SOUTHERLY
VERTICAL WIND SHEAR (VWS) IS CURRENTLY HINDERING CONVECTIVE
DEVELOPMENT OVER THE SOUTHERN SEMICIRCLE BUT IS EXPECTED TO
DIMINISH WITHIN THE NEXT SIX HOURS. UPPER-LEVEL OUTFLOW REMAINS
RADIAL ENHANCED BY STRONG WESTERLIES TO THE SOUTH. ADDITIONALLY,
SEA SURFACE TEMPERATURES OF 29-30C ARE CONDUCIVE FOR FURTHER
DEVELOPMENT. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE KNES AND NFFN DVORAK ESTIMATES OF T2.5,
WHICH ARE CONSISTENT WITH THE OVERALL CONVECTIVE STRUCTURE. NOTE: 
THE 040830Z PGTW DVORAK ESTIMATE IS AT T2.5 (35 KNOTS).

INITIAL WIND RADII BASIS: OTHER (AMSR2 WINDSPEED IMAGE)

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) POSITIONED TO
THE EAST AND SOUTHEAST 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS
   PHFO: T2.0 - 30 KTS
   NFFN: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE (TC) 23P IS FORECAST TO SLOW
AND TURN SHARPLY SOUTH-SOUTHEASTWARD OVER THE NEXT 12 HOURS AS THE
STR TO THE SOUTH SHIFTS EASTWARD AND REALIGNS IN RESPONSE TO AN
APPROACHING UPPER-LEVEL SHORTWAVE TROUGH. TC 23P WILL RAPIDLY
INTENSIFY OVER THE NEXT 24 HOURS FROM 35 KNOTS TO 65 KNOTS AT TAU
24 AS THE SOUTHERLY VWS DECREASES AND UPPER-LEVEL OUTFLOW REMAINS
ROBUST. AFTER TAU 24, TC 23P WILL CONTINUE TRACKING SOUTH-
SOUTHEASTWARD THROUGH THE FORECAST PERIOD ALONG THE WESTERN 
PERIPHERY OF THE STR. TC 23P SHOULD GRADUALLY WEAKEN THROUGH THE 
REMAINDER OF THE FORECAST PERIOD AS THE SYSTEM TRACKS UNDER 
SUBTROPICAL WESTERLIES AND ENVIRONMENTAL CONDITIONS DEGRADE. TC 23P 
WILL BEGIN SUBTROPICAL TRANSITION NEAR TAU 72 AS IT BEGINS TO 
INTERACT WITH A BAROCLINIC ZONE SOUTH OF NEW CALEDONIA AND TRACKS 
OVER COOLER SST (25-26C). ADDITIONALLY, VWS WILL INCREASE TO HIGHER 
LEVELS (30-50 KNOTS) AS THE SYSTEM TRANSITIONS INTO A SUBTROPICAL 
CYCLONE BY TAU 120. THE SYSTEM IS EXPECTED TO SLOW IN THE EXTENDED 
PERIOD AS IT WEAKENS TO 45 KNOTS AND RUNS INTO A STRONG HIGH 
EXPECTED TO BUILD OVER AND NORTH OF NEW ZEALAND.

MODEL DISCUSSION: THERE IS CURRENTLY A BIFURCATION IN THE MODEL
GUIDANCE WITH ECMWF AND THE ECMWF ENSEMBLE MEAN INDICATING A MORE
PROLONGED SOUTHWESTWARD TRACK BEFORE TURNING SOUTHWARD WHILE THE
REMAINDER OF THE GUIDANCE IS SHOWING A TRACK OVER OR TO THE WEST OF
NEW CALEDONIA. THE ECMWF SOLUTION IS ASSESSED AS UNLIKELY SINCE IT
TRACKS THE SYSTEM DIRECTLY INTO THE SUBTROPICAL WESTERLIES BEFORE
STALLING IT AND TURNING IT SOUTHWARD WITHIN THE SUBTROPICAL
WESTERLIES. THE 040000Z ECMWF ENSEMBLE SHOWS A WIDER RANGE OF
POSSIBILITIES WITH A STRONGER TC TRACKING CLOSER TO NEW CALEDONIA
AND A WEAKER TC MUCH FURTHER WEST. OVERALL CONFIDENCE IN THE JTWC 
TRACK AND INTENSITY FORECAST IS MEDIUM.     

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