WDPS31 PGTW 182100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 09P (IRENE)      
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.8S 166.4E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 127 NM WEST-SOUTHWEST OF PORT VILA,
VANUATU
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 23 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 09P IS LOCATED UNDER THE SUBTROPICAL
WESTERLIES WITH LOW TO MODERATE VERTICAL WIND SHEAR. CONSEQUENTLY,
AN 181834Z SSMIS 91GHZ COLOR COMPOSITE MICROWAVE IMAGE SHOWS A
PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER POSITIONED ALONG THE
NORTHWEST EDGE OF THE CORE DEEP CONVECTION. ANIMATED ENHANCED
INFRARED (EIR) SATELLITE IMAGERY DEPICTS THE BULK OF THE DEEP
CONVECTION SHEARING OVER THE EASTERN SEMICIRCLE WITH EXTENSIVE
CONVECTIVE BANDING DISPLACED OVER THE NORTHERN PERIPHERY. THE
INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE
SSMIS IMAGE. THE INITIAL INTENSITY OF 50 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE CONVECTIVE STRUCTURE AND RANGE OF DVORAK
ESTIMATES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: HIGH AMPLITUDE PATTERN WITH ENHANCED
STEERING FLOW BETWEEN A DEEP TROUGH AND THE SUBTROPICAL RIDGE.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   NFFN: T3.5 - 55 KTS
   CIMSS ADT: 53 KTS AT 181730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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: TROPICAL CYCLONE (TC) 09P IS TRACKING QUICKLY
EAST-SOUTHEASTWARD WITHIN THE ENHANCED FLOW BETWEEN A DEEP TROUGH
AND THE STR POSITIONED TO THE EAST. TC 09P WILL WEAKEN STEADILY
THROUGHOUT THE FORECAST DUE TO INCREASING VERTICAL WIND SHEAR UNDER
THE SUBTROPICAL WESTERLIES. THEREFORE, THE SYSTEM IS EXPECTED TO
TRANSITION TO A SUBTROPICAL CYCLONE WITHIN THE NEXT 48 TO 72 HOURS AS 
IT ACCELERATES SOUTHEASTWARD.  

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN TIGHT 
AGREEMENT LENDING HIGH CONFIDENCE TO THE JTWC TRACK FORECAST. 
RELIABLE INTENSITY GUIDANCE ALSO SUPPORTS THE JTWC INTENSITY FORECAST 
WITH MEDIUM CONFIDENCE WITH GUIDANCE INDICATING A SHORT-TERM INCREASE
FOLLOWED BY STEADY WEAKENING. 

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