WDPN32 PGTW 170900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING 
NR 030//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 34.8N 167.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 931 NM NORTH OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) AND A 170424Z SSMIS
91GHZ MICROWAVE IMAGE DEPICT A FULLY-EXPOSED, TIGHTLY-WRAPPED LOW-
LEVEL CIRCULATION (LLC) DEVOID OF ANY DEEP CONVECTION, WHICH 
SUPPORTS THE INITIAL POSITION WITH HIGH CONFIDENCE. THE SYSTEM IS 
CURRENTLY LOCATED UNDER AN UPPER-LEVEL SUBTROPICAL TROUGH WITH 
SIGNIFICANT DRY AIR ENTRAINMENT. MSI ALSO REVEALS THE SYSTEM IS 
INTERACTING WITH A STATIONARY FRONTAL BOUNDARY TO THE NORTH WITH 
COLD-AIR STRATOCUMULUS ADVECTING SOUTHWARD OVER THE WESTERN 
SEMICIRCLE. THE INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH HIGH 
CONFIDENCE BASED ON RECENT ASCAT DATA AND THE KNES DVORAK ESTIMATE. 
UPPER-LEVEL CONDITIONS ARE FAVORABLE WITH LOW VERTICAL WIND SHEAR 
AND WEAK POLEWARD VENTING, HOWEVER, OVERALL CONDITIONS ARE 
UNFAVORABLE DUE TO THE EXTENSIVE ENTRAINMENT OF COOLER, DRIER AIR 
AND COOL SEA SURFACE TEMPERATURES NEAR 24C.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF LOW TO MID-LEVEL
SUBTROPICAL RIDGE (STR) TO THE SOUTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   KNES: T2.5 - 35 KTS
   CIMSS SATCON: 46 KTS AT 170214Z
   CIMSS ADT: 29 KTS AT 170540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 0-5 KTS
   SST: 23-24 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT AND UPPER-LEVEL CONVERGENCE
OVER THE SOUTHERN PORTION OF THE CIRCULATION. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 STORM (TS) 23W IS FORECAST TO RECURVE
AND ACCELERATE NORTHEASTWARD THROUGH THE FORECAST PERIOD WITH
EXTRA-TROPICAL TRANSITION (ETT) COMPLETING BY 180000Z AS THE SYSTEM
GAINS FRONTAL CHARACTERISTICS AND TRACKS NEAR A MIDLATITUDE JET. TS
23W IS EXPECTED TO SLOWLY WEAKEN AS IT TRACKS OVER COOLER SST
(20-22C) WITH INCREASING ENTRAINMENT OF COOLER, DRIER, MORE STABLE
AIR. THERE IS A POSSIBILITY THE SYSTEM MAY WEAKEN BELOW TROPICAL
DEPRESSION STRENGTH PRIOR TO COMPLETING ETT.    

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE REMAINS IN GOOD
AGREEMENT THROUGH THE FORECAST PERIOD WITH A 65NM SPREAD IN 
SOLUTIONS AT TAU 24, LENDING HIGH CONFIDENCE TO THE JTWC FORECAST 
TRACK. INTENSITY GUIDANCE INDICATES A RANGE OF INTENSITY VALUES FROM 
25 TO 40 KNOTS THROUGH THE FORECAST PERIOD LENDING MEDIUM CONFIDENCE 
TO THE JTWC INTENSITY FORECAST.  

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