WDPN32 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING 
NR 010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.3N 152.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 312 NM SOUTH-SOUTHWEST OF MINAMI TORI
SHIMA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A
PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION WITH DEEP CONVECTION
BUILDING OVER THE NORTHWEST QUADRANT. A 120243Z AMSR2 MICROWAVE
IMAGE INDICATES THE BULK OF THE CONVECTION IS SHEARED OVER THE
NORTHERN SEMICIRCLE DUE TO SOUTHERLY VERTICAL WIND SHEAR (VWS). A
120002Z ASCAT-C IMAGE REVEALS A HIGHLY ASYMMETRIC WIND FIELD WITH
EXTENSIVE GALE FORCE WINDS CONFINED TO THE NORTHERN SEMICIRCLE. THE
ENVIRONMENT REMAINS MARGINAL WITH MODERATE SOUTHERLY VWS OFFSET
SLIGHTLY BY STRONG SOUTHERLY UPPER-LEVEL FLOW. THE INITIAL POSITION
IS PLACED WITH HIGH CONFIDENCE BASED ON MSI. THE INITIAL INTENSITY
OF 40 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE RANGE OF
AGENCY DVORAK ESTIMATES AND THE 120610Z ADT ESTIMATE.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) EXTENDING WESTWARD FROM 40N AND 180 TO THE
VICINITY OF KYUSHU. A SECOND RIDGE BUILDING TO THE SOUTHEAST IS
MOVING WEST, PROVIDING A BOOST TO THE WEST-NORTHWEST MOVEMENT. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 41 KTS AT 120610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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: TS 23W IS TRACKING UNDER THE LOW-LEVEL STR TO
THE NORTH BUT IS EXPECTED TO GRADUALLY SHIFT TO A NORTHEASTWARD
TRACK AS THE SECONDARY STEERING RIDGE TO THE SOUTHEAST BECOMES THE
DOMINANT STEERING INFLUENCE. TS 23W SHOULD PEAK AT 50 KNOTS THEN
STEADILY WEAKEN THROUGH THE FORECAST AS IT TRACKS UNDER STRONG
UPPER LEVEL SOUTHWESTERLIES AND OVER COOLER SST. EXTRA-TROPICAL
TRANSITION IS UNLIKELY DUE TO THE LACK OF A STRONG BAROCLINIC ZONE
SO THE SYSTEM WILL LIKELY DISSIPATE BY TAU 120 OR EARLIER.      

MODEL DISCUSSION: DUE TO A FAIRLY COMPLEX, EVOLVING STEERING
PATTERN, NUMERICAL MODEL GUIDANCE IS IN FAIR AGREEMENT WITH HIGH
UNCERTAINTY IN THE RECURVE TIMING AND LOCATION. THE JTWC FORECAST
IS POSITIONED LEFT OF THE MULTI-MODEL CONSENSUS (CONW) THROUGH TAU
36. AFTER TAU 36, THE JTWC FORECAST IS POSITIONED CLOSE TO CONW. 

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