WDPN32 PGTW 112100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.3N 153.9E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 294 NM SOUTH OF MINAMI TORI SHIMA
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A VERY
COMPACT REGION OF CENTRAL CONVECTION, WITH THE MAJORITY OF THE
CONVECTIVE MASS DISPLACED TO THE NORTH OF THE LOW LEVEL CIRCULATION
CENTER (LLCC) DUE TO PERSISTENT SOUTHERLY SHEAR. WHILE THE LLCC IS
OBSCURED IN THE EIR, A TIMELY 111806Z SSMIS 37GHZ MICROWAVE IMAGE
SHOWED THE LOW-LEVEL BANDS WRAPPING INTO THE ILL-DEFINED LLCC, WITH
FLARING CONVECTION SHEARING OFF TO THE NORTHEAST AND LENT HIGH
CONFIDENCE TO THE INITIAL POSITION. THE PREVIOUS BEST TRACK
INTENSITY HAS BEEN LOWERED TO 45 KNOTS BASED ON A REANALYSIS OF
EARLIER SCATTEROMETER DATA. CONSEQUENTLY, THE CURRENT INTENSITY
REMAINS ASSESSED AT 45 KNOTS WITH HIGH CONFIDENCE, BASED ON THE
MAJORITY OF AGENCY ESTIMATES OF T3.0, WHICH ALIGNS WELL WITH THE
ADT ESTIMATE AND THE EARLIER SCATTEROMETER DATA. IN ADDITION TO A
MODIFICATION OF THE INTENSITY TRENDS, THE WIND RADII HAVE BEEN
SIGNIFICANTLY INCREASED IN SIZE BASED ON THE SCATTEROMETER DATA AND
ARE REFLECTED IN THE CURRENT ANALYSIS. ENVIRONMENTAL ANALYSIS
REVEALS A MARGINAL ENVIRONMENT CHARACTERIZED BY MODERATE TO HIGH
SOUTHERLY VWS BEING OFFSET BY WARM SSTS AND STRONG POLEWARD
OUTFLOW. TS 23W IS TRACKING NORTHWEST, IN A COMPLEX STEERING
ENVIRONMENT ON THE SOUTH SIDE OF AN ELONGATED SUBTROPICAL RIDGE
(STR) WHICH IS DISPLACED POLEWARD OF THE CLIMATOLOGICAL POSITION,
AS WELL AS BEING PUSHED WESTWARD BY A SECOND STR CENTERED TO THE
EAST WHICH IS BUILDING WESTWARD.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA. 

CURRENT STEERING MECHANISM: TS 23W IS TRACKING NORTHWEST, IN A
COMPLEX STEERING ENVIRONMENT ON THE SOUTH SIDE OF AN ELONGATED
SUBTROPICAL RIDGE (STR) WHICH IS DISPLACED POLEWARD OF THE
CLIMATOLOGICAL POSITION, AS WELL AS BEING PUSHED WESTWARD BY A
SECOND STR CENTERED TO THE EAST WHICH IS BUILDING WESTWARD.    

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T3.0 - 45 KTS
   CIMSS ADT: 43 KTS AT 111740Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: THE TRACK OF TS 23W, WHILE APPEARING TO BE A
STRAIGHT-FORWARD RECURVATURE SCENARIO, IS ACTUALLY BEING DRIVEN BY
A COMPLEX INTERACTION OF MULTIPLE DEEP-LAYER RIDGES. THE PRIMARY
DRIVER OF TRACK MOTION OVER THE NEXT 48 HOURS WILL BE THE BUILDING
RIDGE CURRENTLY CENTERED NEAR 20N 170E, WHICH EXTENDS SOUTHWESTWARD
TO THE NEAR THE EQUATOR. AS THIS RIDGE BUILDS AND MOVES
SOUTHWESTWARD THROUGH TAU 48, TS 23W WILL MOVE WEST-NORTHWESTWARD
ALONG THE NORTHWEST SIDE OF THE RIDGE, BASICALLY BEING PUSHED
FORWARD BY THE STRONG RIDGE. BY TAU 48, A RIDGE EXTENDING EAST
ALONG ROUGHLY THE 30TH PARALLEL WILL STRENGTHEN, AND CAUSE TS 23W
TO SLOW AS THE TWO RIDGES FIGHT FOR DOMINANCE OF THE STEERING
PATTERN. BY TAU 72, THE SOUTHERN RIDGE IS FORECAST TO RAPIDLY MOVE
WESTWARD, UNDERCUTTING TS 23W AND ALLOWING THE SYSTEM TO ACCELERATE
NORTHEASTWARD ALONG THE NORTHWEST SIDE OF THE RIDGE. MARGINAL
ENVIRONMENTAL CONDITIONS, PARTICULARLY THE SHEAR VALUES, ARE NOT
EXPECTED TO CHANGE MUCH IN THE NEAR-TERM. BY TAU 24 HOWEVER, THE
SHEAR IS FORECAST TO RELAX A BIT, WHICH WILL ALLOW TS 23W TO
INTENSIFY TO 55 KNOTS BY TAU 36. AFTER TAU 48, THE SYSTEM WILL ONCE
AGAIN ENCOUNTER INCREASED SHEAR, AS WELL AS A GENERAL DRYING OF THE
ATMOSPHERE, LEADING TO STEADY WEAKENING THROUGH TAU 120. AT THIS
POINT, THE SYSTEM IS NOT EXPECTED TO UNDERGO EXTRA-TROPICAL
TRANSITION DURING THIS FORECAST PERIOD, BUT RATHER IS FORECAST TO
DISSIPATE UNDER STRONG VWS.  

MODEL DISCUSSION: TRACK GUIDANCE SHOWS SIGNIFICANT AND INCREASING
DISAGREEMENT ON THIS RUN. GFS, NAVGEM AND HWRF TURN THE SYSTEM
SHARPLY POLEWARD BY TAU 12, WITH THE GFS THEN KEEPING THE SYSTEM
SOUTH OF 25N THROUGH TAU 120. THE REMAINDER OF THE GUIDANCE
INDICATES A MUCH WIDER TURN, WITH THE ECMWF ENSEMBLE AND UKMET
DETERMINISTIC NOT TURNING THE SYSTEM POLEWARD UNTIL THE 146E
LONGITUDE. THE JTWC FORECAST TRACK LIES ON THE NORTHERN EDGE OF THE
BULK OF THE GUIDANCE, THROUGH TAU 48, THEN ON THE INSIDE OF THE
ENVELOPE, CLOSELY TRACKING THE ECMWF DETERMINISTIC TRACKER. IN
LIGHT OF THE LARGE AND INCREASING UNCERTAINTY IN THE MODELS, THERE
IS LOW CONFIDENCE IN THE JTWC FORECAST TRACK. LIKEWISE, INTENSITY
GUIDANCE IS MIXED, WITH A LARGE SPREAD BETWEEN THE GFS INDICATING
STEADY WEAKENING FROM THE START, TO THE COAMPS-TC INDICATING A PEAK
OVER 85 KNOTS BY TAU 72. THE JTWC FORECAST IS ABOVE THE CONSENSUS
MEAN BY ABOUT 10 KNOTS THROUGH TAU 48, THEN SLIGHTLY BELOW THE MEAN
THEREAFTER. THE JTWC FORECAST IS SET WITH LOW CONFIDENCE IN LIGHT
OF THE SIGNIFICANT UNCERTAINTY IN BOTH THE TRACK AND INTENSITY
GUIDANCE.   

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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