WDPN32 PGTW 120300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 23W (NAMTHEUN) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5N 153.1E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 286 NM SOUTH 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 COMPACT,
WELL-DEFINED, FULLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC)
WITH FLARING DEEP CONVECTION SHEARED TO THE NORTH AND NORTHEAST OF
THE LLCC. WHILE THE LLCC HAD BEEN TUCKED UNDER THE CONVECTION
EARLIER IN THE DAY, IT HAS BEEN UNABLE TO OVERCOME THE PERSISTENT
SOUTHERLY SHEAR. THE INITIAL POSITION IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE EXPOSED LLCC, AMONGST THE TIGHT GROUPING OF
AGENCY FIXES. THE INITIAL INTENSITY OF 45 KNOTS IS ASSESSED WITH
MEDIUM CONFIDENCE, ON THE HIGHER END OF THE AGENCY INTENSITY
ESTIMATES, IN LINE WITH THE ADT ESTIMATE. A 112330Z ASCAT-B
BULLSEYE PASS INDICATED A MODEST AREA OF 40-45 KNOT WINDS CONFINED
TO THE NORTHEAST QUADRANT. AS DISCUSSED, THE SYSTEM CONTINUES TO
STRUGGLE AGAINST MODERATE TO STRONG SOUTHERLY WIND SHEAR, FLOWING
NORTHWARD IN A TIGHT GRADIENT BETWEEN AN UPPER-LEVEL ANTICYCLONE TO
THE EAST AND A NORTH-SOUTH ORIENTED TUTT-CELL OVER THE MARIANAS.
OUTSIDE OF THE SHEAR, THE ENVIRONMENT IS OTHERWISE FAVORABLE WITH
WARM SSTS AND ROBUST POLEWARD OUTFLOW, WHICH TURNS EAST SOUTH OF
THE 30N LATITUDE.

INITIAL WIND RADII BASIS: PERSISTENCE 

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: 45 KTS AT 112340Z

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

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   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 APPEARS TO HAVE ITS BATTLE WITH THE
PERSISTENT WIND SHEAR, AT LEAST FOR THE TIME BEING, WITH THE
COMPACT LLCC NOW FULLY EXPOSED IN THE MOST RECENT ANIMATED VISIBLE
IMAGERY. THE SYSTEM IS MOVING NEARLY DUE WEST JUST SOUTH OF THE
20TH LATITUDE, ALONG THE SOUTHERN PERIPHERY OF THE DEEP STR TO THE
NORTH. THIS GENERAL MOTION IS EXPECTED TO CONTINUE, WITH A TURN
MORE NORTH OF WEST EXPECTED IN THE NEXT FEW HOURS, THROUGH TAU 36.
THEREAFTER, THE RIDGE TO THE NORTH WEAKENS WHILE THE RIDGE TO THE
SOUTHEAST BUILDS AND MOVES WEST, UNDERCUTTING THE SYSTEM TO THE
SOUTH. IN RESPONSE, TS 23W WILL SLOW AND GRADUALLY TURN POLEWARD
BETWEEN TAUS 48 AND 72, AND COULD BECOME QUASI-STATIONARY DURING
THIS PERIOD. BY TAU 72 THE RIDGE TO THE SOUTH REORIENTS TO A
SOUTHWEST-NORTHEAST AXIS, AND TS 23W WILL KICK OUT TO THE NORTHEAST
ALONG THE NORTHWESTERN PERIPHERY OF THE RIDGE. WHILE OUTFLOW AND
SSTS REMAIN FAVORABLE FACTORS FOR DEVELOPMENT, THE PERSISTENT SHEAR
IS EXPECTED TO REMAIN IN PLACE, WITH A VERY SLIGHT WEAKENING OF THE
SHEAR VALUES EXPECTED THROUGH TAU 24, THUS THE VARIOUS
ENVIRONMENTAL PARAMETERS WILL OFFSET ONE ANOTHER. IN RESPONSE, TS
23W IS FORECAST TO MAINTAIN INTENSITY THROUGH TAU 24, BEFORE
BEGINNING A GRADUAL WEAKENING PROCESS THROUGH THE REMAINDER OF THE
FORECAST AS SHEAR ONCE AGAIN OVERCOMES THE WARM SSTS AND GOOD
OUTFLOW ALOFT. 

MODEL DISCUSSION: TRACK GUIDANCE IS IN POOR AGREEMENT. THE GFS AND
HWRF TURN THE SYSTEM NORTHWARD AFTER TAU 00, SLOWLY DRIFTING THE
SYSTEM INTO THE RIDGE TO THE NORTH. THESE SOLUTIONS ARE CONSIDERED
UNLIKELY AS THE SYSTEM IS NOT STRONG ENOUGH TO PUNCH INTO THE RIDGE
TO THE NORTH. THE REMAINDER OF THE GUIDANCE AGREES ON A GENERAL
WEST-NORTHWEST TRACK, TURNING GRADUALLY NORTHEASTWARD BUT DISAGREES
ON THE SPEED OF THE SYSTEM, AND THE TIMING AND SHARPNESS OF THE
TURN. THE ECMWF AND JGSM REPRESENT THE WIDE OUTLIERS, WHILE NAVGEM
TAKES IN THE INSIDE TRACK. THE JTWC FORECAST HUGS THE SOUTHERN OR
LEFTWARD EDGE OF THE GUIDANCE ENVELOPE THROUGH THE MAJORITY OF THE
FORECAST. IN LIGHT OF THE WIDE SPREAD AND DIVERGENT MODEL
SOLUTIONS, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST TRACK.
INTENSITY GUIDANCE IS ALSO MIXED WITH HWRF AND COAMPS-TC SHOWING
NEAR TERM INTENSIFICATION TO A PEAK OF 60 KNOTS, WHILE THE
REMAINDER OF THE GUIDANCE INDICATING STEADY WEAKENING. IN LIGHT OF
THE RECENT IMPACT OF HIGHER THAN EXPECTED SHEAR AND THE
UNLIKELIHOOD OF THE SYSTEM RECOVERING FROM THE SHEAR, THE JTWC
FORECAST REMAINS CLOSE TO THE MEAN OF THE GUIDANCE SUGGESTING
WEAKENING, WITH LOW CONFIDENCE.   

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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