WDPN32 PGTW 201500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN)         
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.2N 128.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 143 NM SOUTHWEST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A 
PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION (LLC), WITH FLARING DEEP 
CONVECTION OVER THE EASTERN QUADRANT. A 201008Z GMI 89GHZ MICROWAVE 
IMAGE REVEALS A SYMMETRIC, DEFINED LLC, WITH A CLUSTER OF 
DISORGANIZED DEEP CONVECTION OVER THE EASTERN QUADRANT. A 201136Z 
ASCAT-B BULLSEYE IMAGE INDICATES AN ELONGATED CENTER AND SMALL REGION 
OF ELEVATED WINDS (30-35 KNOTS) ASSOCIATED WITH THE DEEP CONVECTION, 
WITH AN ISOLATED 40 KNOT WIND. THE INITIAL POSITION AND INITIAL 
INTENSITY ARE BASED ON THE ASCAT IMAGE WITH HIGH CONFIDENCE. 
ENVIRONMENTAL CONDITIONS ARE MARGINAL, WITH FAIR OUTFLOW OFFSET BY 
DRY AIR ENTRAINMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: LOW- TO MID-LEVEL SUBTROPICAL RIDGE (STR) 
POSITIONED TO THE EAST AND SOUTHEAST, ORIENTED SSW TO NNE. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS ADT: 34 KTS AT 201130Z
   CIMSS AIDT: 40 KTS AT 201130Z
   CIMSS D-MINT: 27 KTS AT 201008Z
   CIMSS D-PRINT: 28 KTS AT 201210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE EASTWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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: TROPICAL STORM (TS) 18W IS FORECAST TO TRACK 
NORTH-NORTHEASTWARD TO EAST-NORTHEASTWARD THROUGH THE FORECAST PERIOD 
AS IT ROUNDS THE NORTHWESTERN PERIPHERY OF THE STR. UPPER-LEVEL 
CONDITIONS HAVE IMPROVED STEADILY OVER THE PAST DAY AS THE SYSTEM 
TRACKED AWAY FROM AN UPPER-LEVEL LOW NEAR TAIWAN, WITH IMPROVED 
EQUATORWARD OUTFLOW EVIDENT IN THE ANIMATED WATER VAPOR RGB IMAGERY. 
ADDITIONALLY, EASTWARD OUTFLOW HAS MAINTAINED. HOWEVER, DRY AIR 
ENTRAINMENT THROUGH THE COLUMN HAS CONTINUED TO HINDER ANY 
SIGNIFICANT CONSOLIDATION. THERE IS A VERY SHORT WINDOW 
(APPROXIMATELY SIX HOURS) FOR THE SYSTEM TO MAINTAIN 35 KNOTS UNTIL
VERTICAL WIND SHEAR INCREASES TO HIGH (25-45 KNOTS) LEVELS AND DRY
AIR OVERWHELMS THE SYSTEM. 

MODEL DISCUSSION: JTWC TRACK CONFIDENCE IS MEDIUM, WITH INCREASING
UNCERTAINTY IN THE EXACT TRACK AFTER TAU 12. IN LARGE PART, THIS IS 
DUE TO THE WEAK NATURE OF THE SYSTEM AND THE COMPLEX LOW-LEVEL 
STEERING INFLUENCE AS IT WEAKENS NEAR LANDFALL. THE CURRENT SUITE OF 
DETERMINISTIC GUIDANCE SHOWS A SPREAD FROM WESTERN CENTRAL KYUSHU TO 
NORTHERN KYUSHU. THE 200000Z ECMWF ENSEMBLE (ECENS) RUN SHOWS THE 
BULK OF THE SOLUTIONS TRACKING THE SYSTEM INTO THE KOREAN STRAIT AND 
NORTHERN KYUSHU. THE 200600Z GFS ENSEMBLE (GEFS) IS SIMILAR, WITH A 
WIDER SPREAD OF SOLUTIONS FROM THE KOREAN STRAIT TO SOUTHERN KYUSHU. 
OVERALL, BOTH ENSEMBLES INDICATE THAT A POTENTIAL DIRECT TRACK OVER 
CHEJU-DO AND THE BUSAN AREA IS AT A VERY LOW PROBABILITY. RELIABLE 
INTENSITY GUIDANCE INDICATES A PEAK INTENSITY OF 35-45 KNOTS OVER THE 
NEXT 12 HOURS, WITH UNANIMOUS AGREEMENT ON A SHARP WEAKENING TREND 
AFTER TAU 12. HOWEVER, THE LATEST HAFS-A RUN IS FORMING SPURIOUS LOW 
PRESSURE AREAS UNDER THE DEEP CONVECTIVE BURSTS, WITH INFLATED 40-45 
KNOT WINDS ISOLATED DIRECTLY TO THESE REGIONS. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM//
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