WDPN32 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN) WARNING 
NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.8N 128.9E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 88 NM SOUTH-SOUTHWEST OF SASEBO, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 18W WITH A LOW-LEVEL CIRCULATION CENTER (LLCC) THAT HAS
TUCKED UNDERNEATH THE DEEP CONVECTION AND CIRRUS FILAMENTS NOW
EXPANDING OUTWARD FROM THE SYSTEM, HINTING AT IMPROVED OUTFLOW. THE
CONVECTIVE STRUCTURE HAS ALSO IMPROVED OVER THE PAST 6 HOURS,
APPARENT BY THE SMALL COMMA SHAPE THAT IS WRAPPING AROUND THE
SOUTHERN AND EASTERN PORTIONS OF THE SYSTEM. ENVIRONMENTAL ANALYSIS
INDICATES THAT 18W IS IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY
MODERATE RADIAL OUTFLOW ALOFT, LOW TO MODERATE (15-20 KTS)
NORTHERLY VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE
TEMPERATURES. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON ANIMATED EIR AND RADAR IMAGERY. THE SYSTEM HAS SOME
SOUTHEASTWARD VORTEX TILT WITH HEIGHT, CAUSING SOME SLIGHT
UNCERTAINTY WITH EXACT CENTER PLACEMENT. THE INITIAL INTENSITY OF
40 KTS IS ALSO ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE LISTED
AGENCY DVORAK FIXES AND OBJECTIVE INTENSITY ESTIMATES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: NORTHWESTERN PERIPHERY OF A SUBTROPICAL
RIDGE POSITIONED TO THE SOUTH-SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 48 KTS AT 201800Z
   CIMSS ADT: 43 KTS AT 201800Z
   CIMSS AIDT: 41 KTS AT 201800Z
   CIMSS D-PRINT: 32 KTS AT 201800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: INTENSITY HELD HIGHER THROUGH
LANDFALL

FORECAST DISCUSSION: 18W IS FORECAST TO TRACK EAST-NORTHEASTWARD,
ALONG THE NORTHERN PERIPHERY OF THE SUBTROPICAL RIDGE, THROUGHOUT
THE FORECAST PERIOD. LANDFALL IS EXPECTED TO OCCUR JUST AFTER TAU
12, NEAR SHIMOSHIMA ISLAND. REGARDING INTENSITY, 18W IS FORECAST TO
MAINTAIN INTENSITY OVER THE NEXT 12 HOURS, UP UNTIL THE TIME OF
LANDFALL AS THE ENVIRONMENT REMAINS OVERALL FAVORABLE. 18W IS NOW
FORECAST TO HAVE GALE-FORCE WINDS, PRIMARILY CONFINED TO THE
SOUTHERN SEMICIRCLE, DURING LANDFALL. HOWEVER, AFTER TAU 12,
EASTERLY SHEAR GREATLY INCREASES AS A RIDGE BUILDS IN OVER KOREA
AND BRINGS IN ENHANCED UPPER-LEVEL LEVEL WINDS DIRECTLY OVER THE
SYSTEM. AS A RESULT, 18W IS EXPECTED TO QUICKLY WEAKEN OVER KYUSHU.
DISSIPATION IS FORECAST TO OCCUR NO LATER THAN TAU 48, NEAR
SHIKOKU. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN MODERATE
AGREEMENT WITH MODELS GENERALLY AGREEING ON THE EAST-NORTHEASTERLY
TRACK. THE 1200Z ENSEMBLE RUNS OF GEFS AND ECENS BOTH HAVE THE MEAN
MAKING LANDFALL IN CENTRAL TO SOUTHERN KYUSHU. THE JTWC TRACK
FORECAST IS PLACED WITH MEDIUM CONFIDENCE, CLOSE TO THE MULTI-MODEL
CONSENSUS. INTENSITY GUIDANCE IS SLIGHTLY SPLIT WITH COAMPS-TC (GFS
BASED) SUGGESTING MARGINAL INTENSIFICATION PRIOR TO LANDFALL WHILE
HAFS-A SUGGESTS A STEADY INTENSITY THROUGH TAU 12. MODELS ALL AGREE
ON WEAKENING AFTER TAU 12. THE JTWC INTENSITY FORECAST IS PLACED
CLOSER TO THE HAFS-A WITH MEDIUM CONFIDENCE.

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