WDPN32 PGTW 200900

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//

SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN)

WARNING NR 005//

RMKS/

1. FOR METEOROLOGISTS.

2. 6 HOUR SUMMARY AND ANALYSIS.



SUMMARY:

   INITIAL POSITION: 30.3N 127.9E

   INITIAL INTENSITY: 30 KTS

   GEOGRAPHIC REFERENCE: 191 NM SOUTH-SOUTHWEST OF SASEBO, JAPAN

   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 10 KTS

   SIGNIFICANT WAVE HEIGHT: 12 FEET



SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:

ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY-

EXPOSED LOW-LEVEL CIRCULATION (LLC), WITH FLARING DEEP CONVECTION.

THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON MSI.

THE INITIAL POSITION IS ALSO SUPPORTED BY THE 200708Z SSMIS 91GHZ

MICROWAVE IMAGE, WHICH REVEALS A DEFINED LLC POSITIONED JUST

SOUTHWEST OF A SMALL REGION OF DISORGANIZED DEEP CONVECTION. THE

INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON

RECENT ASCAT IMAGERY. ENVIRONMENTAL CONDITIONS ARE MARGINAL, WITH

FAIR OUTFLOW OFFSET BY DRY AIR ENTRAINMENT.



INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND

RADII).



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 SATCON: 39 KTS AT 200503Z

   CIMSS ADT: 35 KTS AT 200530Z

   CIMSS AIDT: 43 KTS AT 200530Z

   CIMSS D-MINT: 23 KTS AT 200425Z

   CIMSS D-PRINT: 29 KTS AT 200710Z



FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL

   VWS: 5-10 KTS

   SST: 28-29 CELSIUS

   OUTFLOW: MODERATE EASTWARD

   OTHER FACTORS: DRY AIR ENTRAINMENT



ANALYSIS CONFIDENCE:

   INITIAL POSITION: MEDIUM

   INITIAL INTENSITY: HIGH

   INITIAL WIND RADII: NOT APPLICABLE



3. FORECAST REASONING.



SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO

THE FORECAST FROM THE PREVIOUS WARNING.



FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 18W IS FORECAST TO

TRACK NORTH-NORTHEASTWARD TO 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

FOR SLIGHT INTENSIFICATION TO 35 KNOTS OVER THE NEXT 12 HOURS UNTIL

VERTICAL WIND SHEAR INCREASES TO HIGH (25-40 KNOTS) LEVELS AND DRY

AIR OVERWHELMS THE SYSTEM.



MODEL DISCUSSION: JTWC TRACK CONFIDENCE IS MEDIUM, WITH INCREASING

UNCERTAINTY IN THE EXACT TRACK AFTER TAU 24. IN LARGE PART, THIS IS

DUE TO THE WEAK NATURE OF THE SYSTEM AS IT WEAKENS NEAR LANDFALL. THE

CURRENT SUITE OF DETERMINISTIC GUIDANCE SHOWS A SPREAD FROM WESTERN

CENTRAL KYUSHU TO NORTHERN KYUSHU, WITH JTWC'S TRACK FORECAST ALIGNED

CLOSER TO ECMI, EEMI AND AVNI. THE 191800Z ECMWF ENSEMBLE (ECENS) RUN

SHOWS THE BULK OF THE SOLUTIONS TRACKING THE SYSTEM INTO THE KOREAN

STRAIT AND NORTHERN KYUSHU. THE 200000Z 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 30-35 KNOTS OVER THE NEXT 24 HOURS, WITH UNANIMOUS AGREEMENT ON A

SHARP WEAKENING TREND AFTER TAU 24. THE 191800Z COAMPS-TC (GFS

INITIAL BOUNDARY CONDITIONS) INTENSITY ENSEMBLE SHOWS SOME POTENTIAL

FOR THE SYSTEM TO REACH MINIMAL TROPICAL STORM STRENGTH PRIOR TO

LANDFALL, WITH A HIGH PROBABILITY OF TROPICAL DEPRESSION OR TROPICAL

DISTURBANCE INTENSITY AT LANDFALL AND BEYOND.



FORECAST CONFIDENCE:

   TRACK 00-72 HR: MEDIUM

   INTENSITY 00-72 HR: MEDIUM//

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