WDPN32 PGTW 192100

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

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

WARNING NR 003//

RMKS/

1. FOR METEOROLOGISTS.

2. 6 HOUR SUMMARY AND ANALYSIS.



SUMMARY:

   INITIAL POSITION: 28.6N 126.8E

   INITIAL INTENSITY: 25 KTS

   GEOGRAPHIC REFERENCE: 137 NM NORTH-NORTHWEST OF KADENA AB

   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 07 KTS

   SIGNIFICANT WAVE HEIGHT: 12 FEET



SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:

ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A FULLY

EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH FLARING DEEP

CONVECTION. MODERATE POLEWARD AND EQUATORWARD OUTFLOW IS EVIDENT ON

WATER VAPOR IMAGERY. AN UPPER-LEVEL CYCLONE IS POSITIONED OVER THE

SOUTH CHINA SEA ENHANCING THE EQUATORWARD OUTFLOW, AND THE POLEWARD

OUTFLOW COMPONENT WEAKLY DRIFTS TO THE NORTHEAST. DRY AIR DIRECTLY

SOUTH OF TROPICAL DEPRESSION (TD) 18W HAS CURTAILED DEEP CONVECTION

AND HAS IMPOSED UPON THE VERTICAL STRUCTURE, CAUSING SLIGHT

WEAKENING TO 25KTS OVER THE LAST SIX HOURS. THE INITIAL POSITION IS

PLACED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED ANIMATED

EIR, AND DOPPLER RADAR IMAGERY HIGHLIGHTING THE CLOUD BANDING

SURROUNDING THE CORE OF THE CIRCULATION. THE INITIAL INTENSITY OF

25 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY AND

OBJECTIVE FIXES LISTED BELOW.



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

RADII).



CURRENT STEERING MECHANISM: A SUBTROPICAL RIDGE TO THE EAST.



AGENCY DVORAK AND AUTOMATED FIXES:

   PGTW: T2.0 - 30 KTS

   RJTD: T1.5 - 25 KTS

   KNES: T1.5 - 25 KTS

   CIMSS SATCON: 34 KTS AT 191900Z

   CIMSS ADT: 32 KTS AT 191730Z



FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL

   VWS: 10-15 KTS

   SST: 28-29 CELSIUS

   OUTFLOW: MODERATE DUAL-CHANNEL



ANALYSIS CONFIDENCE:

   INITIAL POSITION: MEDIUM

   INITIAL INTENSITY: MEDIUM

   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: TD 18W WILL TRACK NORTH-NORTHEASTWARD DURING

THE NEXT 24 HOURS. AFTER TAU 24, THE SYSTEM WILL CURVE TO A

NORTHEASTWARD TRACK WHILE ROUNDING THE SUBTROPICAL RIDGE. IN THE

EARLY-TERM FORECAST, DRY AIR WILL BE THE PRIMARY INHIBITOR FOR

DEVELOPMENT. VERTICAL WIND SHEAR WILL BE LOW DURING THE FIRST 24

HOURS, ALLOWING FOR INTENSIFICATION TO 30KTS. AFTER TAU 24,

VERTICAL WIND SHEAR WILL INCREASE TO 20-25KTS, CAPPING

INTENSIFICATION HAVING REACHED A PEAK OF 30KTS. VERTICAL WIND SHEAR

WILL CONTINUE TO INCREASE AS THE SYSTEM TRACKS NORTHWARD TOWARD THE

UPPER-LEVEL JET, RESULTING IN A WEAKENING PERIOD STARTING AT TAU

48. TD 18W WILL FULLY DISSIPATE BY TAU 72.



MODEL DISCUSSION: THE JTWC MULTI-MODEL CONSENSUS MEMBERS ARE IN

GOOD AGREEMENT IN THE FIRST 24 HOURS OF THE FORECAST

PERIOD WITH A CROSS-TRACK SPREAD OF 50NM. HOWEVER, THE SPREAD

INCREASES SIGNIFICANTLY AFTER TAU 24 TO 350NM BY TAU 72.

BIFURCATION BY THE END OF THE FORECAST CONTRIBUTES TO LOW CONFIDENCE

IN THE JTWC TRACK FORECAST. THE UKMET AND ECMWF ENSEMBLE TRACKERS

CURVE TO THE NORTHWEST AFTER TAU 24, WHILE GFS, GEFS, NAVGEM, AND

ECMWF ALL CURVE TO THE NORTHEAST. THE JTWC TRACK CURVES

NORTHEASTWARD BASED ON THE CURRENT DEEP-LAYER RIDGE TO THE EAST

DRIVING THE STORM NORTHWARD AND THEN NORTHEASTWARD. THE JTWC

INTENSITY GUIDANCE IS IN GOOD AGREEMENT, REPRESENTING PEAK

INTENSITIES BETWEEN 25-35KTS BETWEEN TAU 24-36. AFTERWARDS, THE

INTENSITY GUIDANCE MEMBERS CONSISTENTLY REPRESENT WEAKENING,

LENDING TO HIGH CONFIDENCE IN THE INTENSITY FORECAST.



FORECAST CONFIDENCE:

   TRACK 0 - 72 HR: LOW

   INTENSITY 0 - 72 HR: HIGH//

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