WDPN32 PGTW 190900

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

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

WARNING NR 001//

RMKS/

1. FOR METEOROLOGISTS.

2. 6 HOUR SUMMARY AND ANALYSIS.



SUMMARY:

   INITIAL POSITION: 27.1N 127.6E

   INITIAL INTENSITY: 30 KTS

   GEOGRAPHIC REFERENCE: 376 NM SOUTH-SOUTHWEST OF SASEBO, JAPAN

   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 12 KTS

   SIGNIFICANT WAVE HEIGHT: 16 FEET



SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:

AS DEPICTED IN ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI), A

DISTINCT CLOSED LOW-LEVEL CIRCULATION (LLC) HAS FORMED OVER THE

PAST SIX HOURS, WITH LOW-LEVEL CLOUD LINES WRAPPING INTO THE CENTER

AND IMPROVED DEEP CONVECTIVE BANDING. ANIMATED RADAR IMAGERY SUPPORTS

THE INITIAL POSITION WITH HIGH CONFIDENCE, REVEALING EXTENSIVE

CONVECTIVE BANDING OVER THE EASTERN SEMICIRCLE WRAPPING INTO THE

NORTHWEST QUADRANT OF THE LLC. ANIMATED WATER VAPOR IMAGERY SHOWS

EXPANDING POLEWARD VENTING OVER THE NORTHERN AND EASTERN QUADRANTS,

WITH CONVERGENT UPPER-LEVEL FLOW PERSISTING OVER THE SOUTHWESTERN

SEMICIRCLE ASSOCIATED WITH A QUASISTATIONARY UPPER-LEVEL LOW CENTERED

JUST EAST OF TAIWAN. EXTENSIVE DRY AIR IS PRESENT OVER THE

SOUTHWESTERN QUADRANT OF THE SYSTEM LIMITING CONVECTIVE DEVELOPMENT

OVER THIS REGION. SURFACE OBSERVATIONS FROM KADENA AIR BASE INDICATE

SOUTHWESTERLY WINDS AT 10-15 KNOTS, WITH SLP NEAR 1009MB. MAXIMUM

WINDS OBSERVED THUS FAR ARE FROM YORONJIMA (27.0N 128.4E), WHICH

REPORTED ABOUT 22 KNOTS SUSTAINED (1-MINUTE AVERAGE). RECENT ASCAT

IMAGERY INDICATED A SWATH OF 25-30 KNOT WINDS OVER THE EASTERN

SEMICIRCLE, WHICH SUPPORTS THE INITIAL INTENSITY ESTIMATE OF 30

KNOTS.



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.



AGENCY DVORAK AND AUTOMATED FIXES:

   PGTW: T1.5 - 25 KTS

   KNES: T1.5 - 25 KTS



FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE

   VWS: 0-5 KTS

   SST: 29-30 CELSIUS

   OUTFLOW: MODERATE EASTWARD

   OTHER FACTORS: DRY AIR ENTRAINMENT



ANALYSIS CONFIDENCE:

   INITIAL POSITION: HIGH

   INITIAL INTENSITY: HIGH

   INITIAL WIND RADII: NOT APPLICABLE



3. FORECAST REASONING.



SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING

MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.



FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 18W WILL TRACK

GENERALLY NORTHWARD THROUGH TAU 48 ALONG THE WESTERN PERIPHERY OF

THE STR. MODEST INTENSIFICATION TO A PEAK INTENSITY OF 35 KNOTS IS

ANTICIPATED AS THE CORE MOISTENS BRIEFLY AND THE SYSTEM MOVES AWAY

FROM THE UPPER-LEVEL LOW NEAR TAIWAN. AFTER TAU 48, TD 18W WILL

RECURVE AROUND THE NORTHWESTERN PERIPHERY OF THE STR AND WEAKEN

QUICKLY DUE TO INCREASING EASTERLY FLOW AND HIGH (40-45 KNOTS)

VERTICAL WIND SHEAR ASSOCIATED WITH A STRONG HIGH SITUATED OVER THE

KOREAN PENINSULA. ADDITIONALLY, EXTENSIVE DRY AIR WILL PREVAIL ACROSS

THE KOREAN STRAIT AND WESTERN JAPAN, WHICH WILL LEAD TO DISSIPATION

BY TAU 72.



MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN TIGHT AGREEMENT

THROUGH TAU 24, WITH A 40NM CROSS-TRACK SPREAD AT TAU 24. AFTER TAU

24, GUIDANCE DIVERGES WITH EEMI AND AEMI SHOWING TWO EXTREME,

UNLIKELY SOLUTIONS. THE REMAINDER OF THE GUIDANCE IS SPREAD FROM

SOUTHERN SOUTH KOREA TO NORTHERN KYUSHU. THE GEFS AND ECENS ENSEMBLES

ALSO REVEAL A LARGE SPREAD OF SOLUTIONS SUPPORTING THE JTWC FORECAST

TRACK WITH MEDIUM CONFIDENCE. AS MENTIONED, ENVIRONMENTAL CONDITIONS

ARE NOT OPTIMAL THUS RELIABLE INTENSITY GUIDANCE GENERALLY PEAKS AT

35-40 KNOTS NEAR TAU 48, WITH STEADY WEAKENING THROUGH TAU 72.



FORECAST CONFIDENCE:

   TRACK 00-72 HR: MEDIUM

   INTENSITY 00-72 HR: MEDIUM//

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