Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 201026 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 326 AM PDT Wed Sep 20 2017 .SYNOPSIS...The chilly low pressure system that has been over the region the past couple of days will slowly move east through Thursday for a gradual drying and warming trend. High pressure aloft will prevail on Friday for sunnier, drier, and warmer conditions. && .SHORT TERM... Showers have been mainly confined to the southwest quarter of the CWA overnight. There will be a slight northward shift of the precip band during the next few hours as it pivots around the upper level low and becomes east-west oriented. The heavier precip is expected to be south of the CWA. The air mass will be marginally unstable today. It looks like the best chance of thunderstorms will be over the coastal waters. Meanwhile, the chilly upper level trof that has been over the region the past couple of days will move slowly east through Thursday. This will not only cause the flow aloft to become northerly but will also allow for a gradual drying and warming trend. Expect the region to become more under the influence of an amplified upper level ridge centered well offshore on Friday. The main impact will be sunnier, drier, and warmer conditions. .LONG TERM... The medium range solutions were suggesting that the tail end of frontal systems passing to our north might bring a threat of light rain to the northwest part of the CWA this weekend into Monday. The GFS was starting to come more in line with the wetter ECMWF solution. For now, decided to keep the inherited dry forecast but will need to introduce rain if the 1200 UTC cycle continues with the idea of wetter conditions. At any rate, expect the ridge to be a bit dirty (mid and/or high clouds streaming across the area) through Monday. Anticipate the upper ridge to rebound the end of the period for dry weather with near normal temps. && .AVIATION...Upper level trough over the area with light flow aloft becoming northerly this afternoon and evening. Air mass remaining moist and somewhat unstable with a drying trend north portion after 00z. Multiple cloud layers between about 2000 feet and 7000 feet this morning with the 2000 foot deck becoming predominately broken 12z-15z. Local ceilings below 1000 feet until 18z. Ceilings improving this afternoon with the lower cloud layer dissipating leaving a scattered to broken decks around 4000 and 600 feet. Clearing from the north tonight. KSEA...Ceilings 1000-2000 feet early with vfr ceilings by 19z. Southerly winds 4 to 8 knots switching to northwesterly around 19z. Felton && .MARINE...Trough moving over Oregon today with surface gradients remaining light. Weak surface low moving south through the coastal waters Thursday morning. High pressure building in on Friday and remaining over the weekend. Felton && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...Small Craft Advisory from 6 PM this evening to 2 AM PDT Thursday for Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm. && $$ www.weather.gov/seattle An illustrated version of this forecast discussion can be seen at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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