Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 222219 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 320 PM PDT Fri Sep 22 2017 .SYNOPSIS...A weak weather system will brush Western Washington tonight and Saturday morning with clouds and a chance of light rain along the coast. A weak upper level ridge will remain over the region this weekend but will allow a weak weather system to bring more clouds and a chance of light rain to the coast and the northern portions of the area on Sunday. A slightly stronger system could bring light rain to more of the area on Monday. The ridge will build Tuesday or Wednesday with a return to mostly sunny skies and warmer temperatures. && .SHORT TERM...The large upper level ridge just offshore will tilt inland over southern B.C. this weekend. The ridge will be weak enough so that any transient weather systems will passing through the ridge will spread clouds across W WA and some light rain at times. The first system, a weak warm front, tonight and Saturday, will remain mostly just offshore, but will be close enough to bring a chance of light rain to the coast and Olympics and adjacent inland areas. Over the interior mid and high level moisture will keep skies partly to mostly cloudy. The models have finally become a little more consistent and now keep the area mostly dry Saturday night and Sunday. Another weak system may brush the far north part of W WA with some light rain on Sunday. The best chance of rain should be on Monday when another weak warm front moves across the area. Low level moisture should be plentiful, but POPs will still be in the chance range, except for the coast. Rainfall amounts should generally be just a few hundredth of an inch, except maybe a tenth of an inch for the coast. All the cloud cover will keep high temperatures in the lower to mid 60s through Monday, which is a few degrees below normal. Kam .LONG TERM...Tuesday will be kind of a transition day as the upper level ridge builds offshore. There still appears to be a lot of residual low level moisture but this may be clearing out in the afternoon. The models have been generally consistent with the building ridge for mid week. It now looks strong enough to keep skies mostly clear and warm daytime temperatures into the lower 70s in the warmest spots. The models differ a bit on just how quickly the ridge will shift E of the area, but for now the forecast remains dry through Friday. Kam && .AVIATION...High pressure will remain just off the Pacific Northwest coast through Saturday with light northerly flow aloft. The air mass will be mostly dry with residual moisture near the surface. Patchy low clouds and fog will be possible in fog prone locations Saturday morning such as HQM and OLM. Mid and high clouds will increase and hinder any widespread IFR fog/stratus for developing elsewhere. Will plan to cut back the duration of IFR stratus at most TAF sites around Puget Sound. KSEA...VFR conditions. A few hours of IFR or low MVFR possible Saturday morning. Northerly wind to 5 kt becoming light after sunset. && .MARINE...A large area of high pressure will dominate the northeast Pacific including the Western Washington waters through the weekend. A warm front will clip the region on Monday with a chance for small craft winds, mainly in the coastal waters. Northerly or offshore flow develops mid week. dtm && .SEW WATCHES/WARNINGS/ADVISORIES... WA...None. PZ...None. && $$ www.weather.gov/seattle You can see an illustrated version of this discussion at www.wrh.noaa.gov/sew/gafd/latest_webafd.html

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