Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 230337 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 840 PM PDT Fri Sep 22 2017 .SYNOPSIS...A weak north to south moving weather system will brush the coast Saturday with clouds and a threat of light rain for the coast and Olympics. Another weak system Sunday night and Monday moving into British Columbia should bring more clouds and a chance of light rain to mainly northern portions of the area. Then, higher pressure aloft is expected to build offshore Tuesday into mid week for a return to sunshine and warmer temperatures. && .SHORT TERM...The positive tilt upper ridge offshore along about 135W has a lot of moisture moving through it. One weak shortwave along the central B.C. coast this evening is forecast to track south just off the coast through Saturday with mainly mid and higher level clouds and a threat of light rain to the coast and Olympics. There should be enough passing mid level clouds overnight to limit any areas of fog to primarily the south Sound/SW interior Saturday morning. The upper ridge should keep the weather dry Saturday night and Sunday. Anticipate a bit more in areas of fog in the usual fall prone areas Sunday morning. Then the next shortwave moving over the ridge is expected to track onshore into B.C. Sunday night and Monday for additional cloud cover and a threat of light rain mainly across the northern part of Western Washington. This system should weaken further as it moves onshore and inland as heights aloft rise. High temperatures this weekend should remain close to or a little below late September averages. Buehner .LONG TERM...from the previous edition... 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...Western WA lies in between a trough over the Great Basin and a ridge offshore. The flow aloft is northerly. The air mass is dry and stable with mainly mid-high level clouds. Patchy fog may form early Saturday morning, mainly in the valleys. Light rain is possible along the coast where there is weak warm air advection early Saturday morning and afternoon. The interior will remain dry. The ridge will shift inland Saturday night for dry conditions. 33 KSEA...Light south winds and VFR this evening. Patchy low clouds or stratus may form early Saturday morning for brief MVFR conditions. Otherwise dry and VFR through Sat night. 33 && .MARINE...High pressure over the NE Pacific will maintain light onshore flow this weekend. A warm front will clip the area on Monday with possible Small Craft Advisory winds over the Coastal Waters. The flow will turn offshore by midweek as a thermal trough forms along the coast. 33 && .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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