Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 182359 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 359 PM PST Sun Feb 18 2018 .SYNOPSIS...Strong north and northeast wind this evening will spread a cold and dry air mass from western Canada into Western Washington. A widespread hard freeze is expected each night from tonight through Tuesday night with a few record lows possible. Cold northwest flow aloft will continue through next weekend. A weak system dropping down offshore late Tuesday could bring a little snow to the coast and Southwest Washington. Other systems around Wednesday night, Friday, and next Sunday could bring more precipitation, with lowland snow possible with the Wednesday night and Friday systems. && .SHORT TERM...A strong Fraser outflow event is ongoing right now, with high wind gusts around 60 mph measured this afternoon in the San Juans and over Western Whatcom County. The BLI-CYWL gradient is now easing, so northeast wind near Bellingham and the San Juans should respond in the next few hours. Through the Puget Sound lowlands, an uncommonly strong north wind is occurring right now too with gusts to 41 mph last hour at Sea-Tac. The OLM-BLI gradient is currently near -5 mb. Northerly wind in the Puget Sound region will ever so slowly ease this evening and overnight. The strong north winds are quickly bringing a cold and dry continental air mass into the region out of western Canada. With increasingly dry air, the remaining snow showers over the southern half of the forecast area will continue to fade this evening. The last of the Winter Weather Advisories will be allowed to expire at 4 pm. The coldest air mass of the winter will be in place tonight through at least Monday night. A widespread hard freeze is forecast tonight, Monday night, and possibly Tuesday night. A couple of sites could reach record low temperatures. Hoquiam is one such site where a record low is being forecast tonight. Lows in the low-mid 20s will be common over the next two nights. Sunny skies forecast during the day on Monday. Late Tuesday, the ECMWF is consistent with its previous solutions in dropping a system southward off the coast and slopping some precipitation onto the coast and into Southwest Washington. A growing minority of GFS ensemble members bring a few hundredths of precip to Hoquiam late Tuesday as well, and the 18z NAM brings precip to within 25 miles of the coast. If precip moves onshore, the air mass will still be cold enough for snow to sea level. Have about 20-40 percent PoPs late Tuesday for the coast and SW Wa, but have a dry forecast from Seattle metro on north. This system should exit southward early Wed, with a mainly dry but cool day expected. Temperatures on Wed will still be well below normal. Models agree in dropping another shortwave down along the southeast Alaska and B.C. coasts and into Western Washington Wed night and early Thu. The NAM is perhaps most bullish with precip amounts in the warm advection pattern preceding the shortwave. The air mass will still be sufficiently cold for all snow, except perhaps at the immediate coast of Grays Harbor County. Possibilities over the lowlands range from non-impactful flurries to a few inches of snow. The NAM favors neither the coast nor inland, while the EC favors the coast. The latest GFS just has flurries. Ensembles point to a range of possibilities, most on the lighter side. Haner .LONG TERM...A highly amplified longwave pattern will continue, with cold north-northwest flow aloft from Thursday through next weekend. Looks like additional systems dropping down in cold northwest flow aloft around Friday and next Sunday. The Friday system could see a transition from some snow to rain as warmer air spreads inland. Snow levels by next Sunday should rise above the lowlands, perhaps to 1000 to 1500 feet. Still, below normal temperatures. Haner && .AVIATION...Low pressure extended along the Washington/Oregon border this afternoon with cold high pressure over British Columbia. Snow showers over southwest Washington will dry up late this afternoon and evening and clearing skies from the north will push south through the evening, with mostly clear skies over most of Western Washington tonight. Strong Fraser outflow will continue through the evening from Western Whatcom county out through the San Juans. KSEA...Dry cold air and northerly winds as the cold air continues to come down the Fraser tonight. Skies should be mostly clear from here on out til weather systems arrive starting on Tuesday. && .MARINE...The modified arctic front shifted south through Western Washington today and strong northeast pressure gradients with Fraser outflow will continue through the evening, easing later tonight and Monday morning. It looks like a series of weather systems will drop down out of the northwest Tue-Sat so there will probably be about three periods where fairly light southerly winds develop, and then a pretty decent push of NW winds behind each front comes through the region--especially later in the week when the systems seem be be successively stronger. && .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory until 4 PM PST this afternoon for Cascades of Pierce and Lewis Counties-East Puget Sound Lowlands-Southwest Interior. High Wind Warning until midnight PST tonight for San Juan County- Western Whatcom County. PZ...Small Craft Advisory for rough bar until 4 AM PST Monday for Grays Harbor Bar. Small Craft Advisory until 2 AM PST Monday for Admiralty Inlet- Coastal Waters From Cape Flattery To James Island 10 To 60 Nm-Coastal Waters From Cape Flattery To James Island Out 10 Nm-Coastal Waters From James Island To Point Grenville 10 To 60 Nm-Coastal Waters From James Island To Point Grenville Out 10 Nm-Coastal Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville To Cape Shoalwater Out 10 Nm-Puget Sound and Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca. Gale Warning until 4 AM PST Monday for Central U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters Including The San Juan Islands. && $$ www.weather.gov/seattle

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