Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 220007 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 407 PM PST Wed Feb 21 2018 .SYNOPSIS...A weak system southeast across Western Washington this evening, bringing light lowland snow. A break in the weather is expected between systems on Thursday. A stronger system will give wet weather to the area Friday into Saturday with low snow levels. Cool northwest flow aloft will continue into the middle of next week, with cool wet systems around Sunday and then next Tuesday or Wednesday. && .SHORT TERM...Lowland snow will be the main forecast challenge in the days ahead. However, amounts do not look particularly heavy. Cool north-northwest flow aloft will continue for the next week. In the short term, satellite imagery shows the next shortwave in this flow pattern diving S-SE down the B.C. coast and approaching the northwest Olympic Peninsula. The shortwave trough axis will move southeast across Western Washington late this evening, around 06z or 10 pm. In advance of the trough axis, 850 mb progs show some westerly flow off the Pacific and a warm advection pattern, which leads to lift. Meanwhile, the surface reflection will be a weak 1020 mb low which will move S-SE between the coast and 130W, so surface flow will never turn strongly onshore. This will help the low-level air mass stay cold enough for precip type to remain snow, except perhaps at the immediate coast this evening. Many sites have already reported small snow fall amounts, and radar shows the main band of snow now spreading southeast across the northwest Olympic Peninsula. Runs of the HRRR models this afternoon have added a little QPF this evening around Olympia and Tacoma, so have recently expanded the Winter Weather Advisory eastward by one more layer of zones, adding the Admiralty Inlet area, Bremerton area, and Tacoma area. Locales outside of the advisory area are also likely to see some snow but are likely to remain under 1 inch on accumulations. This includes Seattle and Everett. Upper heights will rise nicely on Thu morning, and dry northeast flow aloft will quickly clear out skies in the AM. A sunny Thu afternoon is expected, though the air mass will still be cool. Shortwave ridge axis will move southeast past the forecast Thu night. Northwest flow aloft will bring another shortwave down the B.C. coast on Friday PM, with the height fall center passing turning east-southeast near Vancouver B.C. on Fri evening. A slowly intensifying 1012 mb surface low will pass across the far North Cascades late Friday night. To its south, a turn to onshore flow with southerly gradients will bring a good shot of warm advection and lift on Friday afternoon and evening. With a cold air mass to begin, most lowland locales will start as snow. The coast should quickly change to rain soon after precip onset. Away from the coast, the transition to rain is more likely to occur Friday evening after starting with 1-3 inches of snow during the afternoon. A trailing cold front will move across on Sat morning, with a few post-frontal rain and snow showers continuing. With a cooling air mass behind the front on Sat, some more localized accumulations are possible, especially where there is a focus for lift such as in a Puget Sound Convergence Zone. On a larger scale, weather should be drying out over the course of Saturday. .LONG TERM...Northwest flow pattern will continue. Another system is expected Sunday. Air mass will not be as cold to start with, so precip will come in the form of rain over the majority of the lowlands. However, some post-frontal snow showers are possible on Sunday night. Next similar system expected late Tuesday or next Wednesday. Haner && .AVIATION...A weak upper level trough sliding into Western Washington has brought MVFR ceilings with areas of light snow to the northern portion of the area this afternoon. This will continue to spread south this evening. Snow showers will remain light especially through the metro area where shadowing will help to further limit what is already a weak system. Expect showers to taper off from the north by late evening with ceilings and visibilities lifting to VFR levels after midnight. Flow aloft will remain northerly Thursday as the upper level trough moves south of the area. KSEA...VFR conditions will linger into this evening. Light snow showers associated with the weak system moving in from the north have already begun to affect the terminal. Expect lower MVFR conditions and light snow accumulations - less than an inch of snow by late evening. Ceilings and visibilities will return to VFR levels by 08Z-10Z. South winds 8-10 KT will become easterly 5-7 KT after midnight. && .MARINE...A surface low moving south along the British Columbia Coast and onto the northern Oregon coast this evening will bring light snow showers to area waters this evening. Increasing northeasterly flow behind the low later tonight into early Thursday will bring small craft advisory conditions to the coastal waters as well as the Eastern Strait and Northern Inland Waters. High pressure will move SE across the waters Thursday afternoon followed by another surface low expected to move SE across area waters on Friday. This system is expected to bring widespread small craft advisory conditions on Friday. A set of stronger systems Saturday and Sunday will affect the area and may result in gale conditions over some of the waters over the weekend. && .HYDROLOGY...River flooding is not expected the next 7 days. && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory until 3 AM PST Thursday for Admiralty Inlet Area-Bremerton and Vicinity-Central Coast-Tacoma Area. Winter Weather Advisory until 7 AM PST Thursday for Hood Canal Area-Lower Chehalis Valley Area-Southwest Interior. Winter Weather Advisory until 7 AM PST Thursday for Eastern Strait of Juan de Fuca. Winter Weather Advisory until 3 AM PST Thursday for San Juan County-Western Strait of Juan De Fuca. Winter Weather Advisory until 9 PM PST this evening for North Coast. PZ...Small Craft Advisory from 11 PM this evening to 11 PM PST Thursday for 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. Small Craft Advisory from 6 AM to 6 PM PST Thursday for 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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