Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000 FXUS66 KSEW 210007 AFDSEW Area Forecast Discussion National Weather Service Seattle WA 330 PM PST Tue Feb 20 2018 .SYNOPSIS...Light snow across southwest Washington this evening will shift south into Oregon by midnight. Partial clearing later tonight will allow temperatures to fall with Wednesday morning lows in the low to mid 20s, except upper teens possible in colder valleys. Another relatively weak system will drop south just off the coast Wednesday night and may bring some light snow, mainly to the coast. A break between systems later Thursday && .SHORT TERM...Radar imagery was interesting to watch today as bands of precipitation expanded over the southwest interior and north over parts of central/south Puget Sound. Steadier snowfall and accumulations mainly stayed within the advisory area covering Thurston, Mason, Lewis, and Grays Harbor county. A spotter in Thurston county reported 3.5 inches at 300 pm but also noted snowfall was decreasing significantly. So far, little or no snow has accumulated in the main Tacoma to Seattle/Bellevue I-5 corridor as anticipated. pockets of light snow or flurries are still falling around the Sound, but this should shift south early this evening and end. skies may partially clear later tonight and temperatures should drop back to the low or mid 20s again. Locations that have fresh snow on the ground, mainly the southwest interior, could dip to the upper teens if clouds clear quick enough. Wednesday will be dry most of the day, but another weak low pressure system arriving from the north will begin to spread light snow into the northern coastal areas late in the day. This system is weaker than the current low, but is much closer in proximity to the coast. Almost all models keep most precipitation near the coast and imply an inch or two could fall from Forks to Hoquiam/Aberdeen Wednesday night, with just flurries or very light snow over the interior and not likely to amount to more than a dusting if that. Another break between systems is expect later Thursday and Thursday night. The flow aloft will remain northerly with the long wave ridge still centered out near 140-150W. This should maintain the northerly flow pattern and keep temperatures from warming much until the next stronger system arrives Friday. .LONG TERM...Global and longer range meso models agree on driving a strong front south-southeast through the area Friday. It should produce some breezy or locally windy conditions and a period of onshore flow. Models disagree considerably on the trajectory of the approaching frontal wave and how much temperatures moderate before precipitation develops. The ECMWF has been the most consistent in driving cold air aloft over the area with the frontal precipitation, limiting thickness rises. Unfortunately the 12z ECWMF was not available today, and will be interesting to see the the 00z solution tonight. The GFS moderates temperatures the most with 1000- 850 mb thickness values rising to near 1300M and fluctuating around that value into the weekend. That is often the value referred to for determining if snow levels reach near sea-level. This late in the winter season, thickness values likely need to be lower than 1300M to see snow down to sea-level. There may also be the issue of shadowing off the Olympics depending on track of the approaching front and flow aloft. Much of Puget Sound could be end up with much lighter rain/snow amounts than the rest of Wrn WA. The pattern looks great for orographic enhanced snowfall along the Cascades with snow levels below passes. Ski resorts could pick up over a foot of snow Friday/Friday night and possibly more over the weekend. A cold showery trough will reside over the region over the weekend. Some models bring a system in from the west and introduce a threat of lowland snow on Sunday. Other models like the GFS keep a closed low over the region with rain/snow showers but would not be a steady significant precipitation producer. However, this would maintain the ridge off to the west and could allow additional cold northern stream systems to arrive early next week. Mercer && .AVIATION...Northerly flow aloft will continue tonight, then become more westerly Wednesday as a weak upper trough approaches the area. The air mass is stable with areas of high and mid level moisture over the north, and low level moisture over the south. South of a KHQM-KSHN-KTCM line, precipitation will continue as light snow tonight. North of that line there are light snow flurries and even some rain mixed in. The northwest interior has been dry all day. Northerly winds tonight will gradually push all of the precipitation and most of the clouds out of the region. A weak system Wednesday will increase cloudiness in the afternoon, but any precipitation will probably be confined to the coast. Areas of MVFR or even IFR ceilings with light snow will persist in the southwest interior this evening. VFR ceilings 4-6k ft in the north gradually improve to no ceilings in the far north. Conditions will improve tonight, with gradual clearing from the north. KSEA...VFR ceilings will continue this evening, with light rain or snow showers possible in the vicinity of the terminal. No snow accumulation is expected. Ceilings will rise this evening with just scattered or broken high clouds into Wednesday morning. Northerly winds will increase to 8-12 kt this evening. CHB && .MARINE...Winds will become northerly and offshore tonight as a weak low center moves south off the Oregon coast. Northeast winds will increase to small craft advisory strength in the Northern Inland Waters and through the Strait of Juan de Fuca tonight. Have added a small craft advisory for northerly winds in Puget Sound later tonight. A weaker low will move south through the area Wednesday night. At this time, winds are forecast to remain below 20 kt. A stronger system will arrive Friday and will remain around the area through the weekend. This system is likely to bring at least small craft advisory strength winds to all waters, and could bring gale southerlies or westerlies to some waters. CHB && .SEW WATCHES/WARNINGS/ADVISORIES... WA...Winter Weather Advisory until 6 PM PST this afternoon for the Central Coast. Winter Weather Advisory until 10 PM PST this evening for Lower Chehalis Valley Area-Southwest Interior. PZ...Small Craft Advisory until 3 AM PST Wednesday for Central U.S. Waters Strait Of Juan De Fuca-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-East Entrance U.S. Waters Strait Of Juan De Fuca- Northern Inland Waters Including The San Juan Islands-West Entrance U.S. Waters Strait Of Juan De Fuca. Small Craft Advisory from 7 PM this evening to 4 AM PST Wednesday for Puget Sound and Hood Canal. && $$ www.weather.gov/seattle

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