textproduct: Seattle/Tacoma

SYNOPSIS

Lowland stratiform rain and heavy mountain snow expected to continue today. However, a colder air mass will keep the chance of a rain/snow mix for central Puget Sound through this morning. Conditions expected to remain unsettled this weekend and into next week as an upper level ridge builds over the region, bringing milder temperatures.

SHORT TERM /TODAY THROUGH SUNDAY/

Latest observations show some locations across the lowlands getting light snow or rain/snow mix early this morning. A cooler air mass over the region will have snow levels down to 300-1000 ft early this morning, before rising above 1000 ft late this morning. High uncertainty still remains on snow accumulation for the lowlands. Accumulations will be highly dependent on location, surface and ground temperatures, and where the highest precipitation rates set up. In addition, surface temperatures are expected to remain just above freezing through the morning, leading to highly variable accumulations if there are any. So far light snow has been observed at locations where the temperature has decreased to around 34 degrees or lower, while locations hovering at 36 degrees have just seen rain. There is moderate to high confidence that central Puget Sound may see some snow or rain/snow mix. Expect these wintery conditions to continue through 10 AM. If any snow does accumulate, it will likely be grassy surfaces, elevated surfaces, and the foothills. A stalled frontal boundary in Oregon associated with higher snow levels will drift northwards at times fluctuating snow levels between 1000-3000 ft along the southern interior. These higher snow levels will keep the precipitation type as rain in this area. With the boundary stalled to the south, expect moderate to heavy rain in the southern interior.

Heavy snow will also continue across the Olympics and Cascades through early Saturday morning and will taper off into the weekend. A Winter Storm Warning continues for the mountains. Expect to see an additional 1 to 3 feet of snow for the Cascades passes and 1 to 6 inches for the Olympics.

On Saturday, the frontal boundary finally moves southward and away from Western Washington. Slightly colder air will move into the region, dropping snow levels across the lowlands to 200 to 800 ft Friday night into Saturday morning. This will bring another chance for snow. However, this time there will be limited moisture and will be more showery in nature. Confidence on snow accumulation remains low at this time and again will be highly dependent on where the snow showers develop and precipitation rates.

An upper level ridge will build over Western Washington late Saturday night, drying out conditions briefly before a weak atmospheric river moving into British Columbia clips the region. Snow levels will start out low Sunday morning around 300-1000 ft, but will increase throughout the day as warmer air moves into the region.

MGF

LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/

Snow levels are expected to rise to 5000-8000 ft by late Sunday into early Monday morning and will hover around around these values into next week as well. This will result in a snow to rain transition on late Sunday into early Monday. As a result, mountain snow will be limited to the highest peaks.

The previously mentioned upper level ridge will continue over Western Washington and will bring milder temperatures to the region. A gradual warm up is expected throughout next week. Temperatures early next week will be in the 50s and by midweek will be peaking in the upper 50s to low 60s. Additionally, multiple weak atmospheric rivers will ride along the ridge keeping conditions wet and unsettled.

MGF

AVIATION

Westerly flow aloft will continue through the TAF period as a strong upper level jet remains over western Washington today. Precipitation will continue across the region, though will gradually shift southwards throughout the day. A mixed bag of conditions continues out there early this morning, though the majority of the terminals are trending MVFR to IFR and are either seeing snow falling or a rain/snow mix. While snowfall accumulations are expected to be limited, if rates remain heavy, minor snow accumulations can be expected on paved surfaces, including snow sticking onto airport surfaces through the early morning hours.

Expect lower ceilings and visibilities to hang on through the morning hours with abundant surface moisture and light low level flow. Conditions may further lower to LIFR in any areas of heavier precipitation. Expect some improvement for northern terminals by mid-morning, though expect cigs to remain MVFR/IFR for the majority of terminals through this afternoon. Winds remain light and variable at primarily 6 kt or less.

KSEA...Conditions at the terminal are IFR with light snow falling this morning. Snow may stick to airport surfaces if snowfall rates remain heavy enough, particularly between 10- 16Z. Otherwise, any lingering precipitation will likely transition back to rain between 16-18Z. IFR ceilings and visibilities will continue at the terminal through the morning hours and could drop to LIFR in any heavier precipitation. Expect ceilings to slowly improve to low-end MVFR by late afternoon. Winds are light out of the E/NE at 6 kts or less this morning and will become more S/SE by the afternoon.

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MARINE

A stationary front will remain over the southern portion of the coastal waters today, before moving south out of the area on Saturday as high pressure builds into the coastal waters. Winds and seas have come down below advisory criteria, with both expected to remain below criteria throughout the day. Northerly winds will then increase across the coastal waters overnight into Saturday, with probabilistic guidance indicating roughly a 60-80 percent chance of gusts approaching small craft. Seas will hover between 6-9 ft through Saturday, before subsiding towards 4-6 ft on Sunday. A warm front will then brush the outer coastal waters on Monday, potentially bringing gusty southerly winds to the coastal waters and portions of the interior waters. Seas will then build towards 10-13 ft and hover at this range through midweek. The trailing cold front looks to approach the area waters mid to late week.

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HYDROLOGY

The Skokomish River has fallen below flood stage and is expected to continue to recede over the next few days. While precipitation will continue throughout the day today, snow levels hovering between 1000-2500 ft around Mason County should mitigate any additional flood risk on the Skokomish today.

There does, however, remain a threat of flooding for rivers across the Southwest Interior today and into the weekend with the heaviest precipitation expected to remain over the area today. This, combined with higher snow levels, may bring flooding to rivers in the Chehalis Basin including but not limited to the Skookumchuck, Newaukum and the Chehalis River from Grand Mound to Grays Harbor. Therefore, a Flood Watch remains in effect for these areas.

The threat for flooding will briefly ease on Sunday before an atmospheric river is expected to move inland to the north early next week. This could bring additional flooding concerns to portions of the area- with rivers over the northern portion of the area bearing watching through the first part of the week.

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SEW WATCHES/WARNINGS/ADVISORIES

WA...Winter Storm Warning until 5 AM PDT Saturday for Cascades of Pierce and Lewis Counties-Cascades of Snohomish and Northern King Counties-Cascades of Southern King County- Cascades of Whatcom and Skagit Counties-Olympics.

Flood Watch through Sunday afternoon for Cascades of Pierce and Lewis Counties-Foothills and Valleys of Thurston and Lewis Counties-Foothills of the Western and Southern Olympic Peninsula-Grays Harbor County Coast-Lower Chehalis River Valley-Lowlands of Lewis and Southern Thurston Counties-Middle Chehalis River Valley-Olympia and Southern Puget Sound-Olympics-Willapa and Black Hills.

PZ...None.