AgWeatherNet Weekly Weather Outlook Monday Update for December 21 to December 27, 2020

A stormy Monday followed by quiet weather through Thursday

Mark Ingalls, AgWeatherNet Intern
Joe Zagrodnik, AgWeatherNet Postdoctoral Research Associate

Note: In observation of Christmas and the New Year, this is the last AgWeatherNet outlook for 2020. Outlooks will resume Monday, January 4.

Synopsis:

A low pressure system crossing the state Monday will bring rain and snow to most locations along with a frontal boundary that is oriented east-to-west in central Puget Sound and is currently the dividing line between cold rain with snow mixing in near Bellingham and near-record warm temperatures in Seattle and south. The front will move rapidly through the state this evening, bringing colder temperatures, wind, and rain changing to snow in many places. Following Monday's active weather, some rain and mountain snow showers may linger in the Cascades and Blues into Tuesday, but with a strong incoming ridge will keep conditions dry until the next system begins to impact the coast Christmas morning. Rain and mountain snow showers spread eastward and continue into Saturday with a second system potentially bringing light snow to everyone in Eastern Washington. Rain showers continue in Western Washington Sunday, but Eastern Washington looks to end the forecast period with dry conditions.

Discussion:

Rain and snow showers Monday morning were falling in and west of the Cascades. As is typical with lowland snow events in the Puget Sound Region, determining who will and won't get snow is not an easy task. The biggest consideration for this event is whether precipitation will still be present when colder air filters in later this evening. To the north of the boundary in Snohomish and Whatcom counties, light slushy snow accumulations are possible (especially above 500 feet) and some areas of rain/snow mix down to central Puget Sound. Snow is also possible in lowland parts of Clallam County this afternoon, again, especially above 500 ft.

In lowland areas that do see snow, accumulations are expected to be limited by warm ground temperatures. A survey of AgWeatherNet sites around the sound showed soil temperatures at 8 inches deep around 45 degrees in the Bellingham and Seattle areas Monday morning. Another wildcard in all of this is the Puget Sound Convergence Zone. Models are indicating that the convergence zone will activate late Monday afternoon near Seattle and slide south toward Kent. This will be a brief event with precipitation ending as early as 7 pm, but under the convergence zone heavy rain, gusty winds, and some lightning are possible. Heavy precipitation in the zone will likely lower snow levels to allow hilltop settings to get brief heavy snow.

In the Cascades, the position of the frontal boundary will be the dividing line between widespread mountain snow to the north and snow levels generally above pass level to the south during the afternoon hours. After sunset, temperatures drop before precipitation ends, allowing all the major Cascade passes a shot at heavy snow. East of the Cascades, some rain and snow showers are possible but the big impact of this storm will be the wind, which is expected to gust to 50 mph in some areas ending early Tuesday morning.

On Tuesday, an upper level ridge will begin to build over the Pacific Northwest. Some showers may linger up in the Cascades on Tuesday, but aside from that dry conditions are expected statewide through Thursday. In Eastern Washington, areas of dense fog/freezing fog are likely in the valleys and basins. This ridge will be a cold one, with high temperatures dropping from 50s and 60s for many folks Monday to 30s and 40s by Thursday.

Friday brings with it another rain and mountain snowmaker. This is expected to make landfall on the coast during the morning hours and spread eastward. Precipitation values are expected to be light but it will be followed immediately by a larger system on Saturday. This will be mostly rain for Western Washington, but temperatures in Eastern Washington may be cool enough for some light snow all the way down to basin floor locations like the Tri-Cities and Moses Lake if it is moist enough for them to receive precipitation. Snow in the mountains both Friday and Saturday may to cause travel impacts on the major passes.

Current indications are that Washington will be clipped by a weak ridge next Sunday. This should bring dry conditions to Eastern Washington but westerly flow aloft may be enough to keep shower activity in place in and west of the Cascades. It looks like another system moves into the region on Monday with the pattern of periodically active conditions possibly continuing further into the week.


Important Points About Next Week's Weather

South Central Washington
Remainder of the Week
  • Wind: SW and windy Mon, breezy Tue. Light & variable Wed-Fri.
  • Precipitation: Rain showers possible Mon & Mon night.
  • High Temps: Upper 40s Tue, near 40 Wed, mid 30s Thu & Fri.
  • Low Temps: Near 40 Mon night, low 20s Tue & Wed night, mid 20s Thu night, upper 20s Fri night.
  • Sky Cover: Decreasing clouds Mon night, partly cloudy Tue-Thu, mostly cloudy Fri. Areas of fog.
Weekend
  • Wind: Generally S. Breezy Sat.
  • Precipitation: Light rain/snow possible Sat.
  • High Temps: Upper 30s Sat, mid 30s Sun.
  • Low Temps: Upper 20s Sat night, near 30 Sun night.
  • Sky Cover: Mostly cloudy.

North Central Washington
Remainder of the Week
  • Wind: SW and windy Mon, breezy Tue. Light & variable Wed-Fri.
  • Precipitation: Rain and mountain snow showers ending Mon night. Mountain rain/snow showers possible Fri.
  • High Temps: Low 40s Tue, mid 30s Wed & Thu, low 30s Fri. Cooler in higher terrain.
  • Low Temps: Near 30 Mon night, low 20s Tue-Thu nights, mid 20s Fri night. Cooler in higher terrain.
  • Sky Cover: Decreasing clouds Mon night, partly cloudy Tue-Thu, mostly cloudy Fri. Areas of fog.
Weekend
  • Wind: Generally S. Breezy in the Basin Sat.
  • Precipitation: Lowland rain/snow and mountain snow possible Sat.
  • High Temps: Mid 30s Sat & Sun. Cooler in higher terrain.
  • Low Temps: Mid 20s Sat & Sun nights. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy.

Eastern Washington
Remainder of the Week
  • Wind: SW and windy Mon, breezy Tue. Light & variable Wed-Fri.
  • Precipitation: Rain (possibly mixed with snow in some areas) ending Mon night. Otherwise dry.
  • High Temps: Near 40 Tue, mid 30s Wed-Fri. Warmer in the Lewis & Clark Valley.
  • Low Temps: Near 30 Mon night, near 20 Tue night, low 20s Wed-Thu nights, mid 20s Fri night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Decreasing clouds Mon night, partly cloudy Tue-Thu, mostly cloudy Fri. Areas of fog.
Weekend
  • Wind: Generally S. Breezy Sat.
  • Precipitation: Light snow possible Sat.
  • High Temps: Mid 30s Sat & Sun. Warmer in the Lewis & Clark Valley.
  • Low Temps: Mid 20s Sat & Sun nights. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Mostly cloudy.

Western Washington
Remainder of the Week
  • Wind: W and breezy Mon, breezy on the coast Tue. E gap winds possible Wed & Thu. S and breezy on the coast Fri.
  • Precipitation: Rain and snow showers ending Mon night. Rain and mountain snow Fri.
  • High Temps: Low 40s Tue-Wed, mid 40s Thu-Fri. Cooler in higher terrain.
  • Low Temps: Near 40 Mon night, low 30s Tue-Wed nights, mid 30s Thu night, near 40 Fri night. Cooler in higher terrain.
  • Sky Cover: Decreasing clouds Mon night, partly cloudy Tue-Thu, mostly cloudy Fri. Areas of fog.
Weekend
  • Wind: SW and breezy Sat, E and light Sun.
  • Precipitation: Rain and mountain snow likely Sat, rain and mountain snow possible Sun.
  • High Temps: Mid 40s Sat & Sun. Cooler in higher terrain.
  • Low Temps: Near 40 Sat & Sun nights. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to overcast.

Like us on Facebook at https://www.facebook.com/AgWeatherNet
Follow us on Twitter at https://www.twitter.com/AgWeatherNet
Washington State University