AgWeatherNet Weekly Outlook for December 18 to December 24, 2020

Wet and warm weekend with drying conditions leading up to Christmas

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, AgWeatherNet Field Meteorologist

Note: In observation of Christmas and the New Year, there will be no outlooks on December 25 (Friday), December 28 (Monday), or January 1 (Friday).

Synopsis:

Rain is the name of the game this weekend with repeated frontal passages Friday, Saturday, and Sunday that are positioned to tap into a robust moisture plume in the North Pacific. Snow levels around 4,000 feet will lead to heavy snow on some Cascade passes Friday night, but by Saturday night they will rise to between 6,000 and 8,000 feet limiting snow to only the highest peaks. Windy conditions are also possible in parts of the state, mainly Friday night and Sunday with rising rivers also a concern thanks to heavy rain falling on existing snowpack. By Sunday afternoon, snow levels drop back to around 4,000 feet to blanket the higher passes with more snow. A final system crosses the state Monday to bring another shot at rain, wind, and mountain snow before a ridge builds to dry things out leading into Christmas.

Discussion:

A cold front approaching the coast Friday afternoon will keep rain and mountain snow showers in place in and west of the Cascades for the remainder of the day. As the front moves into Eastern Washington before midnight, rain will increase there with the highest precipitation values being in the foothills of the Cascades, in the Okanogan Highlands, and within about 50 miles of the Idaho border. Behind the front, gusty winds are expected. This will be mainly seen on the coast, along the Strait of Juan de Fuca, the San Juan Islands, and Northwest Interior where gusts to 50 mph are possible at times in exposed areas. Breezy conditions with gusts to 40 mph are possible in the Kittitas Valley and on the Palouse.

A few showers may linger along the Cascade crest in the lead up to another system which is expected to begin impacting the coast midday Saturday. This system will be quite warm, raising snow levels between 6,000 and 8,000 feet overnight into Sunday – well above the major Cascade passes. Heavy rain in this system falling on decent snowpack will lead to rising rivers and streams statewide. As of midday Friday, a flood watch is in place for the greater Puget Sound Region with several rivers draining the Olympics and west side of the Cascades forecast to exceed flood stage following the warm rain event.

This will also be a wind maker as it moves east across Washington with much of the state possibly seeing west/southwest gusts to 40 mph as strong westerly winds aloft mix to the surface. A boundary oriented east-to-west drifts south during the day Sunday, bringing most of Washington into the cold sector of the system and potentially returning snow levels north of US 12 to around 4,000 feet. By this time, though, the bulk of the moisture is expected to have shifted south into Oregon with a few showers in southwest Washington and the Blues.

Another system embedded within the atmospheric river approaches the region Sunday night into Monday lifting the moisture plume back into Washington to bring even more rain and high mountain snow as a warm front moves into the state from the southwest. The cold front associated with this system will then move through the state from the west beginning Monday afternoon. This passage will again lower snow to pass levels with some lowland areas of Washington east of the Cascades seeing a rain/snow mix by Monday night. Primarily locations for this would be areas such as the northern valleys, east slopes of the Cascades, and the Palouse. Before all is said and done with this series of systems, rainfall totals may be in the 3 to 5 inch rain on the Olympic Peninsula, with 1 to 3 inches on the I-5 corridor, and between one and three quarters of an inch throughout most of lowland Washington east of the Cascades.

By Tuesday afternoon a fairly high amplitude ridge looks to build into the Northeast Pacific and shift onshore allowing the Pacific Northwest to dry out ahead of Christmas. Temperatures will be a bit cooler under this ridge, with lowland highs mainly in the mid 30s to the mid 40s. As is typical under strong ridges this time of year, areas of dense fog and freezing fog are possible to end the forecast period. This will mainly be in valleys and basins east of the Cascades, but parts of Western Washington could also see some fog.

Models are indicating a return to active weather with a trough kicking the ridge out perhaps as early as Christmas day to bring rain and snow showers back to Washington. Hopefully a clearer view of this system can be pulled out of the models for the Monday outlook.


8 to 14 Day Outlook, December 25 to December 31, 2020
  • Above average temperatures and above average precipitation are favored for the period.

Important Points About Next Week's Weather

South Central Washington
Weekend
  • Wind: S/SW wind, breezy through the weekend. Gusts to 40 mph at times.
  • Precipitation: Chance of rain showers through the weekend.
  • High Temps: Near 50 Sat, upper 50s Sun.
  • Low Temps: Near 40 Fri night, mid 40s Sat night, low 40s Sun night.
  • Sky Cover: Mostly cloudy to overcast.
Next Week
  • Wind: S & breezy Mon, NW & breezy Tue, light and variable Wed & Thu.
  • Precipitation: Rain showers possible Mon & Mon night, otherwise dry.
  • High Temps: Near 50 Mon, mid 40s Tue, near 40 Wed, mid 30s Thu.
  • Low Temps: Mid 30s Mon night, mid 20s Tue night-Thu night.
  • Sky Cover: Mostly cloudy Mon, partly cloudy Tue-Thu. Areas of fog.

North Central Washington
Weekend
  • Wind: Generally S wind, breezy in the basin through the weekend. Widespread breeze Sun. Gusts to 40 mph at times.
  • Precipitation: Mountain rain/snow showers through the weekend. Lowland rain showers Sat night & Sun.
  • High Temps: Upper 30s Sat, low 40s Sun. Cooler in higher terrain.
  • Low Temps: Low 30s Fri night-Sun night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to overcast.
Next Week
  • Wind: S & breezy in the basin Mon, NW & breezy Tue, light and variable Wed & Thu.
  • Precipitation: Rain and mountain snow showers Mon & Mon night, otherwise dry.
  • High Temps: Near 40 Mon, low 40s Tue, mid 30s Wed, low 30s Thu. Cooler in higher terrain.
  • Low Temps: Near 30 Mon night. Low 20s Tue night through Thu night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy Mon, partly cloudy Tue-Thu. Areas of fog.

Eastern Washington
Weekend
  • Wind: Generally SW and breezy through the weekend. Gusts to 40 mph at times.
  • Precipitation: Rain showers through the weekend with the best chance Fri night & Sat night.
  • High Temps: Mid 40s Sat, upper 40s Sun. Warmer in the Lewis & Clark Valley.
  • Low Temps: Low 30s Fri night, mid 30s Sat & Sun night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Mostly cloudy to overcast.
Next Week
  • Wind: S & breezy Mon, NW & breezy Tue, light and variable Wed & Thu.
  • Precipitation: Rain and mountain snow showers possible Mon-Tue AM, otherwise dry.
  • High Temps: Mid 40s Mon, near 40 Tue, mid 30s Wed & Thu. Warmer in the Lewis & Clark Valley.
  • Low Temps: Near 30 Mon night, near 20 Tue night, upper 10s Wed & Thu nights. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Mostly cloudy Mon, decreasing clouds Tue, partly cloudy Wed & Thu. Areas of fog.

Western Washington
Weekend
  • Wind: Generally S. Breezy through the weekend, areas of gusts to 50 mph.
  • Precipitation: Rain and mountain snow through the weekend.
  • High Temps: Near 50 Sat & Sun. Cooler in higher terrain.
  • Low Temps: Mid 40s Fri night, upper 40s Sat night, mid 40s Sun night. Cooler in higher terrain.
  • Sky Cover: Overcast.
Next Week
  • Wind: S & breezy Mon, NW & breezy Tue, light and variable with possible E mountain gap winds Wed & Thu.
  • Precipitation: Rain and mountain snow Mon tapering off Tue. Otherwise dry.
  • High Temps: Upper 40s Mon, mid 40s Tue-Thu. Cooler in higher terrain.
  • Low Temps: Near 40 Mon night, low 30s Tue night-Thu night.
  • Sky Cover: Mostly cloudy to overcast Mon, decreasing clouds Tue, partly cloudy Wed & Thu.

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