AgWeatherNet Weekly Outlook for December 04 to December 10, 2020

Strong ridging over the West slowly transitioning to more active pattern

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, AgWeatherNet Field Meteorologist
Joe Zagrodnik, AgWeatherNet Postdoctoral Research Associate

Synopsis:

The upper-level ridge that has been providing dense fog in the Columbia Basin and mild conditions for most other places will remain in place through Tuesday. That said, a weak system is expected to slip through the area Saturday afternoon to bring scattered showers mainly in and west of the Cascades and help clear come low-elevation fog in Eastern Washington. These showers may linger in the Cascades on Monday, otherwise conditions restabilize after this for a return to mainly dry conditions until Tuesday afternoon when the first of a couple stronger systems are expected to start impacting the area. Precipitation from this series should be more widespread through Thursday, with rain showers for Western Washington and perhaps a little bit of rain even in the Columbia Basin.

Discussion:

Dense fog in the Columbia Basin and other closed valleys in Eastern Washington with mild conditions elsewhere Friday are the result of a strong upper level ridge positioned over the Western United States. This ridge is expected to remain in place through Tuesday. Despite this, it looks like a weak frontal passage will slip into the state accompanied by a weak shortwave trough on Saturday afternoon. Some rain showers are possible along with mountain snow for the Cascades, but otherwise just expect an increase in high clouds statewide along with a decrease in low-elevation fog in Eastern Washington as most of the precipitation will be focused in Oregon generally south of Salem and Bend. The upper level trough will morph into a closed low that intensifies as it moves south, but this intensification will have no impact here.

Some showers may linger in the Cascades and Olympics into Monday, but otherwise they should be coming to an end midday Sunday. Limited areas of low-elevation fog may make it through this event in Eastern Washington, which should be strengthened by nocturnal temperature inversions Sunday night to resume more widespread fog Monday in the Columbia Basin and other closed valleys east of the Cascades.

Meanwhile, a particularly moist frontal passage will impact southeast Alaska and coastal British Columbia, producing an atmospheric river event that will be aimed at the northern two-thirds of Vancouver Island on Monday. This begins to weaken Monday night, sliding south into Washington on Tuesday to bring a rain and mountain snow event that is more robust than the one on Saturday and Sunday. The southwesterly trajectory of the system Tuesday and Wednesday looks to keep snow levels above pass level. Most showers taper off by Wednesday afternoon, but mountain precipitation may linger into the overnight hours.

Models are in decent agreement in bringing a low-pressure system into the region for Thursday and Friday, with most of them indicating rain starting for Western Washington around midday and spreading eastward. This system is colder than its predecessor with snow levels in the Cascades expected to be below pass levels. It looks as though the waves of precipitation every couple days will continue into the following week, but more details on all of this will be provided in future outlooks.


8 to 14 Day Outlook, December 11 to December 17, 2020
  • Near average temperatures and below average precipitation are favored for the period.

Important Points About Next Week's Weather

South Central Washington
Weekend
  • Wind: Light and variable.
  • Precipitation: Dry conditions expected.
  • High Temps: Upper 30s Sat & Sun. Cooler in dense fog.
  • Low Temps: Low 20s Fri night, mid 20s Sat & Sun nights.
  • Sky Cover: Mostly sunny Fri, increasing clouds Fri night, mostly cloudy Sat & Sun. Widespread dense fog Fri & Sat. Patchy fog Sun.
Next Week
  • Wind: Light & variable Mon-Tue. SW with breezy conditions possible Wed-Thu.
  • Precipitation: Isolated rain Tue & Thu, otherwise dry.
  • High Temps: Near 40 Mon, low 40s Tue, mid 40s Wed, low 40s Thu.
  • Low Temps: Upper 20s Mon night, near 30 Tue & Wed night, low 30s Thu night.
  • Sky Cover: Partly to mostly cloudy. Patchy fog.

North Central Washington
Weekend
  • Wind: Light and variable.
  • Precipitation: Mountain rain & snow possible Sat & Sun.
  • High Temps: Mid 30s Sat & Sun. Cooler in higher terrain and in dense fog.
  • Low Temps: Low 20s Fri night, mid 20s Sat & Sun nights. Cooler in higher terrain and in some valleys.
  • Sky Cover: Mostly sunny Fri, increasing clouds Fri night, mostly cloudy Sat & Sun. Widespread dense fog Fri & Sat. Patchy fog Sun.
Next Week
  • Wind: Light & variable Mon-Tue. SW with breezy conditions in the basin possible Wed-Thu.
  • Precipitation: Lowland rain possible Tue. Scattered mountain rain & snow Tue-Thu.
  • High Temps: Near 40 Mon & Tue, mid 40s Wed, near 40 Thu. Cooler in higher terrain.
  • Low Temps: Upper 20s Mon night, low 30s Tue & Wed night, near 30 Thu night. Cooler in higher terrain.
  • Sky Cover: Partly to mostly cloudy. Patchy fog.

Eastern Washington
Weekend
  • Wind: Light and variable.
  • Precipitation: Isolated rain showers Sun.
  • High Temps: Mid 30s Sat & Sun. Cooler in dense fog. Warmer in the Lewis & Clark Valley.
  • Low Temps: Low 20s Fri night, mid 20s Sat & Sun night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Mostly sunny Fri, increasing clouds Fri night, mostly cloudy Sat & Sun. Widespread dense fog Fri & Sat. Patchy fog Sun.
Next Week
  • Wind: Light & variable Mon-Tue. SW with breezy conditions possible Wed-Thu.
  • Precipitation: Low chance of rain showers Tue & Thu.
  • High Temps: Low 40s Mon-Thu. Warmer in the Lewis & Clark Valley.
  • Low Temps: Upper 20s Mon night-Thu night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Partly to mostly cloudy. Patchy fog.

Western Washington
Weekend
  • Wind: Light and variable Fri & Sat except immediately downwind of Cascade mountain gaps where gusts from the E may exceed 35 mph in some spots. SW and locally breezy Sun.
  • Precipitation: Chance of lowland rain Sat night & Sun. Mountain rain & snow Sat & Sun.
  • High Temps: Near 50 Sat & Sun. Cooler in higher terrain.
  • Low Temps: Low 30s Fri night, mid 30s Sat night, mid 30s Sun night. Cooler in higher terrain.
  • Sky Cover: Mostly sunny Fri, increasing clouds Sat, mostly cloudy Sun. Patchy fog.
Next Week
  • Wind: SE/S Mon & Tue, breezy conditions possible on the Coast and in the N Sound. W Wed & Thu, breezy conditions possible on the Coast and through the Strait.
  • Precipitation: Rain and high elevation snow showers through the week with highest chances in and near the mountains.
  • High Temps: Near 50 Mon-Wed, mid 40s Thu. Cooler in higher terrain.
  • Low Temps: Low 40s Mon night & Tue night. Low 40s Wed night & Thu night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to overcast.

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