AgWeatherNet Weekly Outlook for December 11 to December 17, 2020

A series of transient ridges and frontal passages expected for the period

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, AgWeatherNet Field Meteorologist

Synopsis:

Brief ridging Saturday is expected to move in behind the cold front that moved east across Washington on Friday. Another cold front approaches the region Saturday night to bring precipitation to the region, including possible wintry mix conditions in low elevation areas north of Everett and east of the Cascades before snow levels rise during the day Sunday. A brief drying period spreads west-to-east Monday. It is anticipated that it will also be short lived as frontal passages cross the state every 24 to 36 hours beginning Tuesday and lasting through the end of the forecast period. Current indications are that snow levels during these events will be around pass level, keeping precipitation liquid in the lower elevations. Cross-state travelers should keep up-to-date on pass conditions for at least the next week.

Discussion:

A weak cold front that crossed Washington during the day Friday, brought rain and snow showers to much of the state. Precipitation is expected to taper off Friday evening as the front moves east. Behind the cold front an upper level ridge looks to bring dry conditions statewide on Saturday. Under this short lived ridge, some low elevation fog is likely to form east of the Cascades, especially across the Columbia Basin and Valleys. Otherwise look for partly to mostly cloudy skies.

This ridge is to be pushed east of the Pacific Northwest by another incoming frontal passage that could bring precipitation to the coast as early as midnight Saturday night/Sunday morning. This spreads from west to east through the morning hours on Sunday. For most Western Washington lowlands this will come in the form of rain, but snow levels may be low enough along the Hood Canal and east of I-5 north of about Everett before sunrise, with snow levels rising as the morning progresses Sunday. Eastern Washington locations may also start the day with some snow, freezing rain, sleet, or some combination of all three before low elevation areas gradually transition to rain throughout the course of the day as well.

While snow levels statewide rise during the day Sunday for lowland precipitation to mostly take the form of rain by the afternoon hours. The Yakima and Kittitas Valleys, Spokane area, and the Palouse may remain cool enough to keep some wintry precipitation through the day. In any case, lowland snow and ice accumulations are expected to be minimal as 8 inch soil temperatures remain above freezing, in the mid to upper 30s. The Cascade passes however may see 4 to 8 inches.

Some showers linger into Monday under northwesterly flow aloft, especially west of and in the Cascades. Snow levels will be high enough to keep this as lowland rain and precipitation values are expected to be light. Drying is expected east of the Cascades. Rain from the next frontal system looks to approach Western Washington Tuesday before showers entirely vacate the region, especially along the coast and mountains.

As this system moves over the Cascades Tuesday, it may again start as snow or some form of wintry mix in Eastern Washington, lasting into the following morning. Snow levels are expected to quickly rise Wednesday, limiting low elevation accumulation and restricting frozen precipitation to the mountains. Active weather continues through Thursday with rain and mountain snow showers in and west of the Cascades. Depending on the timing on the next system, Thursday may be a dry day east of the Cascades, but more details on all of this will be provided with better clarity in the Monday outlook.


8 to 14 Day Outlook, November 13 to November 19, 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: Generally N and light.
  • Precipitation: Low chance of rain/snow mix Fri night. Wintry mix Sat night & Sun AM becoming mainly rain Sun PM. Snow and ice accumulations generally under 1”.
  • High Temps: Upper 30s Sat & Sun.
  • Low Temps: Mid 20s Fri night, upper 20s Sat & Sun nights.
  • Sky Cover: Mostly cloudy to overcast. Patchy fog and freezing fog.
Next Week
  • Wind: Generally S, breezy conditions possible Tue-Thu.
  • Precipitation: Slight chance of wintry mix Mon night & Tue AM. Chance of rain Tue-Thu.
  • High Temps: Low 40s Mon, near 40 Tue, mid 40s Wed, upper 40s Thu.
  • Low Temps: Near 30 Mon night, low 30s Tue night, low 30s Wed & Thu nights.
  • Sky Cover: Mostly cloudy to overcast. Patchy fog and freezing fog Mon.

North Central Washington
Weekend
  • Wind: Light and variable.
  • Precipitation: Low chance of rain/snow mix Fri night. Wintry mix Sat night & Sun AM becoming mainly rain in the lowlands Sun PM. Lowland snow and ice accumulations generally under 1”, higher amounts in higher terrain.
  • High Temps: Mid 30s Sat & Sun. Cooler in higher terrain.
  • Low Temps: Low 20s Fri night, mid 20s Sat & Sun nights. Cooler in higher terrain.
  • Sky Cover: Decreasing clouds Fri night, partly cloudy Sat, mostly cloudy to overcast Sun. Patchy fog and freezing fog.
Next Week
  • Wind: Light and variable Mon-Wed. S Thu with breezy conditions possible.
  • Precipitation: Snow possible Mon night, Tue AM snow becoming rain in low elevations, snow (mixing with lowland rain) Tue night & Wed, rain and mountain snow possible Wed night & Thu.
  • High Temps: Mid 30s Mon, upper 30s Tue, low 40s Wed, mid 40s Thu. Cooler in higher terrain.
  • Low Temps: Mid 20s Mon & Tue night, near 30 Wed & Thu night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to overcast. Patchy fog and freezing fog Mon.

Eastern Washington
Weekend
  • Wind: Light and variable.
  • Precipitation: Light snow tapering off Fri night. Rain & snow Sun, snow possible Sun night. Lowland accumulations generally 1-3” Fri, under 1” Sun, higher in higher terrain.
  • High Temps: Low 30s Sat, mid 30s Sun. Warmer in the Lewis & Clark Valley.
  • Low Temps: Mid 20s Fri night, low 20s Sat night, upper 20s Sun night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Mostly cloudy to overcast. Patchy fog and freezing fog.
Next Week
  • Wind: Generally S, breezy conditions possible Tue-Thu.
  • Precipitation: Lingering snow showers Mon AM. Rain/snow possible Tue, snow possible Tue night, rain/snow possible Wed, rain possible Wed night & Thu.
  • High Temps: Mid 30s Mon, upper 30s Tue, near 40 Wed, low 40s Thu. Warmer in the Lewis & Clark Valley.
  • Low Temps: Mid 20s Mon night, near 30 Tue night, low 30s Wed & Thu night. Warmer in the Lewis & Clark Valley.
  • Sky Cover: Mostly cloudy to overcast. Patchy fog and freezing fog Mon.

Western Washington
Weekend
  • Wind: Generally E/SE. Breezy at times on the coast, in San Juan County, and west of Cascade mountain gaps.
  • Precipitation: Rain and mountain snow ending Fri. Rain and mountain snow likely Sat night-Sun night. Lowland wintry mix possible east of I-5 Sat night.
  • High Temps: Mid 40s Sat, upper 40s Sun. Cooler in higher terrain.
  • Low Temps: Mid 30s Fri night, upper 30s Sat night, Low 40s Sun night. Cooler in higher terrain.
  • Sky Cover: Decreasing clouds Fri night, partly to mostly cloudy Sat, overcast Sun.
Next Week
  • Wind: S/SW, breezy conditions possible on the coast and along the strait Tue, widespread breezy conditions possible Thu.
  • Precipitation: Chance of rain Mon, rain likely Tue-Thu. Snow in the mountains.
  • High Temps: Upper 40s Mon & Tue, near 50 Wed & Thu. Cooler in higher terrain.
  • Low Temps: Near 40 Mon night, mid 40s Tue-Thu night. Cooler in higher terrain.
  • Sky Cover: Overcast skies.

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