AgWeatherNet Weekly Weather Outlook Monday Update for December 14 to December 20, 2020

Parade of storms to keep active pattern in place

Craig Oswald, Field Meteorologist
Mark Ingalls, AgWeatherNet Intern

Synopsis:

A series of Pacific low pressure systems look to bring unsettled conditions statewide through most of the week. Western Washington and the Cascades likely won't fully dry out through the course of the week with rain and mountain snow showers trending scattered at best. Washington east of the Cascades will see periods of widespread precipitation, including lowland snow in some regions, as the effects of these weather systems traverse the Pacific Northwest with only brief periods of dry weather in between. Snow can generally be expected at pass levels through the week with levels rising above the passes after Thursday as warmer air makes its way into the state. Windy conditions associated with strong surface pressure gradients due to the deep low pressure centers associated with these systems should also be noted.

Discussion:

A parade of storms is expected to keep active conditions in place over the Pacific Northwest for at least the next week. Scattered showers should continue in and west of the Cascades, becoming more widespread overnight thanks to an incoming low pressure system. This will spread rain and mountain snow east of the mountains by the morning hours Tuesday. The low moving inland will shift mid-level winds to out of the southwest and cause snow levels across the state to rise. On Tuesday morning, snow levels are expected to be low enough to drop some snow along the I-90 corridor in Eastern Washington. As the day progresses, rising temperatures will limit snow to above 3,000 feet by the evening hours.

Precipitation winds down Tuesday night with only scattered showers remaining in and west of the Cascades, generally north of US 12. The next system looks to have a bit more moisture embedded in it and should start impacting the coast before sunrise Wednesday. This will cross into Eastern Washington Wednesday afternoon, leaving scattered showers in its wake to the west. Snow levels in this event are expected to rise to above 4,000 feet, which is around pass level for both Stevens and White but would lead to a transition to rain for Snoqualmie.

Wednesday's system also brings with it the possibility of gusty winds. Much of the state can expect gusts above 20 mph, with stronger gusts in the typical windy spots including the coast, Strait of Juan de Fuca, and Columbia Basin. A few weak troughs transiting the state will keep Western Washington and the Cascades from drying out with scattered showers through Friday, but these aren't expected to bring substantial precipitation to arid portions of the state.

Potent systems make a return Saturday and Sunday, with both days expected to bring areas of heavy rain to Western Washington and modest rainfall values in Eastern Washington. Snow levels will remain high, keeping the I-90 corridor snow free. Heavy rain falling on areas that have recently seen snow may cause flooding concerns on smaller rivers and streams draining the Cascades (particularly west of the crest), so those who live in particularly flood prone areas should pay attention to river forecasts. On top of the rain and snow, windy conditions are also possible over the weekend for many Washington state locations. Models indicate that this progressive pattern can be expected to continue into next week.


Important Points About Next Week's Weather

South Central Washington
Remainder of the Week
  • Wind: S/SW and breezy to windy through Wed. Turning SE Wed night and remaining generally breezy. SW and windy Thu, trending toward breezy Fri.
  • Precipitation: Upland snow and lowland rain/snow starting early Tue. Transitioning to mostly rain through lowland areas further south. Trending dry Tue night. Upland snow/rain and lowland rain Wed afternoon, snow levels higher than Tue. Trending dry Wed night. A scattered rain shower or two possible Fri.
  • High Temps: Near 40 Tue, low 40s Wed, mid 40s Thu, low 40s Fri.
  • Low Temps: Mid 20s Mon night, low 30s Tue night, low 30s Wed night, upper 20s Thu night.
  • Sky Cover: Mostly cloudy. Isolated areas of fog and freezing fog.
Weekend
  • Wind: S/SW and windy.
  • Precipitation: Rain showers Fri night. Chance for a rain shower or two Sat and turning more widespread Sat night with scattered rain showers lingering through Sun.
  • High Temps: Upper 40s Sat, low 50s Sun.
  • Low Temps: Low 30s Fri night, upper 30s Sat night, near 40 Sun night.
  • Sky Cover: Mostly cloudy.

North Central Washington
Remainder of the Week
  • Wind: Generally S and breezy through Wed, windy across the Basin. Turning SE Wed night and remaining generally breezy. SW and windy Thu, trending toward breezy Fri.
  • Precipitation: Mountain snow starting late Mon night and spreading to lowland areas early Tue. Transitioning to rain/snow through parts of the Basin. Trending dry Tue night with some mtn snow showers lingering, especially near the Cascade crest and Idaho border. Upland snow and lowland rain/snow Wed, snow levels higher than Tue. Trending dry Wed night, mtn snow showers again lingering near the Cascade crest and Idaho border. A few scattered mtn snow showers and a lowland rain/snow shower or two possible Fri.
  • High Temps: Mid 30s Tue, mid 30s Wed, upper 30s Thu, mid 30s Fri. Cooler in high terrain.
  • Low Temps: Low 30s Mon night, upper 20s Tue night, low 30s Wed night, mid 20s Thu night. Cooler in high terrain.
  • Sky Cover: Mostly cloudy to cloudy. Isolated areas of fog and freezing fog.
Weekend
  • Wind: S/SW and breezy/windy across the Basin.
  • Precipitation: Mtn snow showers, and lowland rain Fri night, trending more scattered in nature Sat. Widespread mtn snow and lowland rain/snow Sat night with scattered showers lingering through Sun.
  • High Temps: Upper 30s Sat, near 40 Sun. Cooler in high terrain.
  • Low Temps: Near 30 Fri night, low 30s Sat night, mid 30s Sun night. Cooler in high terrain.
  • Sky Cover: Mostly cloudy to cloudy.

Eastern Washington
Remainder of the Week
  • Wind: S/SW and breezy to windy through Wed. Turning SE Wed night and remaining generally breezy. SW and windy Thu, trending toward breezy Fri.S/SW and breezy to windy through Wed. Turning SE Wed night and remaining generally breezy. SW and windy Thu, trending toward breezy Fri.
  • Precipitation: Snow showers with rain/snow further west starting Tue. Transitioning toward rain/snow mix through the day. Blue Mtns to remain as snow. Trending dry Tue night. Mountain snow and lowland rain/snow Wed evening, snow levels higher than Tue. Trending dry late Wed night with a few scattered showers lingering near the Idaho border and Blue Mtns Thu. A scattered rain/snow shower or two possible Fri.
  • High Temps: Mid 30s Tue, near 40 Wed, near 40 Thu, upper 30s Fri. Cooler through the Blue Mtns.
  • Low Temps: Upper 20s Mon night, mid 30s Tue night, low 30s Wed night, low 30s Thu night. Cooler through the Blue Mtns.
  • Sky Cover: Mostly cloudy to cloudy. Isolated areas of fog and freezing fog.
Weekend
  • Wind: S/SW and windy.
  • Precipitation: Rain/snow showers Fri night. Chance for a rain shower/mtn snow shower or two Sat and turning more widespread Sat night with scattered rain showers/mtn snow showers lingering through Sun.
  • High Temps: Mid 40s Sat, mid 40s Sun. Cooler through the Blue Mtns.
  • Low Temps: Low 30s Fri night, mid 30s Sat night, mid 30s Sun night. Cooler through the Blue Mtns.
  • Sky Cover: Mostly cloudy to cloudy.

Western Washington
Remainder of the Week
  • Wind: Generally S/SW and breezy, windy near the coast and sound. Turning W Wed night and remaining breezy to windy. Turning back to S/SW Fri and remaining breezy/windy near the coast and sound.
  • Precipitation: Lowland rain and scattered rain showers through the week. On/off mountain snow through the week with snow levels generally rising during the day. Snow levels expected to be higher than pass levels by Friday.
  • High Temps: Upper 40s Tue, upper 40s Wed, upper 40s Thu, upper 40s Fri. Cooler through the Cascade and Olympic Mtns.
  • Low Temps: Low 40s Mon night, near 40 Tue night, upper 30s Wed night, upper 30s Thu night. Cooler through the Cascade and Olympic Mtns.
  • Sky Cover: Mostly cloudy to overcast.
Weekend
  • Wind: SW and breezy, windy near the coast and sound.
  • Precipitation: Lowland rain and scattered rain showers through the weekend. On/off mountain snow.
  • High Temps: Low 50s Sat, near 50 Sun. Cooler through the Cascade and Olympic Mtns.
  • Low Temps: Low 40s Fri night, low 40s Sat night, low 40s Sun night. Cooler through the Cascade and Olympic Mtns.
  • Sky Cover: Overcast.

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