AgWeatherNet Weekly Outlook for January 08 to January 14, 2021

Atmospheric river event to bring significant lowland rain midweek

Mark Ingalls, AgWeatherNet Intern
Craig Oswald, AgWeatherNet Field Meteorologist

Synopsis:

A strong jet stream across the north Pacific looks to keep the Pacific Northwest in a rather active weather pattern for the upcoming period. A series of upper troughs and associated strong extratropical surface cyclones look to continue to form and move into northern British Columbia/the Gulf of Alaska. This will provide Washington with a series of surface frontal passages and wet weather through much of the week. The trough and associated cold front crossing Washington today (Friday) will continue to move southeast toward the Great Basin Friday afternoon, but not before producing some rain and mountain snow showers throughout the state.

A ridge builds over the West Coast Saturday, under which fog is possible primarily in eastern lowlands with strong easterly winds through the Cascade mountain gaps. A shortwave trough and frontal boundary pass through the Pacific Northwest Sunday as the ridge displaces to the east. This looks to produce some limited rain and mountain snow showers. After that, the next big weather maker comes Tuesday/Wednesday thanks to a weakening front and assistance from an atmospheric river taking aim at the region. Rain and mountain snow from this event are expected to be fairly consistent and heavy at times in and west of the Cascades. The end of the forecast period brings another frontal system into Washington, but model confidence in the timing of this is not high.

Discussion:

A weakening cold front is crossing Washington Friday, driven by an upper level trough that is evolving to become a closed upper level low by the time it reaches Texas' Permian Basin Sunday. This is bringing rain and mountain snow to the state. By the evening hours, most of the precipitation will have moved on toward the southeast, but some lingering showers are possible in the Blues, Cascades, and on the Palouse into the overnight hours. Snow levels are expected to remain around 3,000 feet. While snowfall amounts in the mountains are forecasted to remain around a few inches, some travel impacts on major corridors are possible.

The cold front is followed by the building of an upper level ridge for the West Coast. As is usual under this regime in the winter months, areas of fog and freezing fog can be expected in lower elevations of Eastern Washington Saturday into Sunday morning. The presence of a modest cold pool increases the likelihood of strong easterly mountain gap winds through the Cascades as the higher pressure in the east attempts to fill in areas of comparatively low pressure in Western Washington. Limited areas of gusts in excess of 35 mph are possible in eastern suburbs/exurbs of the Seattle area. Breezy conditions can also be expected in the San Juan Islands and on the coast.

A weak shortwave trough is expected to move through the state beginning Saturday night, as the ridge displaces to the east. Fairly light snow in the mountains and rain in lowland Western Washington is expected in association with a decaying frontal boundary, with lower chances for light snow showers across northern Washington, primarily throughout upland areas. The biggest impact looks to be vertical mixing that occurs as a result of the shortwave, cleaning out most of the cold pools which would then clear most of the low elevation fog in Eastern Washington as well as cut off the aforementioned easterly gap winds blowing through the Cascades. It is possible that some stubborn, but small, cold pools and fog may remain in small and well sheltered valleys/basins.

More rain and mountain snow makes its way into Washington from a weak cold front on Monday. Model guidance indicates that this will be limited to areas north and west of a line generally extending from Portland to Ellensburg and Deer Park. Precipitation looks to move out of the region quickly, but Western Washington won’t have much chance to dry out with a pineapple express taking aim at the state beginning early Tuesday. Pineapple expresses are a type of atmospheric river event where the moisture comes from the region generally around Hawaii, and as such often include much higher snow levels. This will be the case with this event as moisture is channeled between surface high pressure around San Francisco and a strong low heading toward Southwest Alaska.

The low will bring in the first wave of precipitation with a warm front to raise snow levels to around 4,000 to 5,000 feet. Rain and high elevation snow continues along and behind the warm front with a cold front moving across the state from the west/northwest Wednesday to lower snow levels to near 3,000 to 4,000 feet. For Western Washington locations, rain is expected to be steady and periodically heavy. Seattle stands to gain over an inch of rain out of the pineapple express, with higher values toward the mountains. Landslides and some flooding are possible. In Eastern Washington, modest rainfall is possible, though ensembles are significantly more divided on both timing as well as total rainfall. However, it is reasonable to expect liquid equivalent totals of around 0.1" throughout the south central Basin with totals closer to 0.75" or more throughout upland parts of northern Washington and some areas near the Idaho border between Tuesday and Wednesday.

The atmospheric river deteriorates Wednesday night with drier air filtering into Washington from the north. Still, some lingering rain and snow showers will be possible in the Cascades into Thursday. The forecast period ends with a chance of another significant system reaching the Pacific Northwest, but timing on this is currently hard to nail down with the arrival most likely to be on Friday.


8 to 14 Day Outlook, January 15 to January 21, 2021
  • Above average temperatures and below average precipitation are favored for the period.

Important Points About Next Week's Weather

South Central Washington
Weekend
  • Wind: Generally SE yet variable and calm to light.
  • Precipitation: Low chance for a stray rain shower Sun, otherwise dry conditions expected.
  • High Temps: Near 40 Sat, low 40s Sun.
  • Low Temps: Mid 20s Fri night, upper 20s Sat night, upper 20s Sun night.
  • Sky Cover: Partly sunny to mostly cloudy. Areas of fog and freezing fog.
Next Week
  • Wind: SE and light Mon. Becoming windy Tue and shifting to SW, remaining generally windy Wed with some gusts 40+ mph. Return to light and variable Thu.
  • Precipitation: Low chance for a rain shower or two near the east slope of the Cascades late Mon, otherwise dry conditions expected. Scattered rain showers Tue and Tue night. Trending dry for Wed.
  • High Temps: Low 40s Mon, mid 40s Tue, low 50s Wed, upper 40s Thu.
  • Low Temps: Mid 30s Mon night, upper 30s Tue night, mid 30s Wed night, mid 30s Thu night.
  • Sky Cover: Partly sunny to mostly cloudy.

North Central Washington
Weekend
  • Wind: Generally E yet variable and calm to light.
  • Precipitation: A few light snow showers Sat night into Sun, primarily in upland areas. Otherwise dry conditions expected.
  • High Temps: Mid 30s Sat, mid 30s Sun. Cooler in higher terrain.
  • Low Temps: Upper 20s Fri night, low 30s Sat night, low 30s Sun night. Cooler in higher terrain and deep valleys.
  • Sky Cover: Mostly cloudy to cloudy. Areas of fog and freezing fog.
Next Week
  • Wind: E/SE and light Mon. Becoming windy Tue and shifting to S/SW, remaining generally windy Wed with some gusts 40+ mph, especially across the Basin. Return to light and variable Thu.
  • Precipitation: Scattered lowland rain showers and upland snow showers late Mon, primarily along the east slope of the Cascades and the northern mountains with more widespread activity to the west, otherwise dry conditions expected. Rain showers and mountain mixed/snow Tue and Tue night. Trending dry Wed.
  • High Temps: Mid 30s Mon, upper 30s Tue, low 40s Wed, upper 30s Thu. Cooler in higher terrain.
  • Low Temps: Low 30s Mon night. Low 30s Tue night, near 30 Wed night, low 30s Thu night. Cooler in higher terrain.
  • Sky Cover: Mostly cloudy to cloudy.

Eastern Washington
Weekend
  • Wind: Generally SE yet variable and calm to light.
  • Precipitation: Low chance for a stray rain shower/mountain snow shower late Sun, otherwise dry conditions expected.
  • High Temps: Upper 30s Sat, upper 30s Sun. Cooler in the Blue Mtns.
  • Low Temps: Low 30s Fri night, upper 20s Sat night, upper 20s Sun night. Cooler in the Blue Mtns.
  • Sky Cover: Partly sunny to mostly cloudy. Areas of fog and freezing fog.
Next Week
  • Wind: SE and light Mon. Becoming windy Tue and shifting to SW, remaining generally windy Wed with some gusts 40+ mph. Return to light and variable Thu.
  • Precipitation: Chance for a rain/snow shower or two in and near the Blue Mtns late Mon, otherwise dry conditions expected. Rain showers and mountain mixed/snow Tue and Tue night. Trending dry Wed.
  • High Temps: Near 40 Mon, low 40s Tue, mid 40s Wed, mid 40s Thu. Cooler in the Blue Mtns.
  • Low Temps: Low 30s Mon night, mid 30s Tue night, mid 30s Wed night, mid 30s Thu night. Cooler in the Blue Mtns.
  • Sky Cover: Partly sunny to mostly cloudy.

Western Washington
Weekend
  • Wind: E/SE and generally light. Expect gusty Cascade gap winds with some gusts 35+ mph, and some gusty conditions near the sound.
  • Precipitation: Dry Sat. Light rain and mountain snow showers Sat night. Briefly trending dry Sun night.
  • High Temps: Mid 40s Sat, upper 40s Sun. Cooler in the Cascade and Olympic Mtns.
  • Low Temps: Mid 30s Fri night, upper 30s Sat night, low 40s Sun night. Cooler in the Cascade and Olympic Mtns.
  • Sky Cover: Partly sunny to mostly cloudy. A few isolated areas of fog.
Next Week
  • Wind: SE and light Mon with some gusts continuing near the sound and Cascade gaps. Becoming windy Mon night and shifting to S and eventually SW, remaining generally windy Tue with some gusts 40+ mph, possibly stronger along the coast and sound. Trending back to light Wed.
  • Precipitation: Scattered rain showers and mountain snow showers Mon. Precip becoming more widespread and intense Mon night and remaining so through Tue before trending dry Wed. A few mountain snow showers to linger into Thu.
  • High Temps: Low 50s Mon, mid 50s Tue, mid 50s Wed, low 50s Thu. Cooler in the Cascade and Olympic Mtns.
  • Low Temps: Mid 40s Mon night, mid 40s Tue night, low 40s Wed night, low 40s Thu night. Cooler in the Cascade and Olympic Mtns.
  • Sky Cover: Mostly cloudy to cloudy.

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