Back-to-back storms to help replenish snowpack in Northwest

·8 min read

AccuWeather meteorologists say a shift in the storm track across the northwestern United States will allow storms to trek farther south and target areas that largely missed out on the barrage of drenching, warm storms that produced disastrous flooding in parts of the region and British Columbia, Canada, last month.

This shift in the stormy pattern will also bring the return of colder air which will allow a significant amount of snow to reach the mountains with snowflakes possible at lower elevations.

Seattle is coming off its wettest meteorological fall (September through November) ever with 19.04 inches of rain compared to a normal of 11.63 inches. Portions of British Columbia picked up 2 to 4 times their normal rainfall during November alone, and many streams and rivers have been left swollen from the frequent heavy rain that has saturated the soil. Meanwhile, the warmer air associated with the northern storm track has chewed into the snowpack in the region over intermediate and higher elevations.

Snow depth levels across the Northwest as of Dec. 3, 2021.

A break from major storms will last through the end of this week, but more systems are gathering moisture over the Pacific and will aim not only for the coast but areas farther inland this weekend.

There are some different aspects of this storm compared to some that have come ashore in recent weeks, according to AccuWeather Meteorologist Ryan Adamson who noted that the storm on deck for later Sunday to Monday is projected to be more spread out and trek farther south than most recent storms.

"The heaviest precipitation will be to the south of the recently hard-hit areas of northern Washington and southern British Columbia," Adamson stated.

A general 1 inch of rain is foreseen from coastal southern British Columbia through much of the Interstate 5 corridor in Washington and Oregon, as well as the northwestern corner of California. About 2-3 inches of rain with locally higher amounts can occur along the central British Columbia coast; Vancouver Island, British Columbia; Washington's Olympic Peninsula and the immediate coast of Oregon. As with any storm, locally higher amounts are likely, especially along the lower west-facing slopes of the mountains from western British Columbia to western Oregon.

This image, taken on Friday morning, Dec. 3, 2021, shows clouds associated with a storm lurking off the coast of Washington and Oregon. That storm is forecast to swing through the Northwest on Saturday. A larger storm, slated to arrive late Sunday and Monday, was gathering moisture farther to the west over the Pacific. (CIRA at Colorado State/GOES-West)

With the new storm slated to arrive late this weekend and continue into early next week, there is the potential for the rain to aggravate saturated soil conditions and high stream levels in British Columbia and northwestern Washington, but far from the extreme of recent weeks. Still, any persistent downpours can lead to isolated flooding and mudslides.

Soil conditions in the Northwest range from excessively wet along the British Columbia and Washington coasts to severe drought in southwestern Oregon and northwestern California and exceptional drought east of the Cascades in southern Washington and much of Oregon, according to the latest United States Drought Monitor map released on Dec. 2.

The recent onslaught of atmospheric river events was associated with unusually warm conditions due to the storm track's placement so far north in British Columbia. Colder air and snow are needed for the Cascades as well as the ranges farther inland over the Northwest, and the storm late this weekend to early next week should oblige as well as a sneaky storm to unfold across the northern tier of the U.S. and the southern tier of Canada during the first part of the weekend.

The initial storm has the potential to bring several inches of snow to the northern Washington Cascades as well as the Chimney of Idaho and northwestern Montana through Saturday night. Some of the higher terrains in this zone can pick up a foot of snow. A coating to an inch of snow is expected in Spokane, Washington, from the storm. This snow will come on the heels of high temperatures that were in the 50s and 60s in the region much of this week and will serve as a reality check as colder and more seasonable air sinks southward through a large chunk of the Northwest.

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"That first and smaller of the two storms this weekend is likely to cause wintry travel over the northern Washington Cascades," AccuWeather Chief On-Air Meteorologist Bernie Rayno said. Rayno added that people heading over the passes along U.S. Route 2 and Interstate 90 in the Cascades as well as in Idaho and Montana should be prepared for snow-covered roads and delays.

The snow will be intense enough to shovel and plow and could even shut down travel for a time over Stevens and Snoqualmie passes in Washington.

At most lower elevations along the coast in Washington and Oregon, forecasters say this first storm is expected to bring only occasional rain showers and spotty drizzle into Saturday evening. However, just enough cold air may sneak in at the tail end for wet snow to mix in with the rain on the coast north of Seattle and area near and just inland from Vancouver, British Columbia, can pick up a small slushy accumulation of snow.

In terms of snowfall during the second and larger storm from late Sunday to Monday night in the region, the circulation of the system will cause snow levels to vary. At the onset of the storm late Sunday, snow levels are likely to be below most pass levels in the Cascades with a quick accumulation initially. However, a southerly breeze will pull in enough mild air to cause temperatures to rise to near and perhaps a bit above freezing and allow a mixture of rain and snow or a change to all rain for a time late Sunday night and early Monday. Then as the storm pushes inland and northerly breezes bring in colder air, freezing levels will lower and cause a change back to snow over the passes. Untreated wet and slushy areas may freeze.

"The high country of the Cascades is going to be buried with this storm with the potential for a few feet of snow," Rayno said.

Snow from the pair of storms will be a shot in the arm for the ski resorts in the region that took a hit from the recent warm rainstorms, but there is some risk when heavy snow piles up quickly over the high country.

Any time there are fluctuating snow levels, a heavy amount of snow and varying snow density in the Cascades, there is an increased risk of avalanches with the steep terrain, AccuWeather Meteorologist Lauren Hyde said.

It is possible that just enough cold air is pulled in during the departing storm early next week for wintry precipitation at low elevations. Rain may transition to snow near sea level in southwestern British Columbia, including the Vancouver area, with the potential for a few centimeters (1-3 inches of snow) from Sunday night to Monday morning. There may even be an attempt for a change to wet snow in Bellingham, Washington, that can leave a small, slushy accumulation and for wet snow to mix in around Seattle by Monday morning.

"Whether there is more or less than a mix of snow or a small amount of slush will depend on the exact track of the storm," Adamson said.

Adamson explained that a track a bit farther south than anticipated could allow temperatures to drop a bit more which could mean more snow is possible in southwestern British Columbia and part of northwestern Washington. However, a slightly farther north track would mean less chance of enough cold air coming in at the last minute for snow near sea level in the same area.

The same storm is forecast to roll southeastward across the intermountain region of the West early next week with some rain and perhaps a bit of snow for Boise, Idaho, and Salt Lake City. While most of what little precipitation that falls in the Denver area from Monday to Tuesday is likely to be in the form of intermittent rain, it is possible that just enough cold air sneaks in on the backside of the storm later Tuesday into Tuesday evening for a brief period of snow, forecasters say.

The Mile High City is experiencing a snow drought and has not received any measurable snow since April 21. This is the latest on record that snow of at least 0.1 of an inch or greater has not been measured, according to the National Weather Service office in Boulder, Colorado. Should snow avoid Denver early next week, the all-time number of days without measurable snow from 1887 could be broken. That year, there was no measurable snow from Mar. 5 through Oct. 25, or 235 consecutive days. As of Dec. 3, the snow drought was at 226 days.

There could be other attempts to end Denver's run of non-measurable snow days next week. An active storm track may set up that would allow storms to dive from the Northwest to the Rockies. The pattern could also bring much-needed rounds of locally heavy snow to parts of the Sierra Nevada, Wasatch and the central Rockies.

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