Storms to bring rain, snow to northwestern U.S.

The northern Pacific will remain active as storms packing ample moisture in the form of drenching rain and mountain snow move into the northwestern United States starting this weekend, AccuWeather meteorologists say.

The storms will continue to build the high-country snowpack in the Cascades and Olympics and occasionally add to the high terrain snow in the Siskiyous and northern Sierra Nevada.

A potent storm will primarily target Washington and southern British Columbia this weekend, with the bulk of the low-elevation rain and mountain snow expected Saturday due to a minor atmospheric river.

By Saturday evening, between 0.75 of an inch and 1.50 inches of rain is likely to fall along the Interstate 5 corridor of Washington and Oregon, with the greatest amounts in northern Washington. The rain may be heavy enough to cause flash flooding and ponding on the major roads that drain poorly. Heavier rain-up to 3 inches or so-may fall along the immediate coasts of Washington and Oregon, as well as the lower west-facing slopes of the Cascades, Coast Ranges and the Siskiyous.

In the Cascades and higher peaks of the Olympics, a general 1 to 3 feet of snow is forecast to fall into Saturday evening. Travel will be difficult over Stevens and Snoqualmie Passes, with the potential for temporary road closures. Snow may fall at the rate of 1 to 2 inches per hour.

The storm total snow into Sunday evening is forecast to reach between 10 and 20 inches at Stevens Pass and 8 and 16 inches at Snoqualmie Pass. Accumulations will tend to taper off over the Oregon Cascades.

Farther inland, snowfall over the Clearwater, Bitterroot and Blue ranges through the weekend will range from 6 to 12 inches with locally higher amounts on some ridges and peaks.

Throughout the region, snow levels will rise considerably from Sunday to Monday.

Total rainfall in the wettest places-from Friday to early Monday-may approach 4 inches, with up to a couple of inches along the I-5 corridor.

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Periods of rain are in store for most lower elevations from east of the Cascades to the Snake River Basin, with some snow mixing in at the intermediate elevations into early Monday.

A break from the storms is expected from Monday to Wednesday next week, with some sunshine and rebounding temperatures that may lead to significant snowmelt at the intermediate elevations, including pass levels. The runoff could trigger rockslides and even elevate the avalanche risk in the high country.

Later next week, beginning Wednesday night along the coasts of Washington, Oregon and possibly Northern California, the next storm will approach with showers and breezy conditions. Multiple spokes of moisture from the system will push onshore and then inland, bringing rounds of low-elevation rain and mountain snow for the second half of the week and into the weekend.

The main part of the storm may linger over the Gulf of Alaska, stirring up rough seas and coastal waves for days alongside windy conditions. As the pattern evolves, a major batch of moisture-with heavy rain, snow and strong winds-may eventually be flung onshore.

Rainfall has been a few inches below the historical average for the Seattle and Tacoma, Washington, area since Dec. 1. Totals have been close to average in Portland and Eugene, Oregon, and Vancouver, British Columbia. Conditions have been wetter than average in The Dalles and Medford, Oregon, as well as Spokane, Washington.

Seattle typically receives around 18 inches of rain from the start of December to March 20, while Medford tends to average around 9.40 inches for the period.

No significant rain or snow is forecast to reach Southern California through next week.