Potent May storm to drench Pacific Northwest, dump snow on Sierra Nevada

Most winter storms are done for the season, but a late-running system will dump big rain on the coastal Northwest and up to a couple of feet of snow on the Sierra Nevada.

An unusually potent storm will affect the West Coast of the United States through Saturday with areas of heavy rain and feet of mountain snow, AccuWeather meteorologists say.

The precipitation will come when most Pacific storms have ended for the season or are throttling down as the dry season builds.

Storm to drench Pacific Northwest

The bulk of the rain will fall from western Washington to Northern California, where a general 1-3 inches will fall. However, there will be pockets where 3-6 inches of rain can pour down, especially on the west-facing slopes of the mountains in Oregon.

Enough rain will fall to slow travel along the Interstate 5 corridor and may trigger some rock slides and debris flows in the steep terrain.

Portland, Salem and Eugene, Oregon, typically receive about 2.50 inches of rain in the entire month of May. All of that and more may fall from this single storm through Saturday. Farther south, Medford, Oregon, tends to pick up about 1.30 inches of rain during the month and double that amount could fall from this storm. Farther north, Seattle could pick up close to the historical monthly average rainfall of about 1.90 inches.

In the Cascades, snow levels will generally remain above pass levels for most of the storm then dip toward the tail end Saturday night to Sunday morning, when a small amount of slush may occur.

Northern California rain, Sierra snow on the way

Due to the storm's track slicing to the southeast across California, rain will tend to diminish from north to south across the lower elevations of the Golden State from Saturday to early Sunday.

Enough rain will fall on San Francisco and Sacramento to produce a few tenths of an inch. That's enough to create some puddles and slick roads, mainly from Saturday to Saturday night.

In Sierra Nevada, the storm will have a bigger impact.

"One of the greatest impacts from this storm could be along Interstate 80 at Donner Pass, California, where nearly a foot of snow could occur to slow travel over the weekend, especially Saturday night to Sunday morning," AccuWeather Meteorologist Joseph Bauer said.

From 1-2 feet of snow will fall on the ridges and peaks of the northern and central Sierra Nevada from the storm during the second half of the weekend, which will be a nice pad to the plentiful amount of snow that fell over the winter -- most of which will gradually melt this spring and summer, filling streams, rivers and reservoirs.

Snow levels will start at around 8,000 feet in Sierra Nevada Saturday evening but can fall quickly Saturday night and end up at around 5,000 feet Sunday morning.

Sparse rain, mountain snow for Southern California

"The storm, essentially an 'inside slider,' is not recognized as bringing heavy precipitation to Southern California in the winter, let alone the spring and the amount of rain that falls in the region will likely reflect that," AccuWeather Senior Meteorologist Adam Douty said.

"The southern end of the storm will lose steam as it moves into Southern California but could bring enough rain to wet roads in the valleys with more appreciable rain expected in the foothills and mountains," Bauer explained.

Motorists should be alert for slick road conditions in Southern California, where a small amount of rain will mix with the oil and deposits left by traffic from weeks of dry weather. The last time most places in Southern California received rain was in mid-April.

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The historical average May rainfall for Downtown Los Angeles is 0.32 of an inch of rain. Last May, 0.55 of an inch fell. Since records began in the mid-1800s, there have been at least two dozen years where no rain fell at all during the month and at least 40 years where less than 0.01 of an inch fell. Meanwhile, the wettest May on record for Downtown L.A. was in 1921, when 3.57 inches fell.

No travel problems from snow are expected over the Southern California passes from the storm.

"For the mountains around the Grapevine, snow levels may dip to 6,000 feet," Bauer said.

Most of the showery precipitation in Southern California may occur from Saturday night to first thing Sunday morning, when it may least affect outdoor plans.

Storm to become a big 'wind machine'

As this storm swings inland, it will tend to become a wind producer over the Intermountain West, Rockies and High Plains. Over some of the ridges and peaks from the Sierra Nevada to the central and southern Rockies as well as the eastern slopes of the Rockies, the AccuWeather Local StormMax™ wind gust is 95 mph.

Winds may become strong enough to cause property damage, knock over trees and bring down power lines from Sunday to Monday from the Rockies to the High Plains. Where the landscape is dry, the risk of rapidly spreading wildfires will increase.

The same storm is likely to trigger an eruption of severe thunderstorms over the Great Plains on Monday as well.

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