US West Coast stormy siege with flooding rain, mudslides and yards of mountain snow to continue into midweek

By Alex Sosnowski, AccuWeather senior meteorologist

January 05, 2019, 11:01:55 AM EST

A series of potent storms will slam much of the West Coast of the United States with rounds of drenching rain, heavy mountain snow, gusty winds and pounding surf through the middle of next week.

Following the storm into Saturday night, two additional storms will roll ashore.

One storm will hit late Sunday to Sunday night and another storm will roll ashore Tuesday night and last into Wednesday. The biggest and baddest of the bunch is likely to be the storm around midweek.

While the pattern will be a boon for the ski industry and go a long way toward drought relief, each successive storm will hinder travel and raise the risk of flash flooding, mudslides and avalanches.

Download the free AccuWeather app to view the details on each storm and any advisories that are issued for your location or points along your travel route.

The main bombardment of moisture from all of the storms and corresponding heaviest rain and mountain snow will be on Northern California, although heavy amounts of precipitation will also extend to western Oregon, western Washington and British Columbia. The cumulative amount of precipitation will be tremendous from the storm train in these areas.

Storms to unload inches of rain

Rainfall on the western slopes of the Pacific Coast Ranges in Northern California, northern Sierra Nevada and Olympics will be on the order of 5-10 inches with locally higher amounts.

The risk from flash flooding and mudslides will increase with each storm, with the greatest potential for life-threatening conditions likely during the storm spanning late Tuesday to Wednesday. By then the ground will be saturated and the hillsides in recent burn areas are likely to be unstable.

The storm at midweek may bring 3-6 inches of rain alone on the west- and southwest-facing slopes in Northern California.

People should heed all evacuation orders for their area, if given, as there may only be minutes to move to safety.

Motorists are reminded to not attempt to drive through flooded roads or around barriers. The water may be deeper than it appears and/or the road surfaces beneath the water may be weakened or washed away. A foot of water is enough to cause most vehicles to loose contact with the road. Two feet of water can cause even heavy vehicles to float into deeper water.

"Much of, and perhaps all of the rain may stay north of Point Concepcion from this series of storms," according to AccuWeather Meteorologist Maggie Samuhel.

Some rain is in store for coastal areas of Southern California, but the most likely time for wet weather and slick roads in the Los Angeles and San Diego areas will be centered on Saturday night.

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Feet and yards of snow forecast for the mountains

The storms will provide a tremendous boost to the snowpack, which then becomes much of the state's water supply during the warm season.

"Snowfall is likely to be measured in yards over the high country of the northern Sierra Nevada and Cascades and peaks of the Olympics," according to AccuWeather Senior Meteorologist Dan Kottlowski.

As much as 10 feet of snow may pile up on the ridges and peaks of the Northern Sierra Nevada through Wednesday evening.

"Freezing levels will vary with each storm and have a profound effect on travel over Donner Pass, California, Siskiyou Summit, Oregon and Snoqualmie Pass, Washington," according to AccuWeather Senior Meteorologist Kristina Pydynowski.

"At this time, we expect rain and not snow to fall over the Grapevine in Southern California with the siege of storms and especially the storm that has the best chance of bringing significant precipitation during Saturday night," Pydynowski said.

Freezing levels and corresponding heavy snow and wintry travel conditions will exist throughout the duration of the storms over the passes from Northern California to Washington through Monday.

During the storm at midweek, rain may fall initially. However, as colder air moves in during the storm, a change to snow and wintry travel is likely as the storm progresses.

Motorists should expect major delays and possible closures when venturing over the passes. Vehicles attempting to venture over the passes in these conditions must have the proper winter equipment.

Whiteouts are likely with each storm over the passes and high country, but the most drastic sudden change from wet to blizzard conditions and strong winds over the passes is likely with the storm during Tuesday night and Wednesday.

Dangerous conditions forecast from the storms

Gusts over some of the passes in the mountains may reach 90 mph with the storms.

Along with the heavy snow, varying snow levels and strong winds over the mountains will be an increasing risk of avalanches. Snowboarders, skiers and snowmobilers should use extreme caution when doing off-trail adventures over rugged terrain.

Large waves will pound the coast with the risk of overwash and coastal erosion. Never stand on jetties or the beach to watch the storm due to the risk of large, unpredictable sneaker waves.

Localized winds may be strong enough with any of the storms to knock down trees and cause sporadic power outages. Gusts in coastal areas from Northern California to Washington may reach 60 mph.

Travel time suggestions through the upcoming week

For those with travel plans, the period from Monday afternoon to Tuesday midday may offer an opportunity from storm-free conditions. However, a better and longer opportunity for travel is likely from Thursday to Friday.

Another dose of rain and mountain snow may roll in Friday night to Saturday. That storm may bring soaking rain to parts of Southern California as well as Northern California.