Relentless storm train to hammer western US travel

As storms continue to roll in from the Pacific, each will bring a dose of low-elevation rain, mountain snow and strong winds that may not only further strain holiday travel but also lead to other weather-related problems into the start of 2023, AccuWeather meteorologists warn. An atmospheric river may take aim at part of Northern California early on during the New Year's weekend.

A storm that rolled ashore to start the week packed a punch in terms of impacts. At one point, close to 200,000 utility customers were without power from California to Washington on Tuesday, according to The number of outages had dropped to about 15,000 as of Thursday morning.

Winds gusted close to 70 mph at Astoria, Oregon, from the storm during Monday night. However, gusts in some locations from California to Idaho topped 80 mph.

As this storm pushed inland over the West through Wednesday night, heavy snow fell on Utah's Wasatch Range. The same storm then moved into Denver, where a general 6-9 inches of snow fell. The heavy snow caused numerous traffic accidents and even prompted officials to briefly close portions of Interstate 70, bringing traffic to a standstill.

Credit: Colorado Department of Transportation

Following the storm that affected the Pacific coast states from Monday to Tuesday, at least three additional storms are lining up over the Pacific and are on a path toward the region through New Year's Day.

Rainfall from the storms from Thursday to Friday will generally be light, but significant. However, a storm taking aim at the Pacific coast, and particularly California can bring excessive rain, should a strong atmospheric river develop as AccuWeather meteorologists suspect.

An atmospheric river is a plume of moisture that extends from the ocean to the land. Within this firehose of moisture, torrential downpours and significant flooding can occur, should the band of rain persist.

"Northern California, and especially the area from San Francisco to Sacramento and the west-facing slopes of the Sierra Nevada can be hit with several inches of rain from Friday night to early Sunday, AccuWeather Senior Storm Warning Meteorologist William Clark said.

San Francisco may receive 2-4 inches of rain from the storm with the risk of 4-8 inches of rain falling near Sacramento. An AccuWeather Local StormMax™ rainfall of 20 inches is possible on some of the mountain slopes with this atmospheric river setup.

Rainfall of this magnitude on saturated ground can lead to significant small-stream flooding and mudslides in addition to tremendous urban flooding. Some of the short-run rivers that flow from the western slopes of the Sierra Nevada to California's Sacramento and San Joaquin valleys could be subject to rapid and significant flooding.

"It is possible that this single storm brings some reservoirs close to full capacity," Clark said. "Where there are no dams or flood control measures, unprotected areas along some of the rivers can take on water in this situation." Oroville, Folsom and Shasta reservoirs in Northern California were hovering just under one-third capacity as of midweek, according to the California Department of Water Resources.


Through New Year's Day, rain, as well as snow, over the interior West can go a long way toward easing the widespread, long-term drought conditions. Not only will moisture be squeezed out along the coast and over the mountain ranges, but a significant amount of snow will fall on the Colorado River basin. Lake Mead, along the Colorado River, at the Hoover Dam reached historically low levels this past summer at 1,041 feet. Levels this week at the dam are hovering near 1,044 feet. There are hopes that ample snow this winter may help boost levels for Lake Mead and other waterways during the spring thaw.

Storms to date so far this winter season were already building a significant snowpack in many of the mountain ranges of the West, including the Sierra Nevada, where the average to date ranged from 130 to 165%.

Conditions from the Rockies to the Pacific coast range from abnormally dry to exceptional drought, according to the United States Drought Monitor. Some of the worst long-term drought conditions in the entire West were located over California's San Joaquin Valley.

While the long-term drought benefits from the storm train could be substantial for the Western states, short-term problems from flooding can be extensive. Debris flows and avalanches may pose some risk to lives and property at the local level.

"Most of the problems from the storms, especially from the storm that started this week and the one coming for the holiday weekend, will be associated with wet conditions and ponding slowing travel on the highways," AccuWeather Senior Meteorologist Heather Zehr said.

People on the roads from western Washington to western Oregon and much of California and Arizona will need to allow some extra time for their commute due to the rain and poor visibility at times through the holiday weekend.

In some cases, however, rounds of heavy rain can lead to flash flooding in urban areas, as well as on secondary roads over the countryside. Debris flows are possible in recent burn scar locations.

The rainy episodes along with fog and locally gusty winds can lead to airline delays at the major hubs from Seattle to San Francisco and Los Angeles. Even though the storm train will be much less impactful when compared to the massive pre-Christmas storm and severe cold wave that followed, some ripple-effect delays stemming from the West Coast are possible through New Year's Day.

Warmer conditions in the eastern two-thirds of the nation moving forward into New Year's Day will tremendously help with airline travel across the U.S. overall, but there may still be some weather-related issues. The storm moving over the western U.S. on Monday and Tuesday will transition to a large area of rain over the eastern U.S. into the start of the weekend. Mild and moist conditions could also lead to areas of fog east of the Rockies, which can be almost as disruptive as a massive snowstorm should fog focus at one or more of the major Eastern hubs.

Since freezing levels will be somewhat higher than average for late December and early January for most of the storms, some of the snow at intermediate elevations can melt quickly, when combined with the rain. This can lead to flooding and rushing water along some of the short-run rivers that extend across the western slopes of the Cascades, Siskiyous and Sierra Nevada, Zehr said.

Any time there is a combination of fluctuating temperatures, shifting snow levels and gusty winds, like there will be with the storm train into early January, the snowpack over the high country can shift. Where that snow cover breaks loose, avalanches can occur, Zerh explained.

In the mountains, including the passes in the Sierra Nevada and Cascades, motorists should be prepared for strong wind gusts, fog, and episodes of heavy rain and snow.

With two weak storms forecast to seep in on Thursday and Friday, snow levels may rise enough for rain or a rain/snow mix to fall at pass level on Interstate 80 in California, but some slushy travel is still likely, AccuWeather Senior Meteorologist Heather Zehr said.

As a larger storm swings in this weekend, snow levels will climb to above Donner Pass for a time on Saturday, but they are likely to plummet from Saturday night to early New Year's Day. That can lead to several inches of snow with clogged roads as the storm pushes inland.

"The storms from Thursday to Sunday have the potential to unload a general 2-4 feet of snow on the high country of the Sierra Nevada, with locally higher amounts," AccuWeather Senior Meteorologist Brett Anderson said.

"Freezing levels are likely to remain too high to bring accumulating snow over the Southern California passes through New Year's Day," Anderson added.

Meanwhile, 1,000 miles farther north, temperatures will fluctuate but will be a few degrees Fahrenheit lower throughout the storm train into this weekend for the Washington and Oregon cascades. This means that more snow is likely for longer periods of time during the storms over the passes. Motorists should be prepared for substantial delays over Snoqualmie Pass (I-90).

Several storms will continue to roll in from the Pacific during the first two weeks of January, which will be good news for drought concerns and ski enthusiasts, but bad news for travelers from a low-elevation rain and mountain snow standpoint. The storms will help to build the snowpack in the Sierra Nevada, Cascades and various mountain ranges of the western U.S., all for future runoff during the spring season.

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