Storm to bring month's worth of rain to California, yards of snow

A long-duration storm has the potential to unload a month's worth of rain in a few days and deliver blockbuster snow to the mountains of the West Coast from this weekend to next week, AccuWeather meteorologists warn. Heavy rain and snow will be beneficial for drought relief, but it can lead to major problems for travel and pose risks to lives and property.

The storm with eyes for the United States' West Coast moved from the Bering Sea between Alaska and Russia on Thursday to Pacific Ocean waters south of Alaska on Friday. Winds from the jet stream will steer the storm southeastward along the Pacific coast of North America into next week. The storm may even stall for a time or slow down significantly as it nears California, which could enhance precipitation further. Rain totals could climb as high as several inches in central and northern parts of California as the slow-moving storm may deposit yards of snow in the Sierra Nevada.

"As the storm slides from north to south along the West Coast, a firehose effect of moisture, known as an atmospheric river, may lead to rainfall rates close to 1-2 inches per hour for a time and snowfall rates of 2-4 inches per hour over the mountains," AccuWeather Senior Meteorologist Tyler Roys said.

"The magnitude of rain and mountain snow with this storm will lead to dangerous travel and road closures," AccuWeather Meteorologist Rob Richards said, adding that flooding, washouts and mudslides are likely with the storm. The inclement weather is likely to add to shipping delays during a critical time ahead of the holidays as truck, rail and aircraft travel may be hampered.

While impacting the vast area from southeastern Alaska and coastal British Columbia into Friday night, the storm will begin to blast northwestern Washington and spread southward and inland over Washington and Oregon during the first part of the weekend.

The storm's excessive rainfall could add insult to injury as the zone from British Columbia to western Washington has weathered storm after storm this fall. Seattle recorded its wettest meteorological autumn, which spans from September through November, on record with 19.04 inches of rain.

With three weeks to go in 2021, rainfall in Quillayute, Washington, has already topped 100 inches. The city along the Pacific coast on the Olympic Peninsula typically receives 101 inches of rain on average per year, but a few additional inches of rain are expected this weekend on top of the 105 inches observed as of Dec. 9.

A general 2-4 inches of rain with locally higher amounts are forecast to fall along the Washington and Oregon coasts, as well as the lower, west-facing slopes of the Cascades this weekend. Slow travel is likely due to rain and gusty winds along the Interstate-5 corridor. Meanwhile, several feet of snow will pile up in the Cascades with the risk for Snoqualmie and Stevens passes to close for a time.

As cold air arrives in the wake of the storm and moisture lingers, "snow could reach down close to sea level and the valley floors of the Pacific Northwest," AccuWeather Senior Meteorologist Bill Deger said. It is possible that it could snow a bit around Seattle and Portland, Oregon, for a time by midweek.

The storm could hit California especially hard.

The worst of the storm is likely to hit the San Francisco and Sacramento areas from late Sunday to Monday night as the storm's progress slows to a crawl. Normal December rainfall for San Francisco and Sacramento are 4.14 inches and 3.43 inches, respectively. In less than 48 hours, both of those locations could pick up the normal monthly rainfall from this storm. Other locations can also receive a month's worth of rain in a couple of days as the system inches across the Golden State.

With the potential for several inches of rain to fall, flooding and mudslides may not be limited to recent burn scar locations. Urban flooding is a strong possibility in cities such as San Francisco, Sacramento and perhaps even Los Angeles, according to AccuWeather forecasters.

The heaviest rain is expected to roll into Southern California and the Los Angeles area Monday night and may linger through much of Tuesday. Motorists should expect major delays.

"The Los Angeles area has the potential to receive 1-2 inches of rain from the storm," Richards said. Higher amounts are likely, perhaps on the order of 4 inches or more in the San Gabriel, Santa Monica, San Bernardino and San Rafael mountains to the north of the Los Angeles basin. Even areas farther south, such as San Diego, are likely to receive significant rain.

Snow levels are expected to fall and may dip to near and below the passes for a time in Southern California from Tuesday to Wednesday with several inches to a foot or more of snow possible over some of the highest elevations. It is possible, in addition to Donner Pass along Interstate 80 in the Sierra Nevada, that Cajon Pass along I-5 and Tejon pass along I-15 in Southern California could close as well.

If the storm develops to its full potential and lingers for a time over California next week, a general snowfall of 2-4 feet could accumulate in the Sierra Nevada, although higher snowfall amounts of 6 to perhaps 8 feet of snow could be unleashed in the high country.

"Blizzard conditions are likely in the passes as the storm pushes southward this weekend into next week," AccuWeather Senior Meteorologist Brett Anderson said. Conditions will turn dangerous and potentially life-threatening for travelers in the region.

CLICK HERE FOR THE FREE ACCUWEATHER APP

Even though the storm will pose hazards in the short term, there will be some benefits, especially to the ongoing drought. More than 80% of California is in the grips of extreme and exceptional drought, the two highest levels recognized by the U.S. Drought Monitor.

Since the start of 2020, rainfall in much of California has only been about 50% of average, and the long-term drought will not be solved by one storm alone. For example, there are double-digit rainfall deficits in San Francisco and Sacramento. However, the storm may take a bite out of the long-term drought in the region.

"The multi-day storm along the West Coast should prove to be very beneficial to the snowpack across the Cascades and Sierra Nevada initially and later next week for other ranges that make up the Intermountain West," Deger said.

The snow will contain inches of water that can be released later in the winter or spring and run off into area streams and rivers. Rivers such as the Colorado and water levels on Lake Mead could experience much-needed rises should storms continue to dump snow over the West during the winter season. Water levels on Lake Mead had dipped to the lowest levels on record this past summer. The controlled flow of water through Hoover Dam generates a significant amount of electricity for the southwestern U.S., and water levels along the Colorado River are a lifeline for agriculture.

The storm from late this weekend into next week will follow closely behind a storm that was dropping heavy snow from parts of the Sierra Nevada to the Wasatch Range and Colorado Rockies late this week.

For the latest weather news check back on AccuWeather.com. Watch the AccuWeather Network on DIRECTV, Frontier, Spectrum, fuboTV, Philo, and Verizon Fios. AccuWeather Now is now available on your preferred streaming platform.