Atmospheric river to follow 'bomb cyclone' in California as December begins

Alex Sosnowski

Soon after the 'bomb cyclone' pushes east of the Rockies, a new storm will approach the West Coast of the United States this weekend to set the stage for more rounds of heavy rain and mountain snow in California.

The storm will hinder return trips from Thanksgiving vacation as well as the resumption of daily commutes next week.

The weather setup will make the ignition of wildfires unlikely and may even put an end to the risk through the rest of the year. However, enough rain can fall to unleash urban flooding and lead to debris flows in recent burn scar locations, including the Kincade Fire that burned over 77,000 acres in Sonoma County, California, earlier this autumn.

While the heavy mountain snow anticipated will be a further boon for ski interests, snow and slippery driving conditions are likely at Donner Pass along Interstate 80 from Saturday afternoon into Monday night.

A storm at the jet stream level of the atmosphere is forecast to hover just off the California coast later this weekend to early next week. The configuration will create a plume of tropical moisture from the Pacific Ocean to Northern California beginning late Saturday night.

"This persistent plume, sometimes referred to as an 'atmospheric river,' will bring copious amounts of rain to coastal and low elevations and a great deal of snow to the Sierra Nevada of Northern California," said AccuWeather Senior Meteorologist Brett Anderson.

A woman carries an umbrella while walking in the rain in San Francisco, Tuesday, Nov. 26, 2019. Northern California and southern Oregon residents were hit by a 'bomb cyclone' at one of the busiest travel times of the year. (AP Photo/Jeff Chiu)

About 3-6 inches of rain can fall over the lower elevations of Northern California during the early to middle part of next week. However, an AccuWeather Local StormMax™ of 10 inches can occur on the southwest-facing slopes of the Coast Ranges and hills of the Sierra Nevada.

Meanwhile, over in the high country, 3-6 feet of snow can fall from later this weekend to early next week.

"It is possible that Southern California catches an episode or two of heavy rain and mountain snow as well," Anderson said.

"Right now, the best chance of heavy precipitation for the Los Angeles and San Diego areas would be toward the middle of next week," Anderson added.

Rain and mountain snow may avoid much of Southern California until Wednesday.

"In a sense, fire season has come to an end, and mudslide and flash flood season is beginning in California," AccuWeather Senior Meteorologist Dale Mohler said.

The stormy pattern is not likely to end after the middle of next week.

Storms are lining up across the North Pacific, and at least some of these moisture-laden weather systems will reach Northern California during the first half of December.

These webcams show travel at a standstill on Interstate 80 in California near Donner Summit on Wednesday evening. Similar scenes may unfold in the coming days as more mountain snow falls over California. (Images/Caltrans)

With the arrival of each storm and the persistence of the wet pattern, there is the risk of the pendulum swinging the other way from too dry to too wet with perhaps damaging consequences.

The pattern over the next couple of weeks may not be an indicator of what is ahead for the entire winter, AccuWeather forecasters say.

"We do expect some lengthy breaks in the storms from the Pacific Ocean in terms of impact on California," AccuWeather Long-Range Meteorologist Tyler Roys said.

"However, rainfall may end up being above average for the winter in Southern California and slightly below average for Northern California," he added.


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