Storm to set up 'firehose effect' with rain, mountain snow in California

Alex Sosnowski

A storm currently along the California coast is loaded with moisture and will slam the northern part of the state with inches of rain and yards of snow in the Sierra Nevada mountains during the first couple of days of December.

The storm will be double-barreled in nature with the first part set to slam Northern California into Monday afternoon. The second phase is likely to focus on Southern California during Tuesday night and Wednesday.

While recent prior storms have essentially brought an end to the wildfire threat in the short-term, this new setup has the potential to cause flash flooding and mudslides on a more regional basis. The risk of debris flows will be greatest in recent burn scar locations, such as the Kincade Fire in Sonoma County from earlier this fall.

This image, taken on Sunday, Dec. 1, 2019, shows a stream of clouds into Central California that is originating from the tropical Pacific Ocean. (NOAA/GOES-West)

Those heading home from their Thanksgiving visit, grabbing some shopping bargains or resuming their work and school activities should be prepared for adverse weather conditions.

Amid the storm moving into California, over 18,500 people were without power across the state by 7.m. PST Sunday, according to PowerOutage.US.

On Sunday, reports of downed trees continued due to gusty winds. In Monterey County, California, law enforcement indicated trees and power lines were down in the Del Monte Forest area.

Both storms with their atmospheric rivers are likely to produce a firehose effect with a zone of heavy rain and heavy mountain snow.

First part of storm to hammer Northern California

A general 1-3 inches of rain is likely to fall over the lower elevations of Northern California from the first part of the storm into early next week. However, some of the west- and south-facing lower slopes of the Coast Ranges and Northern Sierra Nevada can expect a general 3-6 inches with an AccuWeather Local StormMax™ of 8 inches.

As of 10 a.m. PST, Sunday, Santa Rosa, California, had received 2.54 inches of rain from the storm with 2.27 inches falling on Vacaville, California.

The rain may be intense enough on Monday to impact the morning commutes in San Francisco and Sacramento.

"High winds are likely to hammer the coast, including the San Francisco Bay area," according to AccuWeather Meteorologist Bill Deger.

Winds from the south and southeast can gust to 50 mph.

"Gusts this strong can knock over trees and cause power outages," Deger said.

In terms of snowfall, the freezing level will rise as the storm progresses from Saturday night to Monday. This means that snow will change to rain at intermediate elevations. However, a general 2-4 feet of snow is in store at elevations above 7,000 feet with an AccuWeather Local AccuWeather Local StormMax™ of 6 feet likely above 7,500 feet.

At the intermediate elevations from 2,500 feet to 5,000 feet, a quick meltdown of the snow combined with rain could cause streams and short-run rivers to surge out of their banks early next week.

Some rain will dip southward to around Point Concepcion, but more substantial rain is likely by midweek over much of Southern California.

Second part of storm to target Southern California around midweek

The second part of the storm is likely to spare at least part of Northern California, but focus on Southern California with heavy rain and high-country snow from Tuesday night to Wednesday.

A general 1-2 inches of rain is forecast to fall in coastal Southern California, but at least a few showers are expected to reach past the mountains and into the deserts. An AccuWeather Local StormMax™ of 4 inches can occur over the south- and west-facing slopes of the mountains.

Should the storm race ashore late Tuesday, the evening commute could quickly deteriorate in Los Angeles and San Diego. However, the morning commute on Wednesday is likely to be the worst with areas of urban flooding and debris on some of the roads in the hilly terrain.

It is possible that some rain reaches as far to the north as San Francisco and Sacramento with the second storm.

The same storm at midweek will also send heavy rain and high freezing levels into Arizona, according to AccuWeather Meteorologist Randy Adkins.

"The rain can be heavy enough alone to cause flash flooding in Arizona," Adkins said. "Conditions can be especially bad where the combination of rain and melting snow occur following the recent snowstorm."

Like the storm in Northern California this weekend, the storm in Southern California from later Tuesday to Wednesday can bring strong enough wind gusts to break tree limbs, knock over trees and lead to sporadic power outages.

In terms of snow, the freezing level with this second storm in Southern California will be well above Tejon and Cajon passes. However, 1-2 feet of snow can fall above 7,500 feet in the southern Sierra Nevada with local amounts to 3 feet possible.

The latter part of the storm is most likely to send rain and mountain snow over the interior Southwest by midweek.

Much of Thursday is likely to be free from rain and snow in California. However, a break from storms is likely to be short-lived.

The next storm from the Pacific Ocean is forecast to roll ashore in Northern California late Thursday or early Friday. Rain and mountain snow will push inland and expand southward into next weekend.

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