2nd atmospheric river to flood California with firehose of rain into Christmas

Two major atmospheric rivers will force large amounts of Pacific moisture into California this week. Heavy rains accompanied by flooding, washout and mudslide events will expand and move from north to south across the state, causing at least some disruption to travel and, in extreme cases, some risk to life and property. The Sierra Nevada Mountains will receive several feet of snow due to low ice levels this week, and the region is affected by two atmospheric rivers.

The storm comes at a busy time of year when many people are traveling short or long distances, running errands and planning outdoor activities.

The first atmospheric river is already underway and will continue to focus on northern to central California until Wednesday. A second atmospheric river is expected to form and focus over central and southern California from Tuesday night into Christmas Day. But the continued moisture will lead to more showers and mountain snow even after Christmas, according to AccuWeather.

By Wednesday, much of the Sacramento Valley and San Francisco Bay area will receive 2 to 4 inches of rain, with locally higher amounts. San Francisco is expecting storm rainfall over the next weekend to be one to three times the historical average for December.

Rainfall amounts of 4-12 inches, with AccuWeather Local StormMax™ of 20 inches, on the west and southwest slopes of the Coast Range and Sierra Nevada.

Sometimes, inches of rain fall in a matter of hours, flooding storm drains and causing creeks and short rivers to quickly overflow their banks. As the ground becomes saturated, the risk of mudslides, road washouts and hillside collapse will increase significantly.

A second atmospheric river will spew varying intensities of rain across central and southern California from Tuesday night into Christmas night.

Rainfall amounts are generally expected to be 2-4 inches in the Los Angeles Basin. Downtown Los Angeles received 5.53 inches of monthly rainfall in November, seven times the historical average monthly rainfall. While it hasn’t rained so far in Los Angeles this month, stormy weather Tuesday through Saturday could result in one to two times the average December rainfall of 2.48 inches.

On the southwest slopes of the Transverse Mountains, rainfall amounts of 4 to 8 inches will be expected. Even in many desert areas of southeastern California, rainfall amounts are expected to range from 0.25 inch to 1 inch.

Like the North’s first atmospheric river, rainfall will cause rapid flooding of cities, overflowing of creeks and short-flowing rivers, and cause mudslides, washouts, and landslides in Southern California.

Get the free ACCUWeather app

Is there an app? Unlock AccuWeather Alerts™ with Premium+

Airlines are expected to experience delays and flight cancellations as rounds of heavy rain and high winds impact San Francisco, Los Angeles, San Diego and dozens of secondary airports, which could have a ripple effect on the national aviation industry as crews and aircraft are displaced and flights are forced to be rescheduled.

Storm may intensify rapidly; strong winds imminent

“This storm, which feeds atmospheric rivers, has the potential to become a bomb cyclone off the California coast,” AccuWeather meteorologist Brandon Buckingham said. “Whether the storm intensifies quickly and experiences the required pressure drop of 24 millibars (0.71 inches of mercury) within 24 hours, the storm will strengthen enough to produce strong winds along the northern and central California coast.”

Large waves will hit the California coast with wind gusts of 50-70 mph, and AccuWeather StormMax™ winds reaching 130 mph on ridges and crevices. Storms could cause excessive washing of Southern California beaches.

Winds this strong will cause scattered regional power outages. Loose items around the house, such as Christmas decorations, can become dangerous projectiles or become damaged. When crosswinds are present, the likelihood of a vehicle rolling over increases.

Several feet of snow could cause road closures in Sierra Nevada

During the week, the atmosphere will cool enough for snow levels in the Sierra Nevada and Siskiyous to drop enough for heavy snow to fall at or below mountain pass levels.

While a few inches of snow will fall in Donna Pass, Calif., and along Interstate 80 by Tuesday as some warm air brings showers, several rounds of heavy snow Wednesday night into Friday will bring 1-2 feet of snow that could bring traffic to a standstill.

“We’re likely to see at least 10 feet of snow on the ridges and peaks of the Sierra Nevada this week, which is exactly what they need from a water demand standpoint for the upcoming summer,” said AccuWeather Chief On-air Meteorologist Bernie Rayno.

Ski resorts want plenty of natural snow on their slopes, especially during the industry’s busy holiday season. However, heavy snow accumulations may block roads leading to higher altitude resorts.

Showers, mountain snow will follow atmospheric rivers

Abundant moisture supplies will diminish with the storm later this week and over the weekend between Christmas and New Year’s. However, there is enough power left to produce showers across much of the state.

During this time, snow showers will continue to lead to further snow accumulation in the Sierra Nevada due to the presence of cold air. Snow and accumulation will also occur in the mountains of Southern California, which may cause slippery roads along Interstate 5 and Interstate 15.

The same storm pattern will continue to bring periodic rain and mountain snow to Oregon and Washington, both of which have experienced severe rainfall and mountain snow in recent weeks. Continued wet weather may exacerbate flooding problems in local creeks and rivers.

The same Pacific storm pattern will guide some moisture through the western mountains next weekend in the form of valley rain and mountain snow. Both have been thin so far this season.

Want a higher level of security, ad-free? Unlock advanced, hyper-localized severe weather alerts when you subscribe to Premium+ on the AccuWeather app. AccuWeather Alerts™ are sent out by our professional meteorologists who monitor and analyze hazardous weather risks 24/7 to keep you and your family safer.