Threat of late-week flooding across much of California increases

By Taylor Ward, CNN Meteorologist

The threat of flooding later this week continues to increase across much of California. The Weather Prediction Center now has a Level 3 of 4, moderate risk, for excessive rainfall late Thursday and Friday.

Not only is this event quite strong, but is also much warmer than recent storms, which will cause significant rain on top of a heavy snowpack for many.

The late-week event, known as an atmospheric river, is forecast to be a Category 4 out of 5, according to the Scripps Institute of Oceanography.

“Unlike several of the previous events into the West Coast, this atmospheric river will have a tropical connection that will advect prodigious amounts of both Pacific moisture and also sub-tropical warmth into California,” the prediction center said. “Thus, snow levels are expected to rise well above the levels seen in previous events.”

“A much stronger, wetter, and warmer storm system will impact all of Central California Thursday night into Saturday night,” the National Weather Service in Hanford said. “Excessive rainfall below 8,000 feet and rapid snow melt from 2,000 feet to 5,000 feet may result in flooding.”

The incredible snowpack over California complicates the forecast flood threat from the heavy rain.

“The uncertainty lies in how much rainfall will be absorbed by the snowpack before there is significant release of that water into the rivers,” the prediction center said. “It’s likely some of the (precipitation) will simply be absorbed into the many feet of snow at the highest elevations, but lower elevations, generally below 5,000 ft, appear most likely to not have the snowpack necessary to absorb the multiple inches of rainfall expected.”

“It’s also worth mentioning that while not flooding related, the heavy rain falling into the deep snowpack will result in rapid increases in the weight of the snow on the roofs of buildings, increasing the threat of roof collapses,” the prediction center warns. “Affected communities are urged to remove the existing snow from their roofs to mitigate this.”

Flash flood watches have been issued for much of northern and central California, including San Francisco and Sacramento and cover more than 16 million people. The National Weather Service is forecasting widespread rainfall totals of 2 to 4 inches, with isolated amounts of up to 8 inches.

Officials advise Big Sur residents to stock up with at least 2 weeks of essential supplies

The Monterey County Office of Emergency Services is urging its residents and businesses who are located in Big Sur “to begin stocking up with at least 2 weeks of essential supplies” in anticipation of an atmospheric river weather event that will impact the region this week.

“The National Weather Service (NWS) is predicting that a strong weather system, stronger than recent systems, may arrive late Thursday of this week lasting several days. Current hydrographic estimates have all rivers rising, with the Big Sur River reaching ‘action stage’ and other Monterey County rivers getting close to ‘action stage’ by the weekend,” the news release from the county said.

The Big Sur area is roughly 150 miles south of San Francisco.

The county went on to say “this is a preliminary notice in an effort to provide Monterey County residents and Big Sur residents, in particular, ample time to prepare.”

CNN has reached out to county officials for additional details on preparations.

A flood watch has been issued for the county, according to the National Weather Service’s San Francisco office.

Marin County will increase staff and pre-position water rescue resources

Marin County officials are preparing for this week’s anticipated atmospheric river.

“At this point, we are monitoring expected rainfall both timing and amounts. Our reservoirs are all full from storms earlier this year. With reservoirs full we expect our creeks will rise more rapidly with most of the rain becoming runoff,” Marin County Fire Chief Jason Weber told CNN.

The chief added, “We will up-staff and pre-position water rescue resources and add additional staffing as we get closer based on forecasts.”

Weber said one of the state’s 10 regional Urban Search & Rescue Task Forces is located in the county and will be available to deploy to other counties if they are experiencing significant impacts from the storm system.

The US&R Task Force is comprised of 29 people specially trained and equipped for large or complex urban search and rescue operations, the California Governor’s Office of Emergency Services website says.

A flood watch has been issued for the county beginning Thursday afternoon into Sunday morning, according to the National Weather Service.

Marin County is about 20 miles north of San Francisco.

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CNN’s Joe Sutton contributed to this report.