Officials in California are imploring residents to prepare for a powerful storm set to lash the region with torrential rain later this week as the state continues to recover from colossal amounts of snow that trapped mountain communities.

About 17.5 million people across central and Northern California – including the San Francisco Bay area and Sacramento – and parts of Nevada were under flood watches Wednesday ahead of a storm set to drench the region Thursday with dangerous amounts of rain in most of the places that currently have existing layers of heavy snow from previous brutal storms.

“A storm arriving Thursday will bring a threat of flooding from a combination of heavy rain and snowmelt to lower elevations and foothills in California, especially below 5000 feet,” the Weather Prediction Center said. “And heavy, wet snow at higher elevations will lead to difficult travel and impacts from snow load.

“Creeks and streams in the foothills of the Sierra Nevada will be most vulnerable to flooding from rain and snowmelt.”

In response to the ominous forecast, officials in Marin and Monterey counties in California have begun preparations ahead of the looming storm, which is expected to strike the area as a strong atmospheric river event.

For the Big Sur community in Monterey County, the emergency services office went as far as advising residents and businesses to stock up on essentials that would supply them for at least two weeks. The county has also made sandbags available for residents who need them to protect their property. The Big Sur area, roughly 150 miles south of San Francisco, is one of central California’s renowned tourist attractions with picturesque rugged cliffs, mountains and hidden beaches along the Pacific Coast Highway.

Marin County’s fire department will have staff prepared for rescues in anticipation of possible flooding, county Fire Chief Jason Weber said.

Marin County, where a flood watch is in effect beginning Thursday, is home to one of California’s urban search and rescue task forces, and it will make its resources available for other counties as needed, Weber said.

This week’s severe weather threat comes as much of California has been hit with several back-to-back rounds of heavy snow that made roads impassable for days and knocked out power for thousands of residents as temperatures dropped.

Meterologist Katrina Hand of the Sacramento office of the National Weather service said when the storm first hits that area there likely will be some urban flooding, ponding and flooding from the smaller creeks and streams. Later, as the main rivers rise, more roads will flood.

Strong winds that could knock down power lines are also in the forecast, she said.

In hard-hit San Bernardino County, where mountain residents have been trapped in their homes, crews conducted dozens of rescues over the weekend, the county’s sheriff’s department said on Facebook. On Monday night, the county continued well-checks and community outreach, the sheriff’s department said. Despite the removal of large amounts of snow, some roads remained closed early Wednesday.

In addition to the heavy snow that overwhelmed the state last week, more than a foot of additional snow has already fallen this week in some mountainous parts of Northern California. And Wednesday is expected to bring more to that region, where lower elevations could see between 1 and 6 inches of snow with isolated totals surpassing a foot of snow across highest elevations of the Sierra Nevada mountain range.

The impending atmospheric river event won’t be the first this year to lash California. Late last year and into the new year, multiple rounds of heavy rains from atmospheric rivers devastated much of the state – soaking entire neighborhoods and unleashing mudslides while killing at least 18 people.

Atmospheric rivers are long, narrow bands of moisture in the atmosphere that transport warm air and water vapor from the tropics. They can extend for thousands of miles and dump rain and snow when they make landfall.

The danger of rainfall on top of existing snow

Much of California stands to be impacted by this week’s expected atmospheric river.

The Weather Prediction Center says parts of the state have a level 3 of 4 risk – the second-highest on the center’s scale – of excessive rain Thursday into Friday.

The storm is expected to drop some significant rainfall on top of some heavy snowpacks. The National Weather Service is expecting widespread rainfall totals of 2 to 4 inches, with isolated amounts up to 8 inches.

“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 Weather 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.”

Additionally, the threat of heavy rain seeping into deep snowpack could lead to the snow’s weight to increase, which can cause roofs to collapse, the prediction center noted. “Affected communities are urged to remove the existing snow from their roofs to mitigate this,” the weather agency added.

California Department of Transportation District 3 spokesperson Steve Nelson said this weekend is expected to bring “chaos” on the roads.

Referring to Interstate 80 he said if it’s a wet snow that will create issues with traction. If it’s a regular snowfall, they can keep the highway open to vehicles with chains or four-wheel drive, he added.

According to Nelson, officials were holding all big rigs at Applegate, about 40 miles from Sacramento, and holding all westbound traffic at Truckee, about 30 miles southwest of Reno, Nevada, due to multiple spinouts near the Donner Summit where there is hundreds of inches of snow.

Interstate 80 heads northeast from San Francisco and has been closed several times during recent weather.