Some California residents are urged to prepare 2 weeks of essentials ahead of expected flooding this week

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 16 million people across central and Northern California, including the San Francisco Bay Area and Sacramento, were under flood watches early Wednesday ahead of a storm set to drench the region Thursday with dangerous amounts of rain – in some places on existing layers of heavy snow.

“The combination of heavy rain and snowmelt may lead to flooding,” the Weather Prediction Center said. “Creeks and streams in the western foothills of the Sierra Nevada will be most vulnerable to flooding from rain and snowmelt.”

The ominous forecast comes as much of California has been hit with several back-to-back rounds of heavy snow that have trapped mountain residents in their homes, made roads impassable for days and knocked out power for thousands of residents as temperatures dropped.

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.

Closer to the coast, officials in Marin and Monterey counties have begun preparations days 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 at least two weeks. The county has also made sandbags available for residents who need them to protect their properties.

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

“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,” Weber told CNN.

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 anticipated 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.