Heavy rains and damaging winds swept into California as the first of two back-to-back atmospheric rivers brought the renewed threat of flooding and mudslides to a state already battered by deadly winter storms.

The storm will likely be followed Monday by another atmospheric river, a weather system that delivers a focused plume of moisture off the ocean. And long-range forecasts suggest two more could strike California later this month, State Climatologist Michael Anderson said Thursday.

Excessive rainfall is forecast into the weekend, according to the National Weather Service.

“Areas that normally do not experience flash flooding will flood,” the service said in a statement. “Lives and property are in great danger from Friday into Saturday morning.”

The storms arrive even as parts of the state continue digging out from heavy snows that trapped many mountain residents in their homes. Thursday’s atmospheric river, drawing warm Pacific air from south of Hawaii, will melt some of that snow, raising flooding risks. Officials this week increased the amount of water being released from foothill reservoirs to make room and warned residents to keep a close eye on local conditions.

California Governor Gavin Newsom requested a Presidential Emergency Declaration on Thursday amid “the relentless and deadly storms,” which if approved, would allow for impacted counties to access federal assistance.

FLOODING RISK

As much as 15 inches of rain may fall across the Sierra Nevada snowpack, while coastal areas could get as much as 10 inches from the first storm, said Bob Oravec, a senior branch forecaster at the US Weather Prediction Center.

Eleven river gauges across Northern and Central California were expected to hit flood stage in the coming days, including the Russian River through the wine country, the San Lorenzo River through Santa Cruz and the Salinas River near Monterey.

“The first two days are just huge, and even the third day in the Sierra really has significant amounts,” Oravec said. “Then the second round comes in early next week. For the next week it is really just going to get ridiculous.”

Flood watches, wind advisories and winter storm warnings blanket most of California and its neighbors. California, the most populous US state, has declared a state of emergency in 34 counties. About 69,000 California utility customers were without electricity by 4:15 a.m. local time today, according to the poweroutage.us website.

The rain comes months after a series of atmospheric rivers hit California in December and January, killing at least 19 people, knocking out power to 500,000 homes and businesses and causing billions of dollars in damage. The weather systems, which can stretch for thousands of miles, can deliver as much water as flows through the mouth of the Mississippi River.

Then in late February, an unusually cold storm brought blizzard conditions to the mountains above Los Angeles and coated the San Francisco Bay Area peaks in snow, with flakes even falling at sea level along the state’s north coast.

The moisture has helped roll back the ongoing drought across California. Nearly 99.5% of the state was gripped by drought three months ago and, as of this week, that has fallen to just over 43%, according to the US Drought Monitor’s Thursday report.

But the warmth of the new round of storms poses an increased flooding risk because much snow remains at relatively low elevations that will now see rain. After weeks of snow falling as low as 3,000 feet, the freezing line will jump as high as 7,000 feet in the Sierra by today, Anderson said. Only the highest mountains will get snow.

The cumulative effect of the storms throughout the winter, as well as the warmth this system is bringing, adds to the severity of the impacts, said Paul Pastelok, meteorologist at commercial forecaster AccuWeather Inc.

“When you look at the strength of them, every single one becomes more and more important because of what has already happened,” Pastelok said. “The warmer the system, the worse it is.”