Torrential rains inundated the California coast on Christmas Eve, sparking numerous flash flood and even short-lived tornado warnings, turning roads into rivers and sending debris cascading down hillsides.

Gov. Gavin Newsom declared a state of emergency in Los Angeles, Orange, Riverside, San Bernardino, San Diego and Shasta counties as the rare and dangerous flood threat descended on Southern California, which was devastated by fires just a year ago.

More than 43 million Americans across California, southern Nevada and northwest Arizona are under flood watches on Wednesday amid a rare, high risk for excessive rainfall and flooding.

This includes major metros like Los Angeles, San Francisco, Sacramento, California, San Diego and Las Vegas.

Los Angeles station KABC reported Wednesday the Inland Empire area east of LA had borne the brunt of the first wave of the storm.

Heavy debris flows from heavy rainfall earlier prompted mandatory evacuations in Wrightwood, California, with the main mountain highway — Highway 2 — being impassable, according to the San Bernardino County Fire Department.

Earlier in the afternoon, the fire department said crews were rescuing people trapped in vehicles conducting door-to-door checks in impacted areas and advising residents in danger zones to evacuate immediately.

With more rain on the way, the San Bernardino County Sheriff’s Department on Wednesday evening issued additional evacuation orders for some areas north of the Wrightwood area.

At least one person died in an “apparent weather-related crash,” amid heavy rain in Sacramento County, according to the California Highway Patrol.

The Lancaster Sheriff’s Station reported on Wednesday night that it was “responding to multiple vehicles trapped in flooded roadways due to severe storm conditions.” It added, “If you do not need to be on the road, please stay home until conditions improve.”

A series of atmospheric rivers brought high-intensity rainfall and strong winds, increasing the risk of flooding, landslides and rapidly rising creeks and rivers.

“California is acting early and decisively to do all we can to get ahead of dangerous winter storms. The state has pre-positioned resources, activated emergency authorities, and we are working closely with local partners to protect communities and keep Californians safe,” Newsom said in a statement Wednesday.

A “High Risk for excessive rainfall” is in place for Los Angeles, including I-10 from San Bernardino to Santa Monica and areas north like Highway 101 to Thousand Oaks, I-5 to Burbank, Santa Clarita, and up to Pyramid Lake and all of I-210. Travel on these roads is not recommended as they may become flooded, officials said. Low-lying neighborhoods in these areas could also become flooded, forecasts show.

Being under a “High Risk” designation is rare. This risk is only issued about 4% of days, accounting for one-third of all flood-related fatalities and 80% of all flood-related damages, according to the NWS.

Potential flooding impacts include the threat of significant and widespread urban roadway flooding, a high risk of major rock/mudslides, and rapid rises in creeks, streams and rivers which will likely lead to swift water rescues.

The recent burn scars will be at risk of possibly damaging debris flows. These flooding impacts will likely lead to significant travel delays and road closures during the busy holiday travel period.

Winds are forecast to gust 40 to 50 mph across the area, potentially leading to power outages on Wednesday, according to forecasts. Thunderstorms are also possible.

More than 100,000 customers have been reported without power in California.

The heaviest rain is expected on Wednesday morning and afternoon. Rainfall rates of 1 inch per hour or greater are expected.

Thunderstorms will remain possible, so people will need to watch for cloud-to-ground lightning.

A brief tornado cannot be ruled out today all along all of the California coast, so people should heed any tornado warnings issued.

By 6 p.m. or 7 p.m. PT, the rain will be coming to a brief end before more rain arrives overnight.

Additional rounds of rain are expected on Thursday and Friday, and the flood threat along with mudslides and landslide risks will continue each day as well.

ABC News’ Amanda Morris contributed to this report.

Torrential rains inundated the California coast on Christmas Eve, sparking numerous flash flood and even short-lived tornado warnings, turning roads into rivers and sending debris cascading down hillsides.

Gov. Gavin Newsom declared a state of emergency in Los Angeles, Orange, Riverside, San Bernardino, San Diego and Shasta counties as the rare and dangerous flood threat descended on Southern California, which was devastated by fires just a year ago.

More than 43 million Americans across California, southern Nevada and northwest Arizona are under flood watches on Wednesday amid a rare, high risk for excessive rainfall and flooding.

This includes major metros like Los Angeles, San Francisco, Sacramento, California, San Diego and Las Vegas.

Los Angeles station KABC reported Wednesday the Inland Empire area east of LA had borne the brunt of the first wave of the storm.

Heavy debris flows from heavy rainfall earlier prompted mandatory evacuations in Wrightwood, California, with the main mountain highway — Highway 2 — being impassable, according to the San Bernardino County Fire Department.

Earlier in the afternoon, the fire department said crews were rescuing people trapped in vehicles conducting door-to-door checks in impacted areas and advising residents in danger zones to evacuate immediately.

With more rain on the way, the San Bernardino County Sheriff’s Department on Wednesday evening issued additional evacuation orders for some areas north of the Wrightwood area.

At least one person died in an “apparent weather-related crash,” amid heavy rain in Sacramento County, according to the California Highway Patrol.

The Lancaster Sheriff’s Station reported on Wednesday night that it was “responding to multiple vehicles trapped in flooded roadways due to severe storm conditions.” It added, “If you do not need to be on the road, please stay home until conditions improve.”

A series of atmospheric rivers brought high-intensity rainfall and strong winds, increasing the risk of flooding, landslides and rapidly rising creeks and rivers.

“California is acting early and decisively to do all we can to get ahead of dangerous winter storms. The state has pre-positioned resources, activated emergency authorities, and we are working closely with local partners to protect communities and keep Californians safe,” Newsom said in a statement Wednesday.

A “High Risk for excessive rainfall” is in place for Los Angeles, including I-10 from San Bernardino to Santa Monica and areas north like Highway 101 to Thousand Oaks, I-5 to Burbank, Santa Clarita, and up to Pyramid Lake and all of I-210. Travel on these roads is not recommended as they may become flooded, officials said. Low-lying neighborhoods in these areas could also become flooded, forecasts show.

Being under a “High Risk” designation is rare. This risk is only issued about 4% of days, accounting for one-third of all flood-related fatalities and 80% of all flood-related damages, according to the NWS.

Potential flooding impacts include the threat of significant and widespread urban roadway flooding, a high risk of major rock/mudslides, and rapid rises in creeks, streams and rivers which will likely lead to swift water rescues.

The recent burn scars will be at risk of possibly damaging debris flows. These flooding impacts will likely lead to significant travel delays and road closures during the busy holiday travel period.

Winds are forecast to gust 40 to 50 mph across the area, potentially leading to power outages on Wednesday, according to forecasts. Thunderstorms are also possible.

More than 100,000 customers have been reported without power in California.

The heaviest rain is expected on Wednesday morning and afternoon. Rainfall rates of 1 inch per hour or greater are expected.

Thunderstorms will remain possible, so people will need to watch for cloud-to-ground lightning.

A brief tornado cannot be ruled out today all along all of the California coast, so people should heed any tornado warnings issued.

By 6 p.m. or 7 p.m. PT, the rain will be coming to a brief end before more rain arrives overnight.

Additional rounds of rain are expected on Thursday and Friday, and the flood threat along with mudslides and landslide risks will continue each day as well.

ABC News’ Amanda Morris contributed to this report.