A strong rain and wind storm, carried by an atmospheric river from the Pacific, has been blamed for a third death in southern California as flooding, road closures and debris flows are reported across the region.

A flood watch was also extended through Thursday for almost all of the area, as more than 11in of rainfall was measured in some parts Los Angeles county as of Wednesday night and evacuation warnings were issued for mountain communities in San Bernardino county.

“There is a moderate risk of excessive rainfall over parts of southern California,” the US Weather Prediction Center said in a forecast early on Thursday. “Numerous flash flooding events are possible. In addition, many streams may flood, potentially affecting larger rivers.”

On Wednesday, California’s governor, Gavin Newsom, declared a state of emergency in Los Angeles county and other southern California counties, citing flooding and landslides risks in areas affected by last year’s wildfires.

The Los Angeles mayor, Karen Bass, declared a local emergency. “I am urging all Angelenos to stay safe and be extremely careful on the roads if you absolutely must travel. Please do not take this storm lightly – follow official guidance, plan ahead and sign up for emergency alerts,” she said.

The powerful storm, which is forecast to continue over the Christmas holiday until Saturday, has already prompted evacuations and brought with it road closures, flooded freeways and flight delays.

A 64-year-old man was killed in the City Heights neighborhood of San Diego on Wednesday morning when a 75ft-tall tree fell on him, the San Diego police department said. The family of Roberto Ruiz told NBC 7 San Diego that he stepped out of their home to move his car when part of the tree fell, crushing him.

A woman was also swept nine miles down San Jose Creek before she was rescued, the Los Angeles county fire department said. The woman, whose name has not been released, was first spotted in the creek but when rescue crews arrived she had already been carried downstream.

“She went downstream for a while,” Pauline McGee, a spokesperson for the Los Angeles county fire department, told the LA Times.

In Los Angeles, evacuation orders were issued for about 130 homes considered especially vulnerable to mudslides and debris flows in areas where last year’s wildfires ravaged the Pacific Palisades neighborhood.

The burned vegetation has made the land unable to soak up the water, experts said.

“Those soils are still hydrophobic, which means that rain just runs off like it’s hitting hard dirt or concrete,” said Scott Kleebauer, a meteorologist at the Weather Prediction Center, told Bloomberg. “There are burn scars that have lasted for four or five years before you see any improvement.”

The Angeles Crest Highway, a major traffic route through the San Gabriel Mountains, was closed in two stretches due to flooding.

National Weather Service meteorologist Ariel Cohen said 4-8in of rain had fallen in some foothill areas, and the Los Angeles City News Service reported numerous rockslides in the mountains.

Forecasters also issued a rare tornado warning for a small portion of east-central Los Angeles county due to heavy thunderstorm activity over the community of Alhambra.

As of Wednesday night, rainfall over the region had subsided, but a second wave of the storm system was due to hit on Thursday, forecasters said.