SAN FRANCISCO — A small tornado was whipped up as part of a powerful band of thunderstorms that moved through Northern California during the latest deluge that has downed trees, swollen rivers and prompted evacuations over flood fears.

The tornado was on the ground for about 3/8 of a mile just south of Sacramento, tearing limbs off trees and ripping awnings from buildings late Tuesday, said National Weather Service meteorologist David Rowe. No injuries were reported from the twister, which was about 100 yards wide and registered on the low end of the tornado scale.

The twister got the attention of forecasters at the weather service’s Sacramento office.

“It came as a little bit of a surprise considered that we’re so focused on all the heavy snow and rain,” Rowe said, adding that the Sacramento area received about 2 inches of rain Tuesday.

While the heaviest downpours have moved through, the risk of flooding remains from runoff and showers predicted to continue into Thursday.

About 2,000 people in Wilton, a rural community near Sacramento, were asked to leave their homes Tuesday evening, as emergency crews worked to try to bolster a Cosumnes River levee.

Sacramento County emergency services official Mary Jo Flynn said officials were keeping a watchful eye out in case water spills over the levee. Low-lying buildings are at risk of inundation but Flynn pointed out many of the homes along the path of a possible flood are built on berms or sit on relatively higher ground.

An evacuation center opened in neighboring Elk Grove but some residents said they plan to stay put.

“We have no concerns,” Lill Nichols, who with her husband runs a horse farm near the river, told the Sacramento Bee. “We have animals and can’t evacuate anyway.”

Interstate 80 remained closed Wednesday between the Nevada border and Colfax, California, after more than 6 feet of snow fell along the key mountain corridor. There was no estimate when it could reopen.

Some 3,000 Sonoma County residents were under an evacuation advisory as the Russian River rose again under pounding rain. Officials red-tagged seven homes, ordering residents out, when a rain-soaked embankment came crashing down.

Johna Peterson was one of few residents who ventured out Tuesday in the remote Sonoma County town of Monte Rio. Walking on the bridge across the Russian River, Peterson worried about what the coming hours and days would bring.

“I think it’s going to go higher,” Peterson said. “There’s nowhere for this water to go.”

People were evacuated from businesses and homes in downtown San Anselmo, north of San Francisco, after a rain-swollen creek broke its banks. The Corte Madera Creek was flowing 1 foot over flood stage, the Marin County Sheriff’s Office said.

In Southern California the second fast-moving storm this week dumped steady rain across Southern California. Officials warned residents along Los Angeles-area hillsides scarred by wildfires of the possibility of mudslides, but only minor debris flows were reported Wednesday.

The storms were the latest of back-to-back systems — buffered by a brief respite Monday — that have brought the heaviest rain in a decade to parts of Northern California and Nevada. More showers were forecast for Wednesday morning.

The storms are part of an “atmospheric river” weather phenomenon that draws precipitation from the Pacific Ocean as far west as Hawaii.

A blizzard warning was in effect for parts of the Sierra Nevada, the first issued in the past nine years, said Scott McGuire, a forecaster for the National Weather Service based in Reno, Nevada.

“This is definitely a dangerous, life-threatening situation going on up there,” he said. “People should not attempt to travel at all.”

Forecasters warned of up to 10 feet of snow in the highest mountains, with up to 7 feet of snow around the resorts of Lake Tahoe, high risk of avalanches, and wind gusts to 60 mph. The Sierra ridge had gusts of more than 100 mph.

Many ski resorts shut down Tuesday because of the storm. A number of main roads in the Sierra were closed, including Interstate 80, or required chains.

A series of storms already has added 33 billion gallons of water to Lake Tahoe since Jan. 1.