• The flood threat in parts of California has been upgraded to high risk
  • Officials are concerned about heavy rain and snow melt possibly causing flooding
  • Flood watches have been issued for multiple counties

A weather official warned Thursday that the storm pummeling California will bring about an "unrivaled, unparalleled weather event" that the state has not seen in over 50 years. California Gov. Gavin Newsom has also expanded a state of emergency to more than 20 counties.

"This is an unrivaled, unparalleled weather event, not experienced in several decades, perhaps back to 1969. There will be high water in areas that are usually not impacted, so everyone needs to be ready," said Kris Mattarochia, a science and operations officer at the National Weather Service (NWS) in Hanford, the New York Times reported.

Forecasters are expecting heavy rain from an atmospheric river to melt snow piles in lower elevations through Friday. The weather event could cause flooding across the state.

The NWS Weather Prediction Center (WPC) has reportedly upgraded the flood threat in California to high risk.

Rick Knabb, a hurricane expert at The Weather Channel, said the WPC would issue a "high risk" warning only for flash flooding events in about 4% of heavy rain days, but "those high risk days account for about 40% of flood-related fatalities and more than 80% of flood-related damages. This is a big deal."

The expert noted that this could lead to a rise in snow accumulations in the mountains, adding to the existing concerns about snow melt and heavy rain potentially blocking drainage ditches and causing flooding.

The NWS in Sacramento warned three passes in California could get up to 60 inches of snow Friday: Sonora Pass, Ebbetts Pass and Carson Pass. The Lassen Park area is expected to get up to 80 inches of snowfall.

The California Department of Transportation (Caltrans) has shut down State Route 88/Carson Pass due to the heavy snow.

The WPC also warned the storm will "deliver copious amounts of heavy snow" in high terrains of northern and central California through Friday.

Last week, Newsom proclaimed a state of emergency in 13 counties across the state due to continuous storms. On Thursday, the California governor added 21 new counties to the list, including Inyo, Mendocino, Fresno, Butte, Placer, Sacramento, San Francisco, Stanislaus, Plumas and Monterey.

"With more dangerous storms on the horizon, we'll continue to mobilize every available resource to protect Californians," Newsom said in a press release.

As early as Wednesday, flood watches were issued across parts of the state as weather officials raised concerns about the combination of heavy snow, heavy rain and snow melt potentially triggering widespread flooding through Friday.

The next atmospheric river is expected to hit California early next week.

In this image, water lines remain on a car in a flooded neighborhood in Guerneville, California, Feb. 28, 2019. Justin Sullivan/Getty Images