Northern California braces for atmospheric river storm. What happens to snow and flood risk?

An atmospheric river storm is possible later this week and some of the fresh snow could disappear with the warmer weather system moving through Northern California.

Here’s what you should know:

What is an atmospheric river?

The American Meteorologist Society defines an atmospheric river as a “long, narrow, and transient corridor of strong horizontal water vapor transports that is typically associated with a low-level jet stream ahead of the cold front of an extra-tropical cyclone.”

How much more snow and rain is expected?

According to the National Weather Service forecast, snow is predicted to start Wednesday at 4 a.m. An overnight winter weather alert is in place throughout the Tahoe area. Come Thursday, a combination of rain and snow is predicated through the weekend.

Forecasts are pointing at snowfall to continue through the week, but most of the snow will be around pass level or 6,500 feet, said Katrina Hand, a weather service meteorologist.

Temperatures in Tahoe will dip to 19 degree and reach 42 degrees on Friday, according the five-day forecast.

“Multiple feet of snow can fall over the mountains, but that is confined to the higher elevations,” Hand said.

As for rain, 3 inches is predicated to fall in the Sacramento Valley from Thursday to Sunday, Hand said. Then for the Sierra and foothills, anywhere from 2 to 7 inches of rain is predicated to fall over the four-day period.

The weather service issued a flood watch beginning Thursday afternoon and lasting through Sunday for parts of Northern California, at elevations below 4,000 feet.

The heaviest rain is expected overnight Thursday.

What will happen to mountain snow?

The main concern is for elevations between 2,000 and 4,000 feet, according to Daniel Swain, a climate scientist at UCLA. In a Tuesday tweet, he wrote that the atmospheric river storm could hit Friday and poses concerns for lower elevations that have received above normal snowfall for the year.

The storm “could melt that thinner lower elevation snowpack, causing flooding of streams/smaller rivers,” he wrote.

The system coming in over the weekend is warmer, which will cause rainfall on some of the snow, Hand said.

“Any concerns of snow melts will be 5,000 feet and below,” Hand, with the weather service, said. “The snowpack that’s up there higher than that will likely be able to absorb the precipitation from the storm.”

The Department of Water Resource’s Flood Operations Center along with the CalOES and other state and local partners are monitoring the potential of atmospheric river system, said Jeremy Hill, manager of DWR’s Hydrology and Flood Operations branch.

“The bulk of the snowpack above 5,000-6,000 feet remains cold and will hold the bulk of its water content until later in the Spring,” Hill wrote in an email to The Bee.

At lower elevations, the warm weather system it could result in melted snow.

“A warmer pattern of weather systems could lead to snow melt at lower elevations and generate more runoff than recent colder weather systems,” Hill said.

It could result in “localized flooding impacts in foothill locations, streams and roads in low-lying areas.”

Can I travel to Tahoe this weekend?

“Pretty messy travel” is expected over the weekend with “some flooding concerns, higher elevation snow and gusty winds,” Hand said.

Those that want to enjoy the fresh powder may have to wait a few more days to make the trip up to ski resorts.

The combination of rain and snow will likely lead to messy travel conditions, Hand said.

This story was originally published March 8, 2023 at 5:00 AM with the headline "Northern California braces for atmospheric river storm. What happens to snow and flood risk?."