Atmospheric river heading for California could melt snow, cause floods

First the snow, now the rain.

A significant flood threat is forecast for portions of California over the next few days as a storm with milder air drops heavy rain over higher elevations that have been socked by frequent rounds of heavy snow this winter, according to AccuWeather.

The National Weather Service said the next Pacific storm is expected to arrive in California on Thursday and into Friday and Saturday, and it will be associated with a moderately strong atmospheric river, a plume of moisture stretching back over the ocean.

Heavy rainfall and a mild air mass could cause rapid snowmelt in areas that have received several feet of snow, the weather service said.

Although the deepest snowpack will absorb much of the rain, the shallower snowpack below about 5,500 feet will pose the greatest flood concerns, the weather service added.

But the deep snowpack at higher elevations raises risks of structural collapses, according to UCLA climate scientist Daniel Swain.

"In some ways, it's almost worse that (the snowpack) is absorbing all this water," Swain said during a Wednesday virtual briefing.

TUESDAY'S WEATHER: Snow in the forecast in California, Montana and Plains: Tuesday weather outlook

EXPLAINED: What exactly is an atmospheric river?

How to prepare for flooding risk

"People living near rivers and streams should closely monitor water levels and be ready to move to higher ground if the threat of flooding becomes imminent," the weather service in Hanford, California, said this week. "If you live in a flood-prone area or near rivers and streams, now would be a good time to prepare and have an evacuation plan in place in the event high water becomes a threat to your safety."

Above 7,000 feet or so, several new feet of snow will pile up over the northern and central Sierra Nevada and could cause road closures at pass level along Interstate 80, AccuWeather said.

AccuWeather chief meteorologist Jonathan Porter said, "People and businesses are already cleaning up from previous storms, meaning that the latest storm has the potential to become a disaster upon previous disasters, further compounding any impacts."

Potential risk of structural collapse, avalanche hazards

With current snow loading already causing structural collapses, such as homes, grocery stores, and other buildings, the additional rain could exacerbrate those problems.

"It does not appear likely that this storm is going to melt very much of that very deep snowpack in high elevations," Swain said Wednesday.

Swain advised that residents living in higher elevations should try to remove as much snow as possible from their roofs to avoid density buildup from the rain.

An increased avalanche hazard due to the deep snowpack absorbing rain is also possible, Swain added. And places that rarely see avalanches could be disrupted by the threat.

Slope failures possible

In addition, Swain said that throughout Northern California, this storm will probably bring a significant increase in slope failures – mudslides, landslides and debris flows – because soils are now saturated.

Swain added that although there will be some flood risk associated with this warm and wet storm, he is more concerned about the risk of additional warm atmospheric rivers in mid March. "If we get successive warm atmospheric rivers, that's when big problems would begin," he said.

What is an 'atmospheric river'?

Made visible by clouds, atmospheric rivers are ribbons of water vapor that extend thousands of miles from the tropics to the western U.S. At 250 to 375 miles wide, they provide the fuel for massive rain and snowstorms that can cause flooding along the West Coast.

In general, atmospheric rivers pick up water vapor from the warm, moist air of tropical regions and then drop the water over land in cooler regions as rain or snow.

Multiple atmospheric rivers developed from late December into February in California, AccuWeather said.

Contributing: The Associated Press

This article originally appeared on USA TODAY: California weather forecast: Atmospheric river sparks flood fears