What Is an Atmospheric River and Which Cities and States Should Prepare for the Worst?

Following record-breaking drought and raging wildfires in California, a weather event called an "atmospheric river" will bring heavy rain, snow and wind to parts of the Western U.S. over the next several days.

Hefty precipitation across Northern California on Sunday will help after an exceptionally dry season, but the latest weather is expected to produce dangerous flash floods and mudslides in vulnerable areas.

The National Weather Service (NWS) in Sacramento said Sunday that it will "be a wild 24 to 36 hours across northern California as we will see an extreme and possible historic atmospheric river push through the region."

Experts in the area are predicting a Level 4 or 5 (out of 5) weather event to cover the region with up to 10 inches of rain and 6 feet of snow in some places, as an intense storm system moves up the Pacific Coast and toward the Great Plains.

What is an Atmospheric River?

The NWS describes atmospheric rivers as long, narrow pathways of extremely moist air —sort of like rivers in the sky—that transport water vapor out of the tropics.

Atmospheric rivers that contain excessive amounts of water vapor and strong winds are responsible for extreme rainfall and flooding—a type of severe weather event that can affect the entire length of the West Coast, according to the service. The NWS said atmospheric river events are relatively rare, and have just a 1-in-100 chance of occurring in any given year.

The latest atmospheric river was developing from a cyclone in the Pacific Ocean that pulled deep tropical moisture into the atmosphere, the NWS said. By Sunday morning, the cyclone was intensifying rapidly, with forecasters warning it could turn into a "bomb" or "double bomb" cyclone, meaning that a significant drop in the system's air pressure will allow the storm to gain extreme strength.

"Much of the West Coast is starting to feel the effects of a deepening low off the Pacific NW coast. Conditions will continue to deteriorate as the system approaches. Strong winds, high surf, and heavy rain will lead to major impacts. Heavy snow in the Sierra starting tonight," the NWS weather prediction center tweeted early Sunday.

👀! Much of the West Coast is starting to feel the effects of a deepening low off the Pacific NW coast. Conditions will continue to deteriorate as the system approaches. Strong winds, high surf, and heavy rain will lead to major impacts. Heavy snow in the Sierra starting tonight. pic.twitter.com/JnAloP0UJh

— NWS Weather Prediction Center (@NWSWPC) October 24, 2021

Moisture from the developing atmospheric river has begun affecting northern and central portions of California as well as parts of the Pacific Northwest, the NWS said. Moderate to heavy rainfall has been recorded from San Francisco and Modesto all the way to very northern California, as heavy moisture continues to move ashore.

Where Will It Hit and When?

The storm system is expected to effect much of the Pacific Northwest, with heavy rainfall reaching up to 6 inches or more across northern and central California beginning Sunday and moving into the next two days, the NWS said.

So far, downtown San Francisco has recorded roughly 2 inches of rain, already quadrupling its precipitation amounts from earlier this month, the Washington Post reported.

But since atmospheric rivers carry most of their moisture at mid-levels of the atmosphere, the largest rain and snow totals will be recorded in higher terrains. Five to 8 inches of rain are expected in the Coastal Ranges, with up to a foot of rain and 4 feet of snow predicated in the Sierra Nevada mountain range.

In Sacramento, where rainfall hasn't been recorded for more than six months, the NWS is forecasting between 4 and 6 inches of rain, the Post reported. In the San Francisco Bay Area, Mount Tamalpais has already been hit with 6 inches of rain as the atmospheric river moves inland.

Flash flood warnings are in effect across areas including the Bay Area, Sacramento, and the Sierras, with the NWS warning that the storm system could bring rapid precipitation and dangerous conditions.

"A High Risk of Excessive Rainfall leading to Flash Flooding is in effect for parts of northern California and including the Sierra. Heavy snow will also occur, with much of the Sierra Nevada likely to receive in-excess of 2 feet over the next couple of days. Temperatures will drop significantly in the wake of this system," the weather service said Sunday.

Additionally, the storm system could bring a risk of tornadoes and severe thunderstorms to parts of the Pacific Northwest, including southern Oregon, with strong winds expected across the region.

How Bad Will It Be?

The NWS said Sunday that flash flooding, mudslides, and debris may cause dangerous conditions for people across northern and central California. The issue could become particularly bad in areas that have "burn scars," leftover from wildfires.

Julie Malingowski, a meteorologist at the National Weather Service, told the New York Times that this is because "the vegetation is not there to absorb the rainfall like it normally would, so these areas are a lot more susceptible."

The National Weather Service Bay Area issued a flash flood warning Sunday and said the main concern "will be 2020 burn scars but urban and small stream flooding likely as the heavy rain band passes through Sunday afternoon and night."

Flash Flood Watches in effect Sunday. Main concern will be 2020 burn scars but urban and small stream flooding likely as the heavy rain band passes through Sunday afternoon and night. #AtmosphericRiver #CAwx pic.twitter.com/NfL4Cl0Z9I

— NWS Bay Area (@NWSBayArea) October 23, 2021

Similarly, the NWS in Sacramento issued a flash flood and debris-flow warning for both the Caldor Fire and the Dixie Fire, two blazes that have caused significant damage and together burned more than 1 million acres.

"Heavy rain is starting to move in and this will cause debris flows," the weather service warned. "If you live near a recent burn scar now is the time to prepare and listen to local officials for any evacuation information."

The Weather Prediction Center also said there is a "high risk" of excessive rainfall and flash flooding on the west-facing side of the northern Sierra Nevada range where moisture will both cool and condense, according to the Post.

"Flooding, rock slides, chain controls, overturned vehicles—and that was just this morning. This atmospheric river storm is expected to intensify with heavy rain and significant snow into tomorrow. Do NOT drive if you don't have to," Caltrans, California's State Highway System, warned on Sunday.

California atmospheric river
An atmospheric river is sweeping through California, bringing heavy rain, snow, and winds. Experts have warned that conditions could cause flash flooding and mudslides in vulnerable areas. Here, muddy water flows down the stairs of a home after a storm on February 14, 2019 in Sausalito, California. Justin Sullivan/Getty Images