Atmospheric River Risks Flash Flooding, Mudslides in California Burn Zones

Severe flooding, dangerous debris flows and deadly mudslides loom over parts of Northern California as an atmospheric river targets the region this week.

Heavy rainfall combined with the effects of recent wildfires is set to exacerbate risks, particularly in burn scar zones, where scorched terrain is unable to absorb intense downpours.

“This is a stormy week in the Northwest,” Bernie Rayno, AccuWeather’s chief on-air meteorologist, said in a media advisory on Monday.

“We have a huge storm on the way with soaking rain and mountain snow. A foot or more of rain is possible in some spots by the end of this week,” he added.

From Tuesday afternoon through Friday, Northern California is expected to bear the brunt of the storm, with 8 to 12 inches of rain likely in some areas and localized totals reaching up to 20 inches.

The heaviest rainfall is expected Wednesday through Friday, particularly along the Interstate 5 corridor and Northern California’s mountainous regions.

Burn Scars Raise Flood and Debris Flow Risks

Of particular concern are areas near recent burn scars, including the Park Fire zone, where more than 400,000 acres burned this summer.

Without vegetation to anchor the soil, these charred landscapes are highly susceptible to flash floods and debris flows. According to the National Weather Service (NWS), burned soil behaves almost like pavement, repelling water rather than absorbing it—sending it cascading downhill along with debris it picks up.

“The intense heat from the fire can create a water-repellent layer in the soil, known as hydrophobicity, which significantly reduces the soil’s ability to absorb water,” Brandon Buckingham, a meteorologist at AccuWeather told Newsweek.

“Without vegetation to slow down and absorb rainfall, water quickly runs off the surface, leading to increased runoff and a higher risk of flash flooding. Additionally, the lack of plant roots means the soil is less stable, making it more prone to erosion and further exacerbating the potential for flood damage.”

The NWS warns in its website that “locations downhill and downstream from burned areas are very susceptible to flash flooding and debris flows, especially near steep terrain.”

Even brief, intense rain can trigger catastrophic slides—sweeping mud, boulders, trees, and sometimes vehicles and homes downhill with little warning.

Debris flows, one of the most powerful landslide types, can travel faster than a car and strike unpredictably. These fast-moving mixtures of mud, rocks and debris often occur after wildfires, requiring only short bursts of intense rainfall—about half an inch in an hour—to set them off.

Unlike other landslide types, debris flows can carve new paths, hitting areas that have never been affected before.

The storm’s effects are expected to extend to higher elevations, with heavy snow blanketing the Cascades, northern Rockies and Sierra Nevada. Mountain pass roads may be closed because of snow accumulation, creating additional travel challenges, AccuWeather forecast.

Residents near burn scar areas are urged to remain vigilant. Officials recommend avoiding areas downhill from charred slopes during heavy rain and having an evacuation plan ready.

Buckingham recommended having emergency kits at the ready, alongside important documents such as ID and insurance papers. Making sure you have a charged mobile phone with emergency contact numbers and a portable charger to hand are advised too, as is planning evacuation routes.

The risk of flash flooding and debris flows on burned land can persist for years after a fire, until vegetation has reestablished itself.

Do you have a tip on a science story that Newsweek should be covering? Do you have a question about atmospheric rivers? Let us know via [email protected].

Update 11/19/24 8:46 a.m. ET: The article has been updated with comments from Brandon Buckingham.