Southern California was hit with wildfires this past week, and now rain forecast for the burn scars could mean danger ahead.

We're watching two spots in particular: the burn scars left by the Bryson Fire in Malibu and the Norte Fire in San Clemente and San Juan Capistrano.

Rain is expected to arrive Wednesday afternoon, with the heaviest falling Wednesday night into Thursday morning. On fresh burn scars, it doesn't take much to turn rain into a debris flow.

How a debris flow could happen this week

Here's the number that matters: As little as 0.3 inch of rainfall in 30 minutes has triggered debris flows in Southern California, research from the U.S. Geological Survey has found. Any storm with intensities greater than about 0.4 inch an hour is at risk of producing debris flows.

This is a small amount of rain compared with what is forecast.

(MORE: Landslide, mudslide, debris flow: What's the difference?)

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The National Weather Service's Los Angeles/Oxnard office expects around 2 inches of rain for areas south of Point Conception — including Malibu and the Santa Monica Mountains — with as much as 2 to 3 inches possible in upslope areas and under thunderstorms. 

Areas farther north, toward Santa Barbara County, are expected to see at least an inch. A flood watch is in effect for the Santa Monica Mountains and adjacent coastline from Wednesday afternoon through Thursday afternoon, with rain rates up to 0.75 inches an hour possible.

The Weather Prediction Center has placed the Los Angeles metro area and nearby ranges under a slight risk, a Level 2 of 4, for excessive rainfall Wednesday into Wednesday night. 

Forecasters also noted a moderate risk — one category higher — wasn't out of the question because atmospheric moisture levels are running far above normal for this time of year.

Carol Ciliberti, a meteorologist with the National Weather Service in Los Angeles/Oxnard, told Patch there is "a minor risk of flood and debris flows in our recent burn areas," with about a 10% chance of heavier rain rates that could cause more significant problems.

The risk is that short, intense bursts can drop a lot of rain quickly, even in a storm that looks modest on paper.

Farther south, NWS San Diego expects rain to reach Orange County Wednesday afternoon, spreading inland overnight and continuing into Thursday night, with totals ranging from around 1 inch near the coast to as much as 2 inches in the mountains. 

What makes rain on burn scars dangerous

Postfire debris flows move fast — faster than a person can run. They’re a mixture of water, mud, ash, rocks and vegetation that can surge downslope like flowing concrete. That's a debris flow, and it's different from a slow-moving mudslide.

The danger of burn scars comes down to what fire does to soil. According to the USGS, intense heat can bake a water-repellent layer into the ground. Soil that would normally soak up rain like a sponge instead sheds it, so water runs off almost immediately.

(MORE: How burn scars can trigger dangerous debris flows)

What a debris flow can do:

• Fill a street with mud and boulders
• Slam into a home's foundation or walls
• Bury or block a road, sometimes within minutes of heavy rain starting

Debris flows move fast and give little warning. And you don't have to be under the rain to be in the path of one. Water and debris can travel miles down canyons and channels below a burn scar.

How to stay safe

If you live near a recent burn scar:

• Treat an evacuation warning or order as urgent. Leave before a flow arrives, not after.
• Sign up for local emergency alerts and know your evacuation zone.
• Stay away from creeks, canyons and roads that run below a burn scar when rain is falling upslope, even if it isn't raining where you are.

The fires that started the story

Neither scar is new to the neighborhood's memory. About 16,000 residents were evacuated during the Norte Fire, which burned 570 acres and damaged or destroyed at least seven structures. 

The Bryson Fire burned 146 acres in Malibu's Encinal Canyon. Crews around the Norte scar have been repairing dozer lines and restoring drainage to prepare for the rain, according to the Orange County Fire Authority.

Both fires are largely under control. The Bryson Fire reached full containment Oct. 19. The Norte Fire was 90% contained as of Tuesday morning, according to Cal Fire. 

(MORE: What’s next for Southern California burn scars? Debris flows)

But containment describes the fire, not the hazard left behind. A fully contained burn scar can still produce a debris flow the first time heavy rain falls on it.

The most-cited example is Montecito, California, where a debris flow in January 2018 killed 23 people, injured 167 and damaged or destroyed more than 400 homes, according to the USGS. 

It came just weeks after the Thomas Fire burned the hills above the town and rain arrived on the fresh burn scar.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.