The Los Angeles region has recently been grappling with wildfires and strong winds, and now, a new challenge is on the horizon: an influx of rain.

According to National Weather Service meteorologist Kristan Lund, based in Los Angeles, rain is expected to start by Saturday afternoon and could last until Monday evening. While the region is in dire need of rainfall, the upcoming weather conditions are causing concern among experts due to the heightened risk of mudslides, especially in areas that have recently experienced fires.

“Rainfall is beneficial given our current drought conditions,” Lund explains. “But the situation can quickly become hazardous with heavy downpours or if thunderstorms occur, leading to debris flows.”

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Wildfires alter the landscape in ways that increase the risk of debris and soil being swept away unpredictably. Intense fires leave behind a waxy substance just below the surface, which forms from the decomposition of organic materials like leaves, which are naturally water-repellent. This layer prevents water from seeping into the soil, causing runoff that can carry debris.

“When it rains on these hydrophobic soils, the water simply flows over the surface, dragging burnt remnants with it,” Lund notes. “It’s very concerning.”

Even less intense fires that do not create a hydrophobic layer can still lead to debris flows, according to Danielle Touma, a climate scientist at the University of Texas at Austin. Normally, vegetation absorbs some rainfall, reducing its impact on the soil. However, in burnt landscapes, there’s less vegetation to capture the rain, leading to direct impact on the soil. Additionally, fires often destroy the fine roots that help hold the soil together, making it more susceptible to being washed away by rain.

Lund points out that the three largest recent fires in the Los Angeles area have scorched nearly 50,000 acres, some of which are in hilly terrains prone to mudslides. Despite forecasts predicting moderate rainfall intensity, there’s still a 10 to 20 percent chance of thunderstorms that could bring heavy showers, potentially triggering debris flows.

On a positive note, the anticipated rainfall is expected to aid firefighters in containing the ongoing wildfires. The largest, the Palisades Fire, is 77 percent contained, while the second largest, the Eaton Fire, is at 95 percent containment. The third, the Hughes Fire, is 56 percent contained. Even though a fire may be contained, it can still burn within controlled lines.

The upcoming rainfall may offer temporary relief but will not end the risk of fires in the region. “Unfortunately, we need a widespread rainfall of two to three inches to truly conclude the high fire season,” Lund explains. “It will help, but it’s not enough to pull us out of the high fire season.”

Even after the fires are extinguished, the danger of debris flows remains and can persist for years. Debris flows are particularly concerning within the first two years after a fire, but depending on environmental conditions, they can occur even later. Post-fire recovery includes assessing the landscape to identify high-risk areas for debris flows and implementing protective measures, Touma notes.

Touma’s research suggests that the scenario of recent fires followed by heavy rains will become more common as climate change progresses, causing the atmosphere to hold and release more water. “We should expect an increase in post-fire debris flows in the future, based on weather patterns,” Touma states.