With the heavy rain that Southern California is getting, the National Weather Service is most concerned about the recent wildfire burn scar areas creating an elevated flood risk.
The National Weather Service posted this graphic to explain why those wildfire burn scars are a flood risk.
"Water repellent materials are created during wildfires that prevent the soil from absorbing water. Heavy rain runs off as it would run off of pavement...taking mud, rocks, and other debris along with it. This is why burn scars in steep terrain are so dangerous...and fires are often accelerated and burn hotter up steep slopes or canyons," the National Weather Service Office in Los Angeles writes
In this video, you can clearly see water falling onto the ground but after a recent fire in the area, the ground cannot absorb any of the water. It runs off, collects any loose debris, and heads downhill before coming to a halt downstream. In the same way, when rain comes down to those burn scar areas, boulders, trees, and other large debris gets picked up and they become extremely dangerous.
A more detailed explanation can be found here:
For more information about the dangers of debris flows and what causes them. Here’s WeatherNation’s John Van Pelt