With the strong likelihood of significant high-intensity rainfall over parts of North and Central California on Sunday and Monday comes the increased risk of debris flows (and flash flooding) in recently burned areas.
In the aftermath of a wildfire, there is a huge load of material deposited on the ground. Burned vegetation also changes the water balance of the denuded hillsides from wildfires by increasing the soil's water repellency. All of these factors combine for increased runoff of debris downslope.
There are lots of variables, like vegetation type, slope, and rain intensity that ultimately determine the likelihood and impact of debris flows in each individual burn scar. The USGS produces very comprehensive analyses of these post-fire risks at https://landslides.usgs.gov/hazards/postfire_debrisflow/. However, the basic rules of thumb, mostly used operationally look at rainfall intensity and amount.
Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services
Phone: (650) 712-1876