“Among the most vulnerable spots are Sonoma County’s Mark West Creek area, the ridge between Sonoma Valley and Napa Valley, particularly Mount Veeder, Hogback Mountain and Hood Mountain, and parts of Napa County around Atlas Peak and Mount George.
Some of these places face an 80-to-100-percent chance of a mud or debris flow with just a short, intense burst of rain, defined as about a quarter inch in 15 minutes, according to the Geological Survey.
While mudslides can manifest in different ways after wildfires, the problem starts with a loss of protective vegetation to soak up water and stabilize a slope, coupled with soil that is both less porous and less cohesive after burning. In the most severely charred spots, the soil can develop a slick coating near its surface as a result of gases given off during the fire, making it impenetrable to water.
When the rain comes, the ground is ripe for erosion, with the potential for runoff to snowball as it picks up dirt, branches and other debris. Slides can occur long after the rain, too, when a saturated chunk of hill collapses under its own weight.”