Rainy season poses threat to Bay Area wildfire burn areas

Tuesday's transformation from sunshine to showers highlights the annual transition to rainy season in Northern California. 

“We’re starting to see some cold fronts move out of the Gulf of Alaska. We saw one a few days ago. There’s another one moving through the Bay Area today," said Dr. Jan Null, a San Jose State University adjunct professor of meteorology. He added, "A little more rain than the last one. And that’s sort of the progression we get into this time of year. Each successive storm tends to get a little bit bigger. A little bit wetter."

From the North Bay to the East Bay and further south, it's the kind of brisk wet, weather perfectly suited for ducks, but not so much for people.

“A little bit. The only thing I forgot was a sweater today,” said San Jose resident Emilio when asked if he is ready for the wet weather.

Experts say this year’s La Niña Winter will cool the North Pacific’s waters and could produce drier conditions down south. The Bay Area could see normal rainfall, which may signal another problem.

“We know the Santa Cruz Mountains are notorious for having mudslides, without having this recent burn history,” said Dr. Laura Sullivan-Green on Oct. 20. She is the chairwoman of the San Jose State University Department of Civil & Environmental Engineering.

Burn areas such as those within the multiple complex fire areas that no longer have vegetation and trees could see an increased risk of mudslides and flooding during downpours.

“Heat from the fires impacts the soil characteristics and makes it water repellant. So instead of that water absorbing into the soil, it will then runoff,” said Sullivan-Green. Added San Jose resident Brian YI, “Historically we had that issue over in east San Jose where it got flooded. So I’m hoping this time, this season, with the local government, at least they’re more proactive.”

Experts said the likelihood of a negative event depends on topography and the intensity of rainfall.

“If we have storms that are a bit lighter, with more slow-but-steady rain, we would see less of a risk of that,” said Sullivan-Green. Added Dr. Null, “this particular storm, we’re probably not gonna see those amounts of rain, that high rate of rain that could cause these flows.”

Dr. Null and others caution, this is only the first substantial storm in a season still a long way from ending.