National Weather Service issues weekend flash flood watch for North Bay

The National Weather Service issued a flash flood watch for the North Bay over the weekend, specifically for the 78,000 acres burned by the recent Kincade fire, and areas downstream from that scorched earth.

The flood watch, which is less urgent than a flood warning and means that conditions may develop that could lead to flash flooding, went into effect at 1 p.m. Saturday and will remain through Monday morning, National Weather Service meteorologist David King said.

The flood watch was issued as the first atmospheric river of the rain season arrived in the Bay Area. That storm, spawned by the collision of a low pressure system from the Gulf of Alaska and a plume of moisture from just west of Hawaii, likely will linger until midweek, said King. The North Bay can expect 2 to 3 inches of rain during that time, he said, with higher elevations getting up to 4 to 5 inches.

Within the Kincade burn zone are different areas of severity, said Carolina Walbrun, another National Weather Service metereologist.

“The more severely burned” the land is, “the less it will absorb water. That can create what’s almost a film on top of the soil, and the rain will run over it,” she said.

Severe runoff creates the potential for debris flow, as branches and sediment flow down into culverts and streams.

Areas where water is absorbed, Walbrun said, “will gradually become saturated.” In areas where root systems have been burned or otherwise damaged and weakened, landslides could occur.

The weather service will be more concerned, Walbrun said, if the current rain system stalls over the burn zone, increasing the likelihood of debris flow. In that case, she said, the flash flood watch would be upgraded to a flash flood warning.

“We are monitoring this situation super closely,” she said.

The atmospheric river comes on the heels of an historic cold snap: temperatures in Santa Rosa reached 25 degrees early Friday morning, matching an 88-year record low. Saturday morning’s low was a relatively balmy - but still freezing - 31 degrees.

Describing the rain that had fallen in Sonoma County Saturday afternoon, King said “these are just the initial showers.”

The heavier rains, he said, “are expected to really ramp up tonight, and into tomorrow.”