Flash flood warnings issued for San Francisco, Marin

Last week a wind storm walloped the Bay Area, and now a second set of storms is buffeting the city.

While forecasts don’t anticipate this latest burst of precipitation will last much beyond Thursday, it arrives in dramatic fashion with a pair of flash flood warnings this afternoon, one for San Francisco and the South Bay, and one for Marin County.

The National Weather Service issued a warning for the area from San Francisco down to San Jose and Monterey Bay reading in part:

The combination of saturated soils and recent heavy rainfall will result in excessive runoff into area streams and rivers as well as localized flash flooding. [...]

The primary impacts will be localized ponding of water on low lying roadways with poor drainage, rapid rises on streams and creeks with some likely exceeding bankfull, some river flooding and potential rock and mud slides.

And a second warning encompassing Marin followed minutes later:

Stream gages in Ross Valley are rising rapidly and are approaching flood stages. Due to continuing heavy rain, flash flooding is expected to begin shortly. In some locations along the northern Marin/southern Sonoma County, flooding in lowland valleys is likely already occurring. Flooding is also possible in Mill Valley and other low lying spots across Marin County in or near to the warning area.

The Marin warning is in place until 6:15 P.M. The larger San Francisco and Bay Area warning lasts until Friday morning.

According to NOAA and Department of Commerce literature from the ‘90s, flash floods are the leading cause of flood-related deaths, and can be up to six times more likely in cities, since there’s little exposed ground to absorb rainfall:

Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. [...] During periods of urban flooding, streets can become swift moving rivers, while basements can become death traps as they fill with water.

Chances are that the storm will pass without too much damage. But the danger is very real, particularly on the heels of so much rain the previous weeks. And as the name implies, flash floods may hit with little or no warning.

On the bright side, the latest drought map shows Northern California almost completely free of drought, and almost all of Southern California mired in only “moderate drought.”

• SF Flood Warning [NWS]
• Marin Flood Warning [NWS]
• Flash Flood Power [NOAA]
• State Drought Monitor