Rain on the way

Weather forecasters are predicting a wet and wild weekend on the San Mateo County coast. This after a significant rain event on Friday morning, when as much as .70 of an inch of rain fell on the coast in the span of an hour.

Showers are expected to start on Saturday afternoon, but the real show comes on Sunday. The National Weather Service says there is a 10 to 20 percent chance — what it classifies as a “slight risk” — of excessive rainfall on Sunday that could lead to localized flooding on the coast. The National Weather Service says to expect heavy rain, perhaps as much as two inches over the day.

There is also a wind advisory for the entire Bay Area in effect on Sunday. The winds are expected to be worse in the North Bay but can produce gusts of up to 25 mph.

A flash flood watch is expected in the Santa Cruz Mountains and the coast from 1 to 11 p.m. on Sunday. Experts are particularly concerned about flooding in the CZU August Lightning burn scar.

Meanwhile, a high surf advisory is already in effect from the Sonoma County coast south to Monterey and that changes to a high surf warning on Friday running through Tuesday.

This is the first atmospheric river of the season. The phenomenon are long narrow bands of concentrated water vapor that can hold as much water as in the Amazon river. They begin in the warm waters of the Pacific when water evaporates into the air. When that warm air hits a Pacific storm, the result is a concentrated rain event. Once the concentration of vapor meets the coastal range and the Sierras, ever more water is squeezed out of the system. As much as half of all California precipitation comes from atmospheric rivers.

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