Season’s first ‘atmospheric river’ takes aim at Bay Area

System could deliver as much as 1 to 4 inches of rain to the region

Northern California is off to a slow start to the 2017-18 rainy season, but the expected arrival Wednesday of the first “atmospheric river” storm of the season could quickly make up those early deficits.

The storm is on track to soak the Bay Area with widespread rainfall, including the potential for downpours during the evening commute, according to the National Weather Service.

Fueled by an “atmospheric river,” a type of storm that gets its name from the plume of moisture coming from Hawaii into California, the Sierra Nevada will also see a mixture of rain and snow, according to the weather service. The storm will contain warmer moisture, which means most of the snowfall will occur at higher elevations.

The rain is expected to begin falling late Wednesday morning in the North Bay and then spread south of the Golden Gate, said Scott Rowe, a meteorologist with the weather service. Rainfall totals are expected to range from 2 to 4 inches in the Santa Cruz Mountains, 3 inches in

the North Bay mountains, and 1 to 2 inches in most cities.

The weather service has issued a flash flood watch for the North Bay, beginning Wednesday at 4 p.m, warning that heavy rain will bring an increased risk of debris flows, mudslides and flash flooding, including to the areas burned in the deadly Wine Country wildfires.

While forecasters are still tracking the timing of the system, the brunt of the rain is expected to hit most of the Bay Area after sunset, Rowe said. The weather service also issued a wind advisory for Wednesday, warning many locations in the Bay Area will experience gusts between 20 to 30 mph, with higher elevations reaching up to 50 mph.

Coming off one of the wettest rainfall seasons on record, Northern California so far is lacking in the precipitation department. Rainfall totals throughout the Bay Area are all below-average for the season, which began on Oct. 1, including Santa Rosa (3.04 inches, 78 percent of normal). Others include San Francisco at .93 inches (41 percent), Oakland at .69 inches (32 percent) and San Jose at .54 inches (39 percent).

This season’s current totals are a far cry from this

time last year when a series of October storms kicked off what became one of the wettest water seasons on record in California. For example, at this time last year, Santa Rosa had recorded 7.66 inches of rain, more than twice its current total (3.04 inches).

The precipitation amount so far this water season in the 8-station Northern Sierra Index, a key collection of rain gauges from Lake Tahoe to the Mount Shasta area in the watersheds above many of California’s largest reservoirs,

stood at 79 percent of normal Wednesday morning. So far this season, 4.6 inches of precipitation has been recorded; last season was record-setting, with 94.7 inches during the entire season.

In a typical year, California has between 10 and 15 “atmospheric river” storms which account for up to 50 percent of the state’s rainfall. The arrival Wednesday of the first “atmospheric river” storm of this season could make up most of the early shortfall.

“This system will definitely present itself to make up some short-term deficits,” Rowe said.

A third storm could bring more rain to the Bay Area this weekend, according to the weather service. Meteorologists expect to issue a more detailed forecast in the coming days.