Bay Area braces for freezing temperatures, atmospheric river storm this weekend

The Bay Area is expected to get a couple inches of rain during this weekend’s atmospheric river storm. (National Weather Service) 

As Bay Area residents prepare for some of the coldest temperatures of the season on Friday night, an atmospheric river storm is expected to dump a few inches of rain on the drought-ravaged region.

The narrow, moisture-rich system from the Pacific Northwest will first hit the North Bay, with Sonoma County expected to see precipitation in the early hours of Sunday morning, according to National Weather Service meteorologist Brian Garcia. The storm is likely to be a 2 or 3 on the UC San Diego Center for Western Weather and Water Extremes’ scale, which ranks atmospheric-river storms from weakest to strongest, up to 5.

The storm will reach Marin County around sunrise and by the early daylight hours, the core of the Bay Area is expected to see rain. The system will be focused between Marin County and the Santa Cruz mountains, with costal ranges expected to be the biggest benefactors of the rainfall and to get 6 to 8 inches of rain.

Immediate coastal areas, such as Point Reyes, Half Moon Bay, Pacifica and Santa Cruz are expected to receive 3 to 5 inches of precipitation while lowland areas, such as the East Bay valleys, Daly City and San Mateo, could get 1 to 3 inches. Most of the storm should clear out by Monday afternoon with rainfall ending by Tuesday morning, Garcia said.

Winds are expected to range in the 30 to 40 miles per hour range in the coastal mountains, with lowlands seeing gusts between 20 and 30 miles per hour.

Friday is expected to be one of the coldest nights of the season, with temperatures to drop into the low-to-mid 30s across the Bay Area and to rebound with Sunday’s atmospheric river storm.

The National Weather Service issued a freeze warning for the inland valleys, including the North Bay and Salinas Valleys, which could see frost and temperatures in the upper 20s and mid-30s. The weather service recommends pets be protected and sensitive plants be brought indoors.

Another storm is also brewing for Wednesday.

The “quick-moving” cold front will come through from the north on Wednesday and drop another inch or two of rain in most locations in the region.

That could spell more good news for alleviating some of Northern California’s drought conditions. The majority of the region has been classified under “extreme drought,” meaning that water levels are “inadequate” for agriculture and wildfire, reservoirs are “extremely low” and fire season continues year-round, according to the U.S. Drought Monitor. Portions of Alameda, Santa Clara and Contra Costa counties are under “extreme drought.”

Most of Northern California has been classified as under “extreme drought.” (U.S. Drought Monitor) 

“In terms of the drought, it helps to chip away at it for sure,” Garcia said. “It looks like the system will have some localized flooding but not any widespread flooding. That’s really, really good because we’ll get all the benefits of the rain without any of the impacts of the rain.”

A much stronger Oct. 24 atmospheric river, a 5 on the UC San Diego Center for Western Weather and Water Extremes’ scale, drenched the region with 4.05 inches in one day in San Francisco, marking the city’s wettest October day on record.

“One thing that’s different in this system from the Oct. 24 one is that the soils were very, very dry so they absorbed a lot of the water,” Garcia said. “This time, they won’t be able to absorb as much of the water, so we’ll have more runoff, which have pros and cons. A con is that more runoff in tighter watersheds is going to show higher increases in river heights and river flows so if you’re hanging out around a river, it’ll be very swollen and running fast. A pro is that we’ll have a lot of runoff in watersheds that feed our reservoirs and we’ll see them fill up more effectively than the October system.”

The last time the Bay Area saw significant rainfall was during a Nov. 9 atmospheric river storm that dropped 4.88 inches of rain on Middle Peak at Mount Tamalpais — but as little as 0.09 inches in San Francisco and 0.58 inches at the Oakland International Airport.

“People need to be aware that it’s been a few weeks since we’ve seen some decent rain so there are oils built up on the roadways,” Garcia said. “There will be localized standing water, roadways will be slick. It’s about taking a big dose of patience and allowing yourself and others some extra time. It’s going to be an absolute mess for that Monday commute.”

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