Biggest storm of winter heading for Bay Area
Rain Tuesday night, Wednesday could trigger mudslides in fire-ravaged areas
Winter weather is finally arriving in Northern California. And after weeks of dry, warm conditions and growing drought concerns, it’s coming in hard.
Forecasters say a sizable storm — the first significant atmospheric river event to hit the greater Bay Area this winter season and likely the biggest storm in at least 12 months — will soak much of California starting Tuesday night, continuing Wednesday, and bringing wet roads, downed trees, power outages and the possibility of mudslides.
Chances are growing that residents of the Santa Cruz Mountains, where steep slopes burned in last fall’s CZU Lightning Complex Fire, will have to be evacuated in some communities, officials said Friday, due to the risk of potentially deadly mudslides if rainfall totals are high enough.
The storm, barreling down from the Gulf of Alaska, is expected to bring 1 to 3 inches of rain to Bay Area cities and 6 or more inches in some parts of the Santa Cruz Mountains and Big Sur, according to the National Weather Service. It follows light
rain Friday with more light rain forecast for Monday. The cold system could bring snow to Bay Area peaks.
“It’s quite a turnaround,” said Brayden Murdock, a meteorologist with the weather service in Monterey. “We haven’t seen rain like this in the forecast for some time.”
The last time San Francisco received 1 inch of rain or more was 12 months ago, on Jan. 16, 2020, when it received 1.24 inches. It’s been nearly three years since San Francisco received more than 2 inches, on April 6, 2018, when 2.22 inches fell.
California desperately needs the rain. After a dry winter last year, when rainfall totals were about 50% of normal, this winter has started off worse.
Reservoir levels have fallen below historical averages. Water managers have begun to get nervous that the state may be heading into a new drought. Cities throughout the Bay Area have received only about 20% their historical rainfall averages so far this winter, with Southern California in similar straits.
“Fall and winter have been very dry,” said Chris Orrock, a spokesman for the state Department of Water Resources. “The threemonth period from October to December was the ninth driest going back to 1895.”
This storm could double the seasonal rainfall totals for many Bay Area cities, said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay.
“We’ll start to see some significant bumps,” Null said. “But the totals will still go from about 20% of normal to about 40% of normal — only about half of where we should be,” he said.
The statewide Sierra Nevada snowpack, typically the source of one-third of California’s water supply, on Friday was just 40% of normal for this date. What’s needed, Null said, are more moderate-sized atmospheric rivers in the coming weeks and months.
Often called “Pineapple Express” storms when they originate in the tropics, atmospheric rivers are giant conveyor belts of water in the sky, moisture-rich storms that roll in from the Pacific Ocean a few times a year to fill California’s reservoirs, blanket the Sierra Nevada with snow and sometimes cause deadly flooding and mudslides.
In a typical year, whether coming from Hawaii or further north, atmospheric river storms occur about 10 to 15 times in California. But those few days account for up to 50% of the state’s annual precipitation.
“They are vital to our water supply,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. “They make or break droughts or floods in California.”
So far, this winter California has received no major atmospheric river storms. Because of a persistent ridge of high pressure along the West Coast, several that might have hit have been diverted north to Oregon, Washington and Alaska, where they caused flash floods and avalanche risks in the mountains.
The largest atmospheric river storms carry twice the volume of water per second as the Amazon River and 25 times the volume of the Mississippi River where it flows into the ocean. Ralph’s colleagues plan to fly several research planes from Hawaii into this storm in the coming days. On a scale of 1 to 5 that he developed, with 5 being severe and 1 being moderate, with beneficial rain, Ralph says this atmospheric river event is a 2 so far that could potentially increase to a 3.
A key concern is whether the storm will stall, dumping higher-than-expected amounts of rain in a few hours. If that happens, there could be mudslides in communities across the state that burned in wildfires last year.
Jason Hoppin, a spokesman for Santa Cruz County, said emergency officials plan to monitor forecasts as the storm moves closer. By Monday, they may issue warnings for certain areas in the Santa Cruz Mountains telling people to prepare to evacuate, he said.
“I could imagine we probably will be sending at least some alerts to the community with the understanding that we may have to pull the trigger on some evacuations next week,” Hoppin said. “We’re hoping we don’t have to, but that’s not what the weather forecast right now is telling us.”
Geologists, fire officials and other experts have been surveying the Santa Cruz Mountains and mapping out the areas most at risk of mudslides after the CZU Lightning Complex Fire burned 86,509 acres last August, destroying 1,490 buildings, including homes and businesses from Bonny Doon to Big Basin Redwoods State Park. The areas highest at risk on county maps are communities on the west side of Highway 9 from Ben Lomond north past Boulder Creek.
The U.S. Geological Survey and National Weather Service say that if a storm brings 0.25 of an inch of rain in 15 minutes, mudslides are likely. Those levels could be met early Wednesday morning, based on current forecasts, meteorologists say.
“I am concerned. I can’t say with 100% certainty it is going to happen. But I am worried,” said Jason Kean, a research hydrologist with the U.S. Geological Survey. “Hopefully it’s small. But if the rain rate goes up really high the problems are going to get bigger. We want the rain spread out. That would be the best. We need the rain, just not all at once.”