Kicking winter into high gear, a powerful storm system is forecast to blast California on Sunday and Monday, bringing high winds, soaking rains, flooded roads and downed trees.
It will be wet across the state, but the impacts won’t be the same, experts said Friday.
In a replay of this week’s storm, most Bay Area residents will be see 2 to 3 inches of rain from late Saturday night until Monday morning, with 4 to 6 inches or more dumping on the Santa Cruz Mountains, North Bay hills and Big Sur. Strong winds could cause power outages on Sunday, and the rain is likely to back up storm drains and cause minor flooding on local creeks and some roads.
But in Southern California, the forecast is for a much stronger punch. Rain totals are expected to be twice as high, with up to 6 inches in Los Angeles — half the city’s average yearly rainfall — and as many as 12 inches at higher elevations.
“It’s going to be a big deal in the Bay Area,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay. “But it’s going to be a really big deal in Southern California. We’re not going to get by unscathed. But they are going to get it worse.”
Here are the latest rainfall totals for the atmospheric river that will begin impacting the area tomorrow morning. A flood watch is in effect from 4PM Saturday through 10AM Monday. #CAwx pic.twitter.com/K3WxUwEUU9
— NWS Bay Area 🌉 (@NWSBayArea) February 2, 2024
The National Weather Service issued a flood watch from Sonoma County to Monterey County from 4 p.m. Saturday to 10 a.m. Monday and a high wind warning on Sunday.
Farther south, there are fears of mudslides and significant flooding, particularly on unstable slopes and areas that have burned in wildfires in recent years. That includes Big Sur and in hilly parts of Santa Barbara, Ventura and Los Angeles counties, where some communities could face evacuations if the storm stalls instead of quickly moving through, authorities said Friday.
There is a 40% to 50% chance of the storm dumping up to 15 inches of rain Sunday and Monday in the mountains of southern Santa Barbara County and western Ventura County, the National Weather Service said.
“People need to start preparing now for a major flooding event,” the agency warned Southern California residents.
Although several Southern California waterways, including the Ventura and San Diego rivers, were forecast to exceed flood stage by Monday night, the situation is more moderate in the Bay Area.
Larger rivers that are prone to flooding during the winter, such as the Russian River in Guerneville and the San Lorenzo River near Felton, are forecast to rise steadily on Sunday and Monday but still remain several feet below flood stage. The Pajaro River in Watsonville, which flooded badly last winter when levees failed, has a 21% chance of reaching its flood stage of 24 feet by Monday, the National Weather Service said.
Major South Bay waterways, including the Guadalupe River and Coyote Creek, are not expected to flood in part due to the dry days between storms that have allowed their levels to drop.
“We’re going to see rapid rises in creeks and streams, but we don’t expect any widespread flooding, just nuisance flooding,” said Matt Keller, a spokesman for the Santa Clara Valley Water District.
Dry skies are forecast to return Tuesday and Wednesday in the Bay Area, with a smaller system next Thursday and Friday, and more dry weather expected the following weekend for Super Bowl parties.
This weekend’s storms will provide a major boost to the Sierra Nevada snowpack, the source of one third of California’s drinking water.
The snowpack was just 25% of normal on Jan. 1. But it has grown steadily due to several significant January storms. On Friday, it was 57% of normal.
The incoming storms are expected to unload 3 to 4 feet of new snow in the Lake Tahoe area and a staggering 5 to 6 feet farther south at Ebbetts Pass and Sonora Pass, the National Weather Service forecasts.
“This is going to be a really big storm — certainly the biggest and most intense storm of the year so far, and we’ve seen some decent ones,” said Daniel Swain, a climate scientist at UCLA. “This will probably be the biggest snow event in the Sierra Nevada so far this year. It’s going to be a travel nightmare, but it’s good news from a snowpack perspective.”
The storm also is likely to send rainfall totals soaring in many California cities.
Over the past 30 years, San Francisco has averaged 22.89 inches of rain a year. If the storm brings 2 to 3 inches, as expected, that’s more than 10% of the yearly total in one weekend. In San Jose, which receives an average of 13.48 inches, the storm could bring as much as 20% of the yearly supply. And in Los Angeles, where the annual rainfall average is 12.23 inches, this weekend could deliver up to 50% of a typical year’s rainfall in two days.
“This is quite likely going to result in significant to major flooding in Southern California,” Swain said.
The area of low pressure tracks in San Francisco Bay later on Sunday, but the moisture plume continues into Southern California like a firehose.🌧️
This atmospheric river [🍍Pineapple Express] will be long lasting and dump 8-12″ of rainfall on areas around Los Angeles. pic.twitter.com/vqEmIIak6M
— Ryan Maue (@RyanMaue) February 2, 2024
This winter, El Niño conditions are present. When they occur, changes in wind patterns cause ocean waters near the equator off Peru to be warmer than normal, which can affect weather patterns across the globe.
Historical records going back to the 1950s show that in Northern California, El Niño years have about an equal chance of being wetter than normal as they do of being drier than normal. But in Southern California, El Niño years more often mean wet winters.
Right now, ocean temperatures across the Pacific from California to Hawaii are 3 to 5 degrees Fahrenheit warmer than average, Swain said. That is due to El Niño, and climate change.
When atmospheric river storms — which are bands of moisture hundreds of miles long, often coming from the tropics — come barreling into the West Coast, warmer ocean conditions allow more moisture to evaporate into them, delivering more rain.
“You could imagine a future where California is significantly warmer, with more intense heat waves in the summer and less snow in the mountains,” Swain said, “with relatively little change in average annual precipitation — but large increases in flood and drought risk, as well as wildfire risk.”