This week’s atmospheric river in California delivered up to five feet of snow to the Sierra and drenching rains to the drought-plagued state, along with travel headaches and fierce winds exceeding 80 mph in some places.
A whopping 36.1" of #snow over the last 24 hours at our official 8am measurement! That brings our storm total to 60" with 55" falling in the last 48 hours.— UC Berkeley Central Sierra Snow Lab (@UCB_CSSL) December 14, 2021
We still have snow falling and are expecting snow showers to taper off across the day with some light accumulations. pic.twitter.com/UgbKq9fIlR
Although the system was classified as only “weak to moderate,” it’s an example of how crucial atmospheric rivers are to the state’s water supply and to the trajectory of the rainy season.
A single storm has brought California’s mountain snow much closer to where it should be this time of year. Statewide snowpack is now 76 percent of average for the date; on Friday before the storm, it was just 19 percent. And while an above-normal snow year is needed after the deficits of the past two years, the wettest months of the season lie ahead.
The state experienced record daily rainfall and significant three-day tallies. Three stations in the Bay Area topped 10 inches of rain, although the region largely avoided major flooding. Los Angeles shattered its rainfall record for the date on Tuesday, recording 2.12 inches. Heavy widespread rain also occurred in the coastal mountains of Southern California, where several stations exceeded five inches and a few surpassed seven inches.
The National Weather Service in San Diego received numerous reports of rain-related impacts, including flooding, debris flows, road closures and swift-water rescues. Debris flows in Orange and Riverside counties were downslope of 2020 wildfire burn scars.
October’s extreme atmospheric river kick-started the water year, and the latest storm has added to those gains. Since Oct. 1, San Francisco has received more than 12 inches of rain, more than half of what it normally receives in an entire year. In 2020, less than two inches of rain fell in the same period.
These recent amounts place the current water year in the top 10 wettest to date, with records dating to the mid-1800s. Much of Southern California is running near to above average for the water year; the region had lagged far behind after a mostly dry November.
More rain and snow are on the way as weaker systems affect the West Coast this week. An additional 1 to 2 feet of snow could fall Wednesday and Thursday in the Cascades and the Sierra, where a winter storm warning has been issued.
And there are signs that the storm door could remain open for California, at least through the holiday season.
“We are currently seeing elevated probabilities for atmospheric river activity over Southern California between December 23-26, according to both the NCEP and ECMWF forecast models,” Michael Deflorio, a research analyst at the Center for Western Weather and Water Extremes in San Diego, wrote in an email. Another system is possible in the Dec. 28-Jan. 3 time frame.
“These latest forecasts do represent a change in the forecast models from below normal atmospheric river conditions that were predicted (and that occurred) throughout much of November and the early part of December over Southern California,” he wrote.
Diana Leonard is a science writer covering natural hazards in California.