It’s been years since California has seen a series of storms like those hitting the state now. They’ve caused evacuations, power outages and flooding, all of which are a hazard to people in impacted areas.
“In terms of overall flood risk, one atmospheric river is typically not enough in order to drive severe concerns,” said Paul Ullrich, a professor of Regional and Global Climate Modeling at UC Davis.
But multiple storms in a row is a different story, he said.
“When you have these sequential atmospheric river events, then you really have to be worried about reservoirs overtopping, soil saturation and other drivers of widespread flood damage,” Ullrich said.
This dump of precipitation might also have positive impacts on California’s water supply. Ullrich said he remembers a series of atmospheric river events that hit California in 2016 and 2017 and helped “pull us out of that major drought that we had at the time.”
“Probably, we’re going to see that again this year,” he said.
But although this rash of storms could help the state’s water supply ahead of the summer, researchers say it also reveals weaknesses in the state’s flood-prevention infrastructure and points to more severe weather to come.
California's unpredictable winters getting more extreme
During the fall of 2022, the National Oceanic and Atmospheric Association predicted California would have a relatively dry winter — a prediction that now, of course, has proven incorrect. Ullrich said California’s winters are notoriously hard to predict because of the state’s extreme and variable weather.
However, Ullrich added that the difference between a “dry” and “wet” winter can be very slight in California.
“California is very unique in that so much of its precipitation for the year comes on so few days,” he said. “As a consequence, if you take some of those days away, if you turn them from wet days to dry days, suddenly it changes the whole total annual precipitation received by the state.”
Ullrich said the level of precipitation coming from this storm isn’t unprecedented, for the most part. Overall levels of annual precipitation in Northern California have stayed fairly consistent.
But a warming climate has encouraged more extreme weather events, he said. Warmer temperatures mean the capacity for more water vapor held in the air, which can lead to more precipitation all at once.
“What we are generally seeing is that some of the more extreme events are becoming more common,” he said. “What used to be a 1-in-100 year event is now becoming a 1-in-20 year event, or even more frequent than that.”
What this means for drought conditions
While the current series of storms won’t fix the state’s issues with drought, it does give Northern California a leg up. Precipitation from the storm will likely help replenish reservoirs and better prepare the region’s water supply ahead of next summer.
Nicholas Pinter, associate director of UC Davis’ Center for Watershed Sciences, said that he’s optimistic about the outcome of this storm.
“With the water behind us and the forecast coming in, it's pretty sure that we're going to fill all the reservoirs in Northern California and at least temporarily take a bite out of our drought situation,” he said.
So far, storms have not surpassed the intensity of the atmospheric river that hit California over the New Year's Eve weekend. Pinter said this is a good thing for our water supply and that more negative impacts can be avoided as long as storms don’t become more severe. For now, the heavy rain hasn’t led to flooding of bigger rivers like the Sacramento or San Joaquin, which Pinter said is largely due to the help of nearby reservoirs.
“The hope is that it's going to stay spread out,” he said. “The fear is that this is going to continue, [that] we're going to get … this whole freight train of storms coming in. We're watching very closely what the Pacific is going to send us.”
Pinter said that he’s hoping for smaller, continuous storms moving forward, but it’s hard to predict what will happen next. Among researchers, California is known for its widely variable weather.
Aging infrastructure around California rivers
While large rivers like the Sacramento have not flooded over during this storm, other smaller rivers have seen some flooding, like the Cosumnes River near Elk Grove. This river, like many others in the state, has levees built around it to keep back flooding.
But as this storm has raged on, some of these levees have broken and failed to prevent overflow.
“Some of them have been built decades ago,” said Helen Dahlke, an associate professor in Integrated Hydrologic Sciences at UC Davis. “And we haven't really invested that much funding into upkeeping, just even checking the status of these levees, so there is definitely some aging infrastructure that could potentially become an issue.”
Dahlke said much of this infrastructure was built to control streamflows decades ago, when residents had different expectations of California’s climate. It’s not equipped to the current reality: Warming temperatures due to climate change could potentially ramp up the severity of future storms, which means more streamflow to manage, she said.
But Dahlke added that flooding is a natural part of California’s ecosystem and feeds into the state’s underground water supply; it can’t be avoided completely.
“Before we had settlers and cities moving in, much of the [Central Valley] actually flooded regularly during these storms,” she said. “And that was really the main mechanism for getting water into our groundwater systems.”
Moving forward, Dahlke said that flooding and aging levee breaks are likely, so the next step is to be proactive about responding. Mechanisms to hold back water, like reservoirs, will still be necessary tools, but she said that there’s also discussion about infrastructure that allows rivers to flow more naturally.
“There is, for example, discussion along the river to set back some of these levees to widen the floodplain area so that the river has a little bit more room when it's really peaking,” she said. ”That would also have really positive benefits for groundwater recharge since more areas flooded and then more water could potentially seep into the ground.”
California isn’t an easy state to plan in; Dahlke said researchers have had issues forecasting winters here for decades due to the unpredictability of the state’s weather. All we can do, she said, is focus resources on being as prepared as possible for the next winter.