California Could Flood Like Texas. But Thunderstorms Likely Won’t Be to Blame

A major thunderstorm like the one that produced devastating flash flooding in Texas over the holiday weekend is not likely in the Bay Area or most of California, but climate scientists say that if the perfect weather at the right time of year and geography align, serious flooding can still wreak havoc here.

There are several significant differences between the recent deluge in Texas, which has killed more than 100 people, and the type of flooding that happens in California. First, in the Golden State, it’s the cold winter months that bring flooding, often from back-to-back atmospheric river storms. The instability caused by these rainstorms, which douse the region in water, can generate thunderstorms of varying intensity and trigger flooding.

California typically doesn’t experience massive warm summertime storms because of its Mediterranean climate. The disastrous Texas flooding is a reminder that the ferocity of Mother Nature isn’t always predictable. As the climate continues to warm, resulting in wetter storms, Californians living near waterways need to be prepared for more extreme weather events.

When California floods in the winter, there’s always the possibility of a deadly event depending on where a storm makes landfall. For instance, last November, an atmospheric river parked over the North Bay, causing localized flooding, killing two in Santa Rosa.

“The magnitude of the severity of the flooding absolutely could happen in California, and it is the kind of flooding that we are very concerned about,” said Daniel Swain, a UC Agriculture and Natural Resources climate scientist. “But the physical meteorology involved would be very different.”

How weather messaging works

In Texas, the National Weather Service issued multiple warnings, including its highest level of alert for once-in-a-generation flooding, Swain said. However, the timing of some of the alerts — in the middle of the night — and the regularity of the warnings may have caused people to either miss them or not take them seriously.

Jay Lund, professor emeritus of civil and environmental engineering at UC Davis, likened the immediacy of flash floods to earthquakes.

“They happen right away and with almost no warning,” he said. “You have to have an excellent warning system because if it happens in the middle of the night, most people aren’t going to hear it because they’ll be asleep.”

With the Bay Area’s diverse microclimates, it’s challenging for meteorologists to precisely predict where an atmospheric river will drop the most rain. However, forecasters will alert the public to the possibility of flash flooding and update their messaging as new information becomes available. The National Weather Service’s Bay Area office has issued flash flood warnings for San Francisco during atmospheric rivers.

“For urban areas like San Francisco, if we’re getting 1 to 3 inches an hour rain rates, we can pile up water very fast because that overwhelms storm drains and can cause some pretty significant localized flooding,” said Brian Garcia, warning coordination meteorologist for the weather service’s Bay Area office.

Like in Kerr County in Texas, most California localities don’t use sirens to alert the public about flooding, “because our flooding kind of builds and we can see what is coming in,” state climatologist Michael Anderson said. However, some do, like the Marin County communities of Fairfax, Ross and San Anselmo, which maintain flood horns or sirens that they sound when flooding is imminent. 

California relies heavily on the weather service for messaging about potential flooding from storms. For instance, last December, San Franciscans were startled awake by a blaring weather alert on their phones warning them of a potential tornado.

When it comes to flooding, the weather service issues watches, warnings and advisories. Flash flood warnings also have three different levels, ranging from the base level to catastrophic. Beyond the alerts, the weather service leans on traditional radio broadcasts, local authorities and news outlets to get the word out.

Garcia said the difference between a warning and an advisory is that a warning suggests “there could be trouble,” but an advisory means “the trouble is coming to you.” He recommends that all Bay Area residents sign up for text emergency alerts at alertthebay.org and pay attention to any “action statements” within the message.

“If the action statement says something like get to high ground immediately, that is a cue to take immediate action,” Garcia said. “Whether it’s moving to higher floors, going to the top of a hill, or moving yourself to higher ground.”

Flooding from thunderstorms is possible in California

What distinguishes the Bay Area’s localized flash flooding events from those in Texas is the duration of the atmospheric river, its geographic location and the level of wetness in the system. Atmospheric rivers in California can last for days and arrive in a succession train, while thunderstorms last for a few hours at most.

“Texas can get these systems that consist of thunderstorms that don’t move very much over a period of time, producing an enormous amount of rainfall,” said John Monteverdi, emeritus professor of meteorology at San Francisco State University. “That’s different from the kind of flooding that happens when the Russian River floods, maybe once every two or three years.”

Still, a big flash-flood-producing thunderstorm in California isn’t entirely out of the picture and can occur during the summertime in the Sierra Nevada or the deserts across the southeastern part of the state.

“The kind of thing that happened in Texas could also happen in California,” said Nicholas Pinter, associate director of the UC Davis Center for Watershed Sciences. “Anyone out hiking in confined, rugged topography needs to be aware that we have this risk of flash flooding in California, kind of similar to Texas.”

While the Texas thunderstorm covering a broad geographic area and producing a wall of water is “not typical of California,” the “wettest precipitation events are getting wetter” and in turn elevating flood risk, which is in line with the effects of human-caused climate change on storms in both states, said Noah Diffenbaugh, a climate scientist at Stanford University.

“We know we’re in a climate where the kind of intense precipitation that leads to flooding is more likely overall for a given storm,” he said. “Our infrastructure in many cases was not designed and built for the most intense conditions that are now occurring.”

California has experienced numerous wet years that resulted in flooding, including the Great Flood of 1862 and other extreme events in 1955, 1964, 1986 and 1997, as well as more recent occurrences such as 2017 and 2023.

For a major flood to occur anywhere in California, a specific set of ingredients — geographic location, soil moisture and storm intensity — is required, according to Anderson, the state climatologist. He pointed to the winter of 2023, when nine atmospheric rivers hit the state over 18 days.

“That’s almost one every other day, and that led to substantial flooding across the state,” he said. “So, what we look for is different than what Texas has to look out for.”

The primary issue is that during a series of winter storms, the soil across the region can become oversaturated, leading to flooding in Santa Rosa, the Russian River watershed, and the Pajaro Valley, among other areas. Last November, a weeklong atmospheric river “embedded with thunderstorms” hammered parts of the Russian River watershed with “40% of [its] annual rainfall out of that one storm,” Anderson said.

Wildfire-scorched areas are also susceptible to flooding during atmospheric rivers or heavy rains that fall during a monsoon.

“Fire creates a hydrophobic layer on the soil that magnifies the impact of flooding because the water cannot infiltrate it,” said Anna Serra-Llobet, a researcher at the Center for Catastrophic Risk Management at UC Berkeley.

Serra-Llobet said she worries about flooding next winter across wildfire burn scars, especially in Southern California and the Sierra Nevada, as those areas will be primed for flash floods full of ash and debris. In the case of the Los Angeles fires, those floodwaters could hit urban areas.

She said there needs to be more public outreach on how to respond during a flood.

“People don’t understand the risk where they live, and I think we need more drills to be more prepared,” she said. “Creating a risk culture could help many communities to be more proactive and effective in acting during a disaster.”