California’s climate-change-and-El Niño-intensified extreme weather siege may only be beginning.
After the state endured its hottest year-to-date period on record and a multi-million-dollar battering by Hurricane Marie in September and Hurricane Rachel in October, much greater destruction could be on the way this winter. El Niño is continuing to warm coastal waters, and because water expands as it heats, sea levels could rise to unprecedented heights during the coming king tides at Thanksgiving, Christmas, and late January. In concert with the rising seas will likely come a battering by record-wet atmospheric rivers when the jet stream assumes its usual California-centered configuration during the coming very strong (“super”) El Niño winter. Atmospheric rivers, like rivers in the sky, carry moisture out of the tropics.
If it comes to pass, a disaster this winter in California would strain FEMA, which has been decimated under the Trump administration. Since 2025, “the agency has so far lost 17 percent of its staff, including one-third of full-time employees responsible for the agency’s day-to-day operations,” according to the Brennan Center. And it’s an open question how much the federal government would help: According to a March analysis by politico.com, President Donald Trump has approved just 23% of blue state requests for disaster aid, compared to 89% for red states.
An unusually high risk of a record-wet winter for California
In an October 12 post at weatherwest.com, climate scientist Daniel Swain shared alarming odds of a record-wet winter for California. He analyzed the winter forecasts from 10 different seasonal forecast models; eight of them gave at least 10% odds that one or more months from January through March 2027 would be the wettest on record in Southern California, Central California, or both.
“In a few locations along the Central Coast, the odds are depicted as being as high as 40-50%,” Swain wrote. “Technically, a dry winter is still possible, though it is very unlikely (in my estimate, 5-10% or less).”
The latest run of the European Copernicus climate service (C3S) October 14 outlook, which combines 10 seasonal forecast models from nine weather agencies, concurred with Swain’s analysis: About 13% of the 454 model ensemble members explicitly predicted California’s wettest winter on record; only 8% showed a drier-than-average December through February period statewide.
The Golden State has a long history of cataclysmic floods, which have occurred about every 200 to 400 years — most recently in the Great Flood of 1861-62, which swamped the 300-mile length of the Central Valley. And climate change has made these extreme floods more likely.
In fact, climate change has already doubled the risk of a megastorm, and more than tripled the risk of a trillion-dollar megaflood of the type that occurred in the 1860s, according to a 2022 study that Swain co-authored. The study authors found that the odds of a lesser megastorm costing tens of billions of dollars — less precipitation than in 1861-62, but more than in California’s recent wettest winters — were about 1% (26% over a 30-year period).
I asked Swain on Bluesky for the odds of such a lesser megastorm this winter. He replied, “It is reasonable to interpret that the risk is at least 10x baseline, and quite possibly higher.” That implies at least a 10% risk this winter of a California flood costing over $10 billion (2023 dollars), which would be the state’s costliest flood on record.
A separate 2022 study estimated that a 1% per year likelihood flood affecting only the Los Angeles area would likely inundate property worth about $56 billion containing a population of about 425,000 to a depth of a foot or more, with the risks disproportionately higher for non-Hispanic Black and other underresourced populations.
Timing of the rains will be crucial
Despite their obvious power, strong El Niño events have historically been a mixed bag for severe flood impacts in California. San Francisco-based meteorologist Jan Null wrote by email that a very wet winter won’t necessarily bring destructive flooding if the storms are strung out over the winter.
For example, the strong El Niño winter of 1997-98 had more than double California’s average rainfall — the second-largest amount ever — but ranks only No. 13 for most destructive, because the rain was spread over almost twice the number of rainy days as usual. Conversely, the strong El Niño winter of 1982-83 had less rain but more significant damage (ranking No. 4), as multiple big storms occurred in a short period of time. And in the winter of 2015-16, predicted heavy rains from El Niño never materialized, despite much higher-than-usual odds.
Record sea surface temperatures will intensify
Sea surface temperatures along the California coast from San Francisco Bay southward are about 2-6 degrees Celsius (3.6-10.8°F) above average and are record-warm for this time of year along the stretch from San Francisco Bay to Los Angeles.
Why? Partially because California has experienced its hottest year-to-date period on record. In addition, an oceanic “Kelvin wave” has transported warm water from the tropics along the coast over the past few months. The Kelvin wave was generated by strong westerly winds from the intensifying super El Niño, which pushed record-warm waters from the western Pacific against the western coasts of North and South America. The Coriolis effect caused by the planet’s rotation then pinned the bulge of warm water against land, forcing it to spread along the Pacific coast. These warm waters will spread farther north and south later this autumn and into the winter.
Swain wrote that water temperatures along much of the California coast could average about 8-10°F above average this winter. “These are, again, pretty astonishing numbers, and would blow records from 1982-83, 1997-98, and 2015-16 right out of the water,” he wrote. “In addition to (alas) potentially very serious marine ecosystem impacts, these extremely warm near-shore waters will likely add considerable additional moisture to incoming winter storms.”
These warm waters will mean that much of the winter precipitation in the mountains of California will fall as rain, with snow restricted primarily to the highest elevations.
The unusual ocean warmth will also boost atmospheric instability this winter, increasing the risk of coastal thunderstorms, possibly bringing severe thunderstorms with flash flooding, damaging straight-line wind gusts, and a few isolated tornadoes. The unusually active jet stream expected this winter will contribute to this risk.
Record coastal flooding has already occurred and will worsen
Average water levels at Los Angeles were already running one to four inches above expected in the spring of 2026 and rose an additional 9.5 inches between May and September because of the arrival of the Kelvin wave. Water levels have ranged about one to two feet above expected levels so far in October. (An additional sea level rise of six to eight inches since pre-industrial times has already been factored into the “expected” water levels reported by NOAA.)
These high water levels contributed to millions of dollars of coastal damage as a result of the large swells from Hurricane Marie in September and Hurricane Rachel in October. Nearly the entire Balboa Peninsula near Los Angeles was inundated up to two to three feet by Hurricane Rachel’s waves in October.
Add in king tides
The highest tides of the year — the king tides — occur during the full moon of certain months. The king tides during July — which hit at the same time that a two- to three-foot swell from a Pacific storm arrived at the coast — brought San Diego its highest water levels since its tide gauge was installed in 1906.
More king tides are expected November 24-26, 2026, December 22-25, 2026, and January 20-23, 2027. These king tides are almost certain to cause coastal flooding, even on calm sunny days.
“Even a moderate storm could bring water levels higher still, and a strong storm would likely produce widespread damaging, and probably historic, coastal flooding,” Swain explained.
Join me for a Wednesday evening Zoom discussion on California’s winter weather
I’ll say more about the risks of a California megaflood during a 30-minute Zoom call at 7:30 p.m. EDT Wednesday, October 21. This call is primarily a fundraiser for the “Weather Under Fire” documentary I reviewed last week. The movie’s director, Sarah Gibson, and star Chris Gloninger will speak. We will show a few clips from the film and answer questions in the chat.
Here is the link for the Zoom meeting on Wednesday, October 21, at 7:30 p.m. EDT / 4:30 p.m. PDT. To receive an invite, please email WeatherUnderFire@gmail.com.
Please help get the word out on this great project by sharing the link to my review, the fundraiser website, and/or the Zoom call.
Related
My 2023 three-part series on California megafloods:
Part One: The other ‘big one’: How a megaflood could swamp California’s Central Valley
Part Two: If a megaflood strikes California, these dams might be at risk
Part Three: Climate change is increasing the risk of a California megaflood
The El Niño Flood Watch site provides a clickable map for hundreds of points along the North and South American west coasts, showing how many days this winter coastal flooding can be expected, plus a calendar with flood risk percentages for each day.
Bob Henson and Irene Sans contributed to this post.
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