MENDOCINO CO., 9/21/26 — Two camps are shouting about this winter. One says Mendocino County is going to drown. The other says nobody can know, because strong El Niños have come up dry before. Daniel Swain, a University of California climate scientist, says both are wrong.
The skeptics start from something true, Swain said in a phone interview Sept. 14. An El Niño of any strength guarantees nothing, and its pull on California weather is strongest in Southern California and weakest along the North Coast. Mendocino County sits at the weak end. In a weak or moderate El Niño, Swain said, the skeptics would have a point: “It’s not very predictive of anything in particular” north of the Bay Area.
But the skeptics’ case comes with a caveat, Swain said, and this year the caveat is the main point.
“It’s a rare case where the caveat is the story,” said Swain, who works at the California Institute for Water Resources at the University of California Agriculture and Natural Resources. “This is not an ordinary El Niño event.”
The ocean isn’t doing a weak or moderate El Niño. The National Oceanic and Atmospheric Administration’s Climate Prediction Center, in its Sept. 17 outlook, put the odds of a very strong event this fall and winter above 90%. It gave a 75% chance that by the October-through-December season the event will be stronger than any on the books since 1950. Sea surface temperatures in the eastern equatorial Pacific were running more than 5 degrees Fahrenheit above normal.
“We are now predicting with pretty high confidence,” Swain said, “the strongest El Niño event on record by potentially a significant margin.”
There is no historical analog for this event, he said, because nobody has seen one this big. But models that run the physics of the ocean and atmosphere three, six and nine months out, he said, are in near-universal agreement that the likelihood of a wet or very wet winter across most or all of California is exceptionally high, “probably the highest that it’s ever been looking forward from September.”
“This winter, that signal extends all the way up to the Oregon border,” he said. He put the odds of a wetter-than-average winter on the Mendocino Coast “north of 80 to maybe even 90%,” and the odds of a dry one at roughly 1% to 4%.
“If there was an 80 or 90% chance of rain, would you bring an umbrella?” he said. “I think almost everybody would.”
A January through March forecast that’s the boldest ever
The Climate Prediction Center’s own map looks tamer. Its December-through-February outlook, updated Sept. 17, gives Ukiah, Fort Bragg and Covelo a 33% to 40% chance of above-normal rain, the lowest tilt the agency draws. Point Arena gets 40% to 50%. Swain’s answer is to look at the January-through-March outlook, which puts every Mendocino County town at 40% to 50%. The agency’s written discussion says the wet signal peaks at 50% to 60% near the California coast through the winter and may be raised again in October. Swain, who spoke three days before the update, was reading the August maps, which gave the county the same numbers.
Look past the early-winter map, he said. “Look at January through March, because it paints a significantly different picture, a much more aggressively wet one.”
Swain said odds that rise from the first map to the second are backward from how forecasts usually behave. Confidence normally drops the further out you look, he said, and here it climbs. That matches his own expectation that the effects arrive in December at the earliest and might take until January to fully kick in. He called the center “scientifically conservative,” an agency that rarely makes a bold call even when the evidence supports one. “Their January through March forecast for California is, by their standards, I think the boldest forecast they’ve ever issued for precipitation.”
Wet isn’t the same as flooded
A wetter-than-average winter doesn’t have to mean the whole county is underwater, Swain said. “You could have a wetter than average winter and have relatively unremarkable flooding, right, if it’s reasonably spaced out.”
But later in the call he said a wetter-than-average winter “will probably feature at least one very or extremely wet individual month,” and that’s when he’d expect flood risk to peak.
“Will it be a risk of terrible flooding or more routine, like, oh, 101 is under a foot of water again, type of flooding? That is not really possible to predict at this stage,” he said.
On the Russian River near Hopland, the river crested at 16.57 feet during the very strong 1997-98 El Niño, according to U.S. Geological Survey and National Weather Service gauge records. The New Year’s flood of 2005-06, not an El Niño year, hit 26.81 feet, and February 2019, in a far weaker El Niño winter than 1997-98, reached 22.95. On the Navarro, 1998 peaked at 27.97 feet. The 2005 flood reached 39.81 feet and in 2019 reached 37.16.
Last winter, with no El Niño to blame, the Russian at Hopland reached 18.55 feet on Jan. 5, about 3.5 feet over the minor flood stage. At that level the weather service expects water on Highway 175 at the bridge approaches and on Highway 222 near Ukiah. The Navarro touched 23.08 feet the same day, which is the stage at which the service says Highway 128 “will be closed.” Those figures are provisional.
Swain also discussed a piece of confusion he said he can’t trace: that atmospheric rivers, the long moisture plumes that bring about 60% of California’s precipitation, run on their own schedule and have nothing to do with El Niño. Part of that is true, he said. El Niño doesn’t mainly work by adding atmospheric rivers. It brings more storms of other kinds, which keeps the ground soaked between them, and it loads the atmospheric rivers themselves with more water.
“It doesn’t decrease the number of atmospheric rivers, but it increases everything else, and it makes atmospheric rivers more intense,” he explained.
A higher ocean and a warmer one
The coast gets a second problem the valleys don’t. El Niño lifts sea level along the West Coast by a foot or two for the winter, Swain said, about as much as climate change has added so far. “It’s kind of like suddenly doubling the amount of climate change-related sea level rise along the Mendocino Coast.” Low roads along estuaries, river mouths and bays, which he said turn up at points along Highways 1 and 101, could be at greater risk of flooding, not just from rain but from the higher ocean itself.
The water is also warming. It started along the Mendocino Coast in the last couple of weeks, Swain said, and it will keep warming relative to normal through the fall and winter. The ocean doesn’t swing the way the air does, he said, so a few degrees for a few months is a big deal for kelp, for the nutrients that cold-water upwelling brings and for where the fish are. “For fisheries, he said, the biggest influence is probably the water temperature, not so much the rain.
What the county can and can’t do
Howard Dashiell, who runs the Mendocino County Department of Transportation, answered questions by email. His list of trouble spots is the floodplain: the Russian and its creeks at Hopland, Talmage, Vichy Springs, Redwood Valley and Potter Valley, plus the flat bottoms of Willits Valley and Covelo Valley, where water backs up because the creeks that drain them are full. Dashiell calls it a backwater curve. “Even a recently cleaned ditch will back up from high water levels downstream,” he wrote, and “there is nothing the county can do about floodplains.”
The department will do its best to clean ditches and culverts before storms, Dashiell wrote, though big storms often wash new debris in. Usually, he wrote, the first big rain flushes the system, and the rest of the winter goes better. The department has seven corporation yards and crews spread along 1,014 miles of county road. Dashiell put that at about one road worker for every 25 miles, fewer overnight. “MCDoT does not have any additional resources regardless of El Nino,” Dashiell wrote, “so we will do what we can just as we do in any other winter preparation.” Crews respond around the clock, he wrote, but unlike Caltrans the county doesn’t have the staff for a full-time overnight patrol shift.
A $460,000 storm-drainage study won’t help this winter. Dashiell said the grant paying for the study was applied for years before this forecast, and it’s very specific: it covers a drainage system designed in the ‘70s and ’80s in one northwestern Ukiah subdivision and is meant to line up future construction money.
His advice for a wet night: stay home if you can, drive in daylight, slow down. County road closures post here. State highways are on Caltrans’ QuickMap and the road line at 1-800-427-7623.