Rain season 2025-26 is here, and, as if on cue, two storms made for a gloomy, damp start to the week.
What happens after that is another story. Will Napa County have a fourth consecutive wet rainy season? Will it eke out the south county norm of 25 inches? Or will it be hit with a duster portending a return to drought?
Stakes aren't as high as in some years, given the U.S. Drought Monitor reports the Bay Area and Napa County aren't in a drought. But much of Southern California is, and the state is always a dry rainfall year away from facing water supply challenges.
Predicting a rain season outcome is a bit like speculating what team will win the World Series or Super Bowl. There's guesswork involved, but experts can make educated guesses.
An aside: rainfall for the annual rain season in California is measured from Oct. 1 through Sept. 30, not the calendar year. That means the small amount of rain that fell on Monday and Tuesday counts toward the past rain season. Rain falling on Wednesday counts toward the coming one.
Atmospheric rivers a wild card
Mike Pechner of Golden West Meteorology in neighboring Solano County said this looks to be a La Niña rain season, referring to a climate pattern in the Pacific Ocean. That means a 40% chance of a wet year and 60% chance of a dry or normal year.
"It looks like it may start out very promising ... but December on some of the long-range data I have looks dry," Pechner said.
December is an important month for rainfall. Receiving 1 or 2 inches at Napa State Hospital instead of the usual 5 inches or so could mean a below-normal rainfall year, Pechner said.
"Tough to say if this is going to be a good year," Pechner said. "My indication right now is if it comes up dry December and it's normal the rest of the year, we're short on rainfall. How much, I'm not sure."
The wild card is atmospheric rivers, he said.
"We seem to be getting more ... just one of those would change the whole picture. Change everything," Pechner said.
An atmospheric river is a band of moisture-laden tropical air that fuels rainfall. One event can bring several inches of rain. An extreme example occurred in October 2021, when various parts of Napa County received 6 to 14 inches in a few days.
Pechner said signs point to a wet start of the rainy season before that possibly dry December. Not all the signs are from computer forecast models.
"I can tell you this -- I saw leaves falling here in Cordelia on oak trees in late August, over a month ago," he said. "That gave me an indication that it is an early fall weather pattern."
Fall rains could put an early end to the fire season. About 4 inches are needed, Pechner said.
"It might take all of October or November to do it," he said. "But instead of December or late November, it could be a couple of weeks early this year."
The National Weather Service Climate Prediction Center also tries to peer into the future. For December, January and February - historically the three wettest months of the year - its prediction map has Napa County on the line of equal wet/dry chances and being on the dry side.
In other words, flip a coin. Also in the predicting business is the Old Farmer's Almanac. Unlike Pechner, it forecasts a warm, dry fall for Napa County. That is to be followed by a mild, dry winter.
Late November, mid-January and late February should be stormy periods, the almanac predicts.
Rain season hits home in Napa
Whether the rain season is wet or dry hits home in Napa County. Rain supplies water for local homes, businesses and vineyards, as well as wildlife and the environment. Wine country can't make wine without water.
Whatever the outcome, the city of Napa should be in decent shape.
The city has two major water supplies. One is its own Lake Hennessey reservoir in the mountains east of Rutherford that fills with local rainfall. The other is annual State Water Project allocations from the Sacramento-San Joaquin Delta that originate largely from Sierra Nevada snowmelt runoff.
Lake Hennessey is 76% full. The city has leftover State Water Project allocations to supplement whatever allocations it might get for next year.
"We're OK as we head into this time of year," city Utilities Director Joy Eldredge said.
Ironically, an extremely wet rainy season could cost the city a portion of its State Water Project supplies. California stores the city's leftover State Water Project allocations in San Luis reservoir south of the Delta. This is essentially banked water as a hedge against dry years.
But carryover allocations apparently aren't the highest priority under state and federal rules for the reservoir, which stores high Delta flows for the State Water Project and Central Valley Project. Eldredge said the city could lose banked water if San Luis reservoir fills.
The city of Napa also has the relatively small Milliken Reservoir near Silverado. Yountville, St. Helena and Calistoga are served by local reservoirs. American Canyon has no reservoir and depends on State Water Project allocations.
Finally, local rains provide water for local aquifers. Napa County wants to reduce overall groundwater pumping in the Napa Valley subbasin by 10% from the average seen between 2005 and 2014 ensure the subbasin isn't overdrawn. Dry years intensify groundwater concerns.
Looking back isn't easy
With a new rain year arriving, it's also time to look back at the 2024-25 rain year. Figuring out whether it was a gusher, so-so or dry isn't as easy as it might sound.
The gold standard for south Napa County rain season totals is the weather station at Napa State Hospital. Records there date back to 1893.
But the National Weather Service reports a mere 21.3 inches of rain fell at the hospital last rainy season as of Friday, only 77% of normal. That's on the dry side.
How can that be? After all, such February sights as Lake Berryessa spilling into the Morning Glory spillway and Napa River water surging through the city of Napa's Oxbow Commons bypass - both for the first time since 2019 weren't mirages.
The answer lies with Napa State Hospital station record keeping. Each month typically has a few days of missing data.
Missing for last rainy season were two days, one in November and one in February, when atmospheric river storms pounded Napa County.
Given that, the National Weather Service reported rainfall for Napa State Hospital appears to be short several inches. In fact, AccuWeather reports the hospital received 25.27 inches.
The National Weather Service reports 46 inches fell at Angwin northeast of St. Helena, or 113% of normal. Napa County's OneRain website reports 27.62 inches fell at the city Napa corporation yard on Jackson Street, or 109% of normal.
A big winner in the rain sweepstakes was Mount Veeder. OneRain reports 56.22 inches fell there, 133% of normal. However, as a result, Mount Veeder Road suffered millions of dollars of damage at several locations.
All in all, the 2024-25 rain season seems to have been slightly wetter than usual for most of the county, with some spots well over the norm.
How did prognosticators do for last rainy season, which also had La Niña conditions? A year ago, Pechner predicted 90% to 105% of normal rainfall, the Climate Prediction Center predicted equal chances of wet or dry and the Old Farmer's Almanac predicted below-average rainfall. Pechner came the closest. ■
You can reach Barry Eberling at 707256-2253 or [email protected].