Abundant rainfall finds North Coast region wet with little idea of the future

Thirty-six inches in 23 days in the remote community of Venado, west of Healdsburg.

That’s just one of the whopping rainfall totals — though likely the most impressive — to result from the recent storms that bombarded California over a three-week period.

The rains caused wide-scale flooding and mudslides that proved distressing and, in many cases, disastrous, as years of drought gave way to an onslaught of atmospheric rivers.

Cazadero, another typical wet spot in the rural hills of Sonoma County, got more than 33 inches of rain over the same 23-day period from Dec. 26 to Jan. 17.

While Oakland, San Francisco airport and Stockton just experienced the wettest 23-day periods since rainfall records have been kept, the North Bay’s higher historic precipitation levels set too high a bar for records to be broken in most areas, meteorologists said.

But the drastic, abrupt shift from drought to plentiful rain was no less remarkable in Sonoma County.

Guerneville saw 30.21 inches in the 23-day time frame; Cloverdale, 23.98; Healdsburg, 23.75; and Lake Sonoma, 27.76.

At Santa Rosa’s Charles M. Schulz-Sonoma County Airport, 18.76 inches fell, more than three times the 25-year mean of 5.42 inches for the entire month of January.

The airport already has received 24.44 inches since Oct. 1, the start of the water year, and is nearly three-fourths of the way to seasonal normal rainfall for 2022-23.

If we get to 33.78 inches or more, it will be the first time the region has had a statistically normal rain year since 2018-19. That season, a stunning 46.5 inches of precipitation was measured in downtown Santa Rosa, and flooding on the Russian River caused $155 million in damage around the county.

So what do this month’s rains signal for the rest of the year? The short answer is no one knows.

At this moment, the gates at Coyote Dam are open, as the operators of Lake Mendocino seek the sweet spot between hoarding long-awaited rainfall and leaving enough capacity to receive runoff from potential storms that may still be ahead.

There are tools and guidelines to inform the decision about when to close the gates again. But reading the future is still largely a guessing game. Beyond about a week or maybe two, current forecasting skill offers little certainty about future conditions, experts say.

The rest of the month is predicted to be dry, but January falls in the middle of the rainiest part of the year, typically December through February and, sometimes, March. So there’s still plenty of time for more rain.

March of 1991, for example, was what’s been called a “Miracle March,” bringing 13.74 inches of rain to downtown Santa Rosa after five months in which only 7.69 inches total had fallen.

In January 1909 — the wettest January since record-keeping began in 1902 — a record 18.45 inches of rain that fell in what was otherwise a fairly unimpressive winter, though that February was above the mean at 8.74 inches.

But look no further than last January for early season rainfall that suddenly, dreadfully ground to a halt.

The first three months of 2022 were the driest in recorded California history and contributed to what would be the driest three-year period — the rainfall seasons ending 2020, ‘21 and ‘22.

“Historically, December, January, February, March — those are the wet months,” National Weather Service meteorologist Ryan Walbrun said. “But it’s just chaos, right?

“Some years we get it early, late, whatever. There’s not necessarily going to be a defined pattern.”

This winter, the proverbial “storm door” opened with a bang and allowed a series of back-to-back-to-back atmospheric rivers to enter the region, beginning late Dec. 26 and finally tapering off early this week with a final cold front dropped less than inch overnight Wednesday.

The North Coast avoided some of the more severe ramifications, which were worst along the Central Coast, though at least four of the state’s 20-storm related deaths occurred in Sonoma and Mendocino counties.

The rain has really helped the area and much of the state rebound from three years of drought, however, filling Lake Mendocino to capacity for this time of year and recharging Lake Sonoma.

“The 10-to-14-day forecast is trending drier right now, but that could change quickly,” said Don Seymour, principal engineer with Sonoma Water. “I think, in the big picture, the reservoirs, particularly Lake Sonoma, our largest reservoir, it’s almost full now. Even if it trended dry, we’re just in such a different positions than we’ve been in recent years. We’ve basically filled that reservoir.”

It was a quick turnabout. Just last month, Lake Sonoma reached the lowest level in its history, at 96,310 acre-feet. It has more than doubled its storage since then, with nearly all of the gains coming after Dec. 26.

By Tuesday, the reservoir near Cloverdale held almost 232,723 acre-feet and was still gaining runoff from the surrounding watershed. Full water-supply capacity is 245,000 acre-feet. (An acre-foot is equal to 325,851 gallons, or about the amount of water needed to flood most of a football field one foot deep. It can supply the indoor and outdoor needs of three water-efficient households for a year.)

It’s a similar story at Lake Mendocino. Storage there dropped below 13,000 acre-feet — it’s second lowest level ever — in October 2021, days before a giant atmospheric river dropped record one-day rainfall on the region.

After recent storms, it reached a key threshold Jan. 8 of 68,410 acre-feet, which requires transfer of control from Sonoma Water to the U.S. Army Corps of Engineers, which must manage the reservoir for the purpose of flood control. A week later, water storage had hit almost 96,000 acre-feet.

On Monday, the Army Corps began high-flow releases from the dam, allowing about 9,000 acre-feet to flow down river by Thursday, though it was expected to begin ratcheting down releases in the coming days.

Under new operational measures designed to maintain as much water supply as possible while ensuring there’s still room to take more if heavy rain comes, the Corps has discretion to keep the water at about 11,650 acre-feet above flood level, or a little over 80,000 acre-feet, as long as the forecast is clear.

On Thursday afternoon, the reservoir held about 86,600 acre feet.

Nick Malasavage, operations and readiness chief for the Corps’ Golden Gate District, said he expected releases would be reduced until they reach minimum stream flow required — the level required by federal regulators to support aquatic life and basic water needs.

Having healthier reservoirs also should restore river flows to more normal levels this summer, which would benefit fish and other river species after several warm seasons with critically low, minimum flows.

“I’m just elated, because we’ve stepped back from the brink,” said Don McEnhill, executive director of the Russian Riverkeeper. “I wouldn’t ever say we’ve ended the drought, but we’ve certainly taken a step back.”

McEnhill said the sudden, drastic shifts from extremely dry to extremely wet weather are exactly what climate scientists had been predicting for decades, and he feared that residents would too easily forget the importance of water conservation, given current abundance.

But he noted that Lake Mendocino was full in spring 2019 and then two years later, almost empty despite drastic measure imposed to try to prevent that from happening.

“I just look at this as what it is: a temporary respite,” he said.

You can reach Staff Writer Mary Callahan (she/her) at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.