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Rain-year recap: The numbers are in

By Shannon Lee

The official water-year has come to an end and the numbers are in. Our unofficial Glen Ellen rain gauge at Two Moon Family Farm recorded a total of 37.25 inches from Oct. 1, 2021, to Sept. 30, 2022. The average annual rainfall for Sonoma is 31 inches; for Santa Rosa it is 36.23 inches. In this context, our year seems about “average.”

Our 2021–2022 rain record, however, absolutely looms over last year’s paltry 14.06 inches and considerably edges out the 24.88 inches recorded for the year before that. And yet, it doesn’t come close to the 2018–2019 barn-busting 60+ inches for our end of the valley.

This year’s rain had an interesting pattern of arrival, with the wettest month being October, when we had more than 14 inches of rain, most of it falling between Oct. 18 and Oct. 27. You may recall the crazy midmonth storm that dumped 10+ inches in one 24-hour period (Oct. 24-25, 2021). Very unusual.

The second wettest month this past year was December, with slightly more than 11 inches of rain. Also notable is that we had very little rain in January (less than an inch), and not a drop of measurable rain in February.

In addition, we had atypical measurable rain in September. Long-time valley residents (and the vintners) will tell you this is an unusual pattern, and the historical data bear that out as well. Sonoma’s historical average for rainfall in October is only 1.64 inches, and our typical wet months are December, January, February, and March, in that order. Our historical “very dry” months are July, August, June, and September, in that order (all under a quarter-inch). You can see the comparisons between this year and historical averages on the graph.

Perhaps most interesting, and despite having a relatively hot summer, stream flow never dwindled to the low or dry levels that it did last year. This is likely due to a strong rain showing in April, where we accumulated nearly 4 inches, compared to an average of just above 2 inches. In addition, one-day heavy storms in both June and September boosted the creeks at critical moments in the season.

It is impossible to say with certainty what this next water year will bring, but forecast modeling is available. Researchers predict that winter ’22/’23 will continue as a La Nina cycle, with the chances of a shift more likely heading into early spring. It is unclear what that means for our area because we don’t have a clear pattern of heavy versus light rainfall with La Nina/El Nino cycles. In Southern California, El Nino means the potential for heavy rains and La Nina means dry conditions, but here in the North Bay, the two wettest years on record for Santa Rosa were both La Nina years, 2016–2017 (60.40 inches) and 1889–1890 (56.07 inches).

Maybe we’ll have an October surprise like last year, but the reliable forecast that takes us now into late October shows zero rain and highs in the mid 70s to low 80s.

To close on an exciting note, this author just celebrated a birthday and received an “official” rain gauge as a gift. We are now going to be part of a network of rainfall reporters via CoCoRaHS (Community Collaborative Rain, Hail & Snow Network). The collaborative has sent us the official gear to install, and by signing up we are agreeing to report data each day at the same time. The information goes into a searchable, public-facing database. You can see the real-time maps at the website cocorahs.org.

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