Sonoma Valley rainfall roundup
By Shannon Lee
Our rain year is really off to the races. The atmospheric river near the end of last month was impressive and brought more than 12 inches to Glen Ellen in less than 24 hours. The wetness of that October storm was truly something, but combined with the early timing, the event broke records around the Bay Area.
Each fall the official rain year closes out on September 30 and the new one begins on October 1. Average annual rainfall for Sonoma County is approximately 39 inches, but local areas show a lot of variation. The city of Sonoma has an average of 30.7 inches, while Santa Rosa’s average is 38.1 inches. Thus far, Rain Year (RY) 2021-2022 is looking very promising. At our place we have logged 15.88 inches since October 1.
On our Glen Ellen property, we’ve been recording rainfall data since the fall of 2018. I’m very diligent about checking the gauge here on the flank of Sonoma Mountain just north of the SDC campus. It’s not an official meter but corroborates well with other locals who keep track and falls in line with several nearby official setups.
Starting the recording tradition in RY 2018-2019, as it turned out, was very fortuitous and exciting because that year we received twice our normal average rainfall. Surprisingly, the first measurable rainfall that year did not arrive until after December 21, but by March 20 we had logged 54 inches. By the close of the rain year on September 30, 2018-2019 racked up 67.5 inches.
Then came two very low rainfall years. In RY 2019-2020 and RY 2020-2021, the rain began in November but didn’t amount to much and by March it was clear that our totals were going to be far below average. RY 2019-2020 closed at 24.88 inches, and RY 2020-2021 closed at an even more dismal 14.06 inches.
As you can see, only 6 weeks into RY 2021-2022 we’ve already surpassed the rainfall from last year. Sounds good right? Unfortunately, our early and strong October storm doesn’t mean that the rains will continue. In fact, oceanographers and meteorologists have determined that we are in a La Niña cycle.
The opposite of El Niño, La Niña tends to bring drier and warmer conditions to the southwest and cool, wet conditions to the north. Our region is in the interface between these two regions and as such, our rain year potential is very hard to predict. On the east coast, La Niña conditions can lead to more tropical cyclones. Winter 2020-2021 was also a La Niña. Our most recent El Niño was 2018-2019; that year we received twice the annual rainfall.
The wettest rain years for our county (as recorded in Santa Rosa) were: 2016-2017 (60.40 inches), 1889-1890 (56.07 inches), 1982-1983 (55.66 inches), 1940-1941 (52.15 inches), and 1981-1982 (47.70 inches). The strongest ever El Nino was the winter of 19821983, and 1940-1941 was also an El Niño year. But, the two wettest years in our county’s official records were La Niña years, so it is quite possible to have a wet year even with those oceanographic conditions.
The El Niño/La Niña cycles have been ongoing but there is a new wrinkle. Our area’s wettest months have historically been December through February. A winter rainfall pattern, with February having the largest number of rain days. There are suggestions that this is shifting due to warming. We may be leaning into lower annual rainfall totals overall in addition to a new pattern that is bimodal, with an initial rainfall pulse in the fall and then another in the spring. This shift would also impact our patterns of humidity and wind directions/strengths. This has big implications for both natural ecosystems and our systems of agriculture. Viticulture scientists are watching these patterns carefully.
Although 14 inches in 1 week was incredible, and brought the end of fire season, it does not mean that our drought is over. Lake Sonoma is currently at ~50% of capacity. Total water supply capacity (when full) is 245,000 acre-feet. The October atmospheric river brought 13,000 acre-feet into Lake Sonoma, just a dent. All water conservation recommendations and targets are still in place.