Why Annual Rainfall Is the Wrong Metric to Understand California Weather

After our lovely, wet December, the last six weeks have been almost completely dry. The last week has seen headlines that feel like flashbacks to 2015 or 2016, including Brush fires rage in Southern California amid record heat, worsening drought (Washington Post), California’s Drought-Relief Dreams Are Quickly Drying Up (Bloomberg), and As drought continues, Southern California offers millions to buy Sacramento Valley water (Sacramento Bee). And yet, here at Tablas Creek, 2021 finished with above-average rainfall at nearly 30 inches.

Although we do get more rainfall than most of the Central Coast, we're not alone in having 2021 be an above-average rainfall year. The city of Paso Robles recorded 16.75", about 119% of their 75-year average. Downtown Los Angeles saw 14.27" of rain, which was 98% of their 60-year average. So why are climatologists and reporters so downbeat about our current totals?

The secret to making sense of this is understanding the difference between the calendar year and the water year. In a place like California where nearly all our annual precipitation comes in the winter, the break between one calendar year and the next is not a meaningful way of looking at current conditions. Our rainfall distribution is such that in the winter, we're a rain forest climate, while in the summer, we're a desert (climate graph from the site climate-data.org, on which I spend probably more time than is healthy):

As you can see, the calendar year ends just as the water year is really getting going. Take the winter of 2020-2021. We saw a punishingly dry early winter that left things in late January looking more like we'd expect in November than in March. We shared a piece on our social media in mid-January that I found really dramatic: a photo taken from the same spot on the same day in 2022 (exceptionally green) and 2021 (entirely brown):

When the rain did come last winter, it came with a vengeance, in the form of an atmospheric river that dumped more than a foot of rain on us in roughly 48 hours on January 27th and 28th. That ended up being most of the rain we got last winter, with just over an inch more before summer came. But still, the 13.85" we got January-June was more than 80% of our averages for those months. Combine that with the 14.2" of rain we got in our wet October and December, and you end up with a year that looks like it was 12% wetter than average. Instead, what you really have is a function of the calendar. Last winter's total of 15.09" was just 60% of a normal rainfall year. And unless some unforecasted rain comes before the end of February, we'll be below 80% of normal rainfall for this rainfall year too.

There's still time. March is often one of our wettest months, averaging more than four inches of rain. As recently as 2018, a "Miracle March" turned what looked like a truly scary rainfall picture into something closer to average. And thanks to our wet December, we're worlds ahead of where we were that winter. But unless it rains again soon, we're likely to see early budbreak as drier soils and the higher soil temperatures they allow cue the grapevines to begin their growing season. And more visibly, scenes like the below one will start to move from green to gold as the grasses go to seed in preparation for the long, dry summer:

None of that would be the end of the world. But we know that the clock is ticking on this rainfall year, with only another couple of months to go. So while 2022 is just getting started, the 2021-22 rainfall year is already on what feels like its final lap.