Assessing the Historic January 2021 Winter Storm on California's Central Coast

I sit here in my office at the vineyard, looking at a sunny landscape outside, listening to birdsong coming in through my open window. It could be a California Chamber of Commerce commercial. What a change from the last 72 hours, during which we saw the largest storm in our 31-year history drop 12.71" of rain on us. For perspective, that's almost half of what we expect to receive in a normal year (26") and 828% of the total that we'd received so far since fall. Since we installed our weather station in 1996, we've had just five months that exceeded the rain we received in this storm.

Before I dive into how the rain came down, what things look like now, and what we think its long-term impacts will be, let me set the stage. I wasn't out here the past two days, but Senior Assistant Winemaker Chelsea Franchi took a break from overseeing a bottling run we couldn't reschedule to capture this great photo of our flock sheltering under our solar panels during the height of the storm Wednesday:

The rain started Tuesday around 8pm, with four-hundredths of an inch that hour. Both rain and wind built in intensity, with our strongest gust (40.8mph) coming at 4am Wednesday. That hour was also the first hour where we got over a quarter-inch of rain. We would exceed that total for the next 22 hours, ending 2am Thursday. By that point we had accumulated 9.42" of rain, nearly double the 5.36" that we would expect in an average January. At that point the rain slowed down, but we had measurable rainfall every hour for another 20 hours, ending at 10pm Thursday evening and picking up another 3.02". And while the bulk of the storm had moved past us, we did get some more showers early Friday morning, accumulating an extra 0.27" in total. The final tally of 12.71" is the most we've ever seen from one storm, eclipsing the 9.6" we received in one storm in October of 2009. 

What drove this storm? A meteorological phenomenon called an atmospheric river, where a relatively narrow plume of moisture stretches from southwest to northeast across the Pacific, funneling tropical moisture toward the California coast. This is different than a typical storm, which is centered around a low pressure system and rotates counter-clockwise. While those storms can bring intense rainfall, they don't generally pummel the same spot for hours on end. Atmospheric rivers are a critical part of California's water systems, and even moderate atmospheric rivers can produce significant coastal rainfall and Sierra Nevada snow. Powerful atmospheric rivers can bring staggering amounts of precipitation, refilling reservoirs and aquifers but also causing flooding and mudslides. A satellite photo shared by the Western Weather Group, which provides forecasts for the Paso Robles Wine Country Alliance, shows the storm system clearly:

Remarkably, this picture could have been taken any time Wednesday or Thursday, as the plume of moisture barely moved for two days. The Adelaida area where we are was the center of the storm's bullseye, accumulating triple or quadruple the already-significant totals that most of the rest of the AVA saw. And that water adds up. Our area drains into Lake Nacimiento, and the storm raised the lake's level by 24 feet since Tuesday, and its capacity from 21% to 39%. That may not sound like much, but it's a massive lake whose surface area grew by 1,000 acres since Tuesday, adding more than 21 billion gallons to its volume. 

I got out into the vineyard today, and found that it held up better than I'd feared it would to this onslaught of rain. As you would expect given that it had been so dry that our cover crops weren't well established yet, there was some surface erosion in some of our blocks, like this Cinsaut planting:

 Or at the edge of this old block containing Counoise and Mourvedre:

The roads, which are less porous than the vineyards, are always at greater risk of erosion. That's true both in the case of unimproved roads, like this one running down from our old nursery buildings toward Last Tablas Creek:

And even on the vineyard roads where we laid straw to slow the flow of water and limit erosion the many hours of flowing water were enough to overwhelm our preparations:

The calcareous soils that underlie Tablas Creek are so absorbent that the vast majority of the water soaked in. I only found one vineyard block that had any standing water in it: a head-trained Grenache block near the north end of the property:

But most of the vineyard looked like it was in great shape. This old Grenache block is representative:

The water that did run off ended up in Las Tablas Creek, eventually flowing into Lake Nacimiento. Today, the creek was hardly a raging torrent:

That relatively gentle flow, even on the last day of a historic storm, should tell you one more thing in addition to providing evidence of how absorbent our soils are. This winter, up until now, has been exceptionally dry. Normally, by the end of January we'd expect to have received about 13.5" of rain. Before this storm hit, our winter total sat at a paltry 1.37". Thanks to our recent atmospheric river, we've recovered from about 10% of normal rainfall to about 105% of normal rainfall. But even with this storm, what we've seen is only about average for a winter at this point.

What's next? Hopefully, a few days of sun to transfer the water from saturated surface soils to deeper layers, and to give the cover crop a chance to get growing. We have gotten a little growth, but it's variable and nowhere particularly well established. This section, which we left fallow this year in preparation for a new Mourvedre planting this spring, is probably the farthest along:

But before too much longer, we'd like some more rain. It looks like there's a little in the forecast for next week. A moderate storm of a couple of inches every 10 days or so would be just about perfect. If you've got sway with any weather gods, please let them know.

Meanwhile, we'll be grateful that we've made up three months of rainfall deficit in three days. We'll be looking forward to the vineyard being noticeably greener in under a week. And we'll be listening to the splash and burble of Las Tablas Creek flowing after 10 dry months. It may not be rushing torrent, but it's a more than healthy start.