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October 2019

Does a dry fall mean a dry winter season? Less than you might think.

Last week, I wrote about the dry, cool beginning to winter that we're seeing this year in Paso Robles. In my research for the piece, I made a surprising and reassuring discovery. It turned out that having a totally dry October, as we did this year, didn't have any predictive effect on our future rainfall for those winters. There were five such Octobers in the 23 years since we installed our weather station in 1996, and for the rest of the rain season (November-May) we averaged the same 22.2" of rain that we did in the 18 not-totally-dry Octobers. Yes, we missed out on the rain we didn't get that month (an average of 1.5") but it didn't appear that the conditions that produced these dry months lingered in any meaningful way later into the winter.

That got me wondering: was that true for future months? Did low rainfall in October and November mean we were likely to see a drier December-May? Did low rainfall October-December mean drier January-May? Or were the weather patterns truly independent, as my first-pass analysis last week suggested? It turned out that a dry month or months does have some predictive effect, but it's less than you might think. I'll present my findings below, but first a note on my methodology.

I decided first that using "totally dry" as my measuring point wouldn't be reasonable. We do of course have winter months without any rainfall, but after October they're rare. So, I decided that for a period to qualify as "dry", we'd need to have seen less than half of our annual average winter rainfall to date. So, for the period through October, "dry" meant less than 0.85" of total rainfall since July. For the period through November, "dry" meant less than 1.8" of rainfall since July. And for the period through December, "dry" meant less than 4.1" of rainfall since July. This does mean that the results are correlated, since this is a cumulative total, but it seemed better than counting a winter like 2009-10 as "dry" through November because we received less than 1/10" of rain in November, while ignoring that we received a nearly 10" storm in October.

So, what predictive effect does a dry early season have? About 15%, in my calculations. Here's a quick recap of the averages. At the bottom I've added in some graphs that highlight how the dry early seasons have played out.

Period Avg Rain, Rest of Winter Avg Rain (Dry Years) Avg Rain (Wet Years) # of Dry Years
Through October 22.49 20.84 24.28 12
Through November 20.49 16.92 22.05 7
Through December 16.04 14.31 16.52 5

As you might expect, the data is noisiest when you're looking at early-season results, both because there are more dry years (12) and you'd expect to have received a lower percentage of your total precipitation. In the below graph, I've marked years that met my definition of dry through October with orange columns. Wetter years are blue:

Wet and dry winters at Tablas Creek  through October

While the averages are still somewhat predictive, there are some very wet winters that followed dry early seasons, including last year. Looking at years that are dry through the end of November shows a more obvious correlation:

Wet and dry winters at Tablas Creek  through November

In the seven years where we had less than 50% of our average rainfall at the end of November, we only saw one year where we recovered to even hit our long-term average (2005-06). Some of that is the lower average future rainfall (23% less) but a lot of it is that we'd already gotten through enough of the rainy season that the difference between what we've banked in a dry year (1.2") and our average from our wet years (4.7") starts to become more significant. Looking at the data through the end of December doesn't change the picture that much:

Wet and dry winters at Tablas Creek  through December

It's probably unsurprising that when it's been dry through December, we're likely to be looking at a dry winter. But even more than in the previous chart, the biggest difference isn't in the future rainfall we'd expect (we received an average of 13% less rain January-June in those years that were drier through December) but in how much rain we've banked, or not. We'd normally expect to have received 8.17" of rain through the end of December, a little more than a third of our annual total. In the five notably dry years, we'd only accumulated an average of 2.7" to date, while in the other eighteen years we'd averaged nearly four times that already: 9.6".

What does this mean for us this winter? In practical terms, not much. Like always, we're at the mercy of the weather patterns, and what we've seen so far this fall has been consistently dry, with a persistent area of high pressure diverting storms well north of us. But for the first time, forecasts are starting to sound more hopeful, and it looks like there's a chance that this pattern will break down by the end of the month: 

For all that, I feel like the results of my digging into the weather details have done some good for my state of mind. Each week without rain at this time of year feels long. And as nice as it is to be able to go out and enjoy the beautiful sunny afternoons, I enjoy them less because of this nagging feeling that it's wrong, and we really need the rain. Knowing that the predictive effects of past early season dry spells have been modest, and that we have 90% of our rainy season in front of us (and fully two-thirds after January 1st) is a good reminder to be patient. Storms will be coming. Fingers crossed that they'll come soon.

Dark clouds over Tablas Creek Nov 2015


A Dry, Chilly Beginning to the 2019 Winter Season

Those of you who have been following California's recurring struggles with fire won't be surprised to hear that it's been dry. Record low relative humidity has been a major contributing factor to the wildfires that have raged in both northern and southern California. Here in Paso Robles, we haven't seen the same high winds that fueled this year's terrible fire season, and one somewhat comforting fact is that those sorts of wind events are quite rare here, as the Santa Ana winds that affect southern California don't often make it this far north, while the Diablo winds that affect northern California don't typically make it this far south. Still, it's been really dry, with no rain yet, relative humidity dropping into the mid-single digits the last few days, and the lowest dew point I can ever remember seeing (-3°F) on Halloween. There have been several mornings over the last couple of weeks where I've gone out to my car in the morning, had it be below freezing, and yet have no frost on the windshield because it's just been too dry.

What may not be so obvious from the news coverage is that it's been quite cold so far as we've transitioned from fall to winter. We've already had several frost nights in our lower-lying spots, and the vines are mostly in dormancy. Check out the difference in the look of one of our old Roussanne blocks from a month ago:

Roussanne block October 2019

To today:

Roussanne Block November 2019

And we haven't even been the coldest spot in Paso Robles. If you look at the Paso Robles Wine Country Alliance's weather summary for October 31st (the day with the amazingly low dew point) you can see that there were several local weather stations that measured lows around 20°F and one that even dropped into the teens. And yes, even on this day with some very cold mornings, we had warm afternoons. The diurnal temperature swings, always big here in Paso Robles, reach extreme (50+°F) levels during this season:

Weather Summary - October 31 2019

Even with the cold nights, the hilltops are still showing nice fall colors, and are likely to until we see a hard freeze. This photo, of one of our Syrah blocks, is just up the hill from the Roussanne block I photographed above:

2019 Fall Foliage - Syrah

How unusual is all this, and what does it mean for our prospects for a wet winter? Not particularly, and not much. We see our first rainfall of the winter by the end of October about three-quarters of the time. The rest of the years, which have included 1996, 1999, 2003, 2005, and 2017, it's been dry into November. A lack of early rainfall has not particularly impacted what the rest of the winter has looked like; those five years have averaged 22.5 inches of winter rainfall, only about an inch and a half different from the 24.2 inch 20-year average. As the 1.5 inch difference is exactly what we've averaged in our last twenty Octobers, there's essentially no correlation between a dry October and a dry winter. That's a reassuring thought.

While we'd like it wet, the cold is a good thing, as it forces the vines into dormancy and keeps them from expending extra energy at a time of year when there's no fruit to ripen. And we'll take what we can get, while we wait for the long-term forecast to show some hope for rain. We'd like to get our cover crop growing, so our sheep have something to eat. And rain would put to bed any lingering worries about this year's wildfire season. There no rain forecast for the next two weeks: more dry weather, with sunny, warm days in the upper 70s and low 80s, with chilly nights dropping to around freezing. But while our vines would like to usher in the rainy season, that sounds like pretty ideal weather for people. If you're coming to Paso Robles between now and Thanksgiving, it sounds like you can expect conditions to be pretty great.

Meanwhile, we'll look forward to the greens, yellows, and oranges of the harvest season transitioning to the softer browns of fall, like the Mourvedre block below. It may not have autumn's drama, but it's beautiful in its own right.

Late fall 2019 Mourvedre