Did 2.6" of rain in July really just happen?

Yesterday, we hosted a seminar on dry-farming.  In the rain.  In Paso Robles.  In July.

And it wasn't just a little rain, either.  As we were talking about how we've been working to plant increasing acres of vines without any irrigation infrastructure, and how we've been weaning even our established vines off of needing regular water, we were in the process of accumulating more rain -- seven times more rain -- than Paso Robles had received in any July day in its history.  In fact, we received four times more rain yesterday than Paso Robles has ever received in an entire month of July before.

Of course, July is the driest month of the year in Paso Robles, averaging less than 0.2" of rain for the month.  But residents of other parts of the country may not realize just how unusual summer rain is in the Central Coast of California.  Maybe a graph will help (from the useful site climate-data.org):

The unusual storm was caused by the remnants of Hurricane Dolores, which instead of remaining offshore or moving inland toward Phoenix (both more common paths for Pacific ex-hurricanes) wandered slowly and more or less directly north up California's coast, bringing unusually hot, moisture-laden air to parts of the state where humidity is almost totally foreign.  This moist, unstable air spurred a series of thunderstorms (themselves rare in this area) whose path took them directly over Paso Robles.  The town recorded 3.55", the highest total in the county and more than a quarter of the average annual rainfall the city can expect to receive.  It's also more than the town averages in any month during the year; for those of you who like me need a translation from the metric, 3.55" is more or less 90mm... about 10% more than Paso Robles receives on average in January, its wettest month.  We saw a little less rain out here than they did in town, but at 2.6" we still received more than we did all but two months (December and February) this past winter.

Given how much rain we received, and how fast it came down, you might wonder how the vineyard fared. It came through with flying colors. We rarely see much erosion here, due to the porosity of the clay-limestone bedrock, and we didn't see any in the vineyard. The little we saw was confined to a few vineyard tracks, where the occasional tractor traffic compresses the soils enough that water runs down rather than soaking in. From this afternoon:

We don't anticipate any serious negative consequences from the rain. After a last round of showers and thunderstorms that are forecast to blow through this evening, it's supposed to dry out. It looks like we'll have a few sunny, breezy, relatively cool, low humidity days, and then it will warm up to normal for the season (highs in the 90s and lows in the 50s, with virtually no humidity).  This should serve to dry out the surface layer of the soil, and prevent too many weed seeds from germinating.  Plus, both the low humidity and the warm days will serve to prevent mildew from becoming a problem.

On the positive side, this is a time of year when the vines are nearly all under some stress.  A dose of rain now helps alleviate this stress, and gives the vines the energy to make their final push toward ripeness.  And the amount of rain that we received was enough to get through the topsoil and replenish the moisture in at least the upper layers of the calcareous bedrock, which will provide a reservoir for the vines over the next couple of months.  You can see how rich and soft the soils look after the rain, and also how void of erosion channels they are even near the bottom of a steep hillside:

If the rain had come a month later, with some grapes nearly ripe, we would worry that the water might dilute the flavors and even cause grapes to swell and split.  But now, with veraison barely started, that's not a risk.

It's worth remembering that most other wine regions, including the Rhone, see summer rain.  The same chart as above, for Chateauneuf-du-Pape, shows that while rain is a bit less common in the summer than in other seasons, it's hardly rare:

So, we'll enjoy the unusual moisture in the air, and feel thankful that we got it now and not during harvest.  If this is a precursor to what is sounding likely to be a strong el nino winter, so much the better.  I'll leave you with one final photo, taken in the middle of yesterday's rain, and looking more like an impressionist painting than a summer Paso Robles landscape: