One of the benefits of being a member of the Paso Robles Wine Country Alliance is their commissioned agricultural forecast, which we receive each day. Normally this covers the recent weather and then looks forward both short and medium-term, and it's one of the tools we use to decide what vineyard work (or, during harvest, what harvesting) we're going to look to do next. But occasionally they send out a special message that looks at a long-term trend of interest. Earlier this fall, when it became clear that we're going to experience a second consecutive winter driven by a la nina pattern, they distributed a note with the prediction for this winter's temperatures and precipitation.
La nina patterns happen when the water surface temperatures of the central and eastern tropical Pacific Ocean are cooler than normal. Changes in these ocean temperatures impact weather patterns around the globe. The note sent to us included predictions for Paso Robles, but also NOAA-prepared maps of the entire United States. I thought their implications were interesting enough to share. First, the temperature forecast:
According to the projections, coastal California should be significantly cooler than normal, at least a 40% greater chance of it being cooler than average than it does of it being warmer than average. During the winter, this doesn't much matter. We know it can get cold in Paso Robles, and we typically get somewhere in the neighborhood of 50 below-freezing nights each winter. That we may get an extra dozen frosty nights, or that the nights may bottom out in ther teens rather than the twenties, doesn't matter to dormant grapevines. But in the spring, cold nights can hurt. This past winter was a la nina pattern, and we had some of the coldest spring temperatures we've ever seen, including the damaging two-day frosts of April 8th and 9th.
It does not seem that the cooler-than-normal temperatures are confined to the winters, either. Both the 2010 and 2011 growing seasons were significantly cooler than normal, with degree day readings at Tablas Creek roughly 10% lower than the 10-year average. And a cooler winter can actually delay budbreak and make the vines less susceptible to spring frosts. But if you get a warm March, look out.
The precipitation forecast is equally interesting and probably even more important:
In a la nina winter, the northern United States, particularly the Pacific Northwest, gets pummeled by rain. The south tends to be dry. Paso Robles is right in the middle, but the timing of the rain in a la nina winter is unusual and tends to be front-loaded in the first few months of the rainy season (November-January). So while Paso Robles is around its winter-to-date average for rainfall, we'd hope to be above our average with a drier-than-normal late winter likely.
While the impact of la nina on short-term weather patterns is typically ambiguous, what we've seen so far this winter has certainly borne out the predictions. After some rain on November 20th, we've settled into a three-week-long cold, dry, sunny pattern, with most nights dropping down well into the mid-20s and daytime highs around 60. Our precipitation this winter (about 4.5 inches so far) is maybe half an inch below average. And the long-term forecast doesn't suggest a lot of change in store. We're supposed to see a chance of some showers tonight and tomorrow, but nothing significant. A similarly meager storm is supposed to arrive at the end of the week. But the forecast holds little promise of the sorts of big winter storms that we need to replenish surface and ground water, and as we move from early- to mid-winter, we know that our chances of these big storms go down.
With these longer-term forecasts in mind, it seems unlikely that we'll enjoy a third consecutive winter of significant rainfall. It seems more likely that we may see a recurrence of the spring frosts that have proven our most significant viticultural challenge at Tablas Creek. I'll be keeping fingers crossed that I'm wrong on both counts.