There is an ongoing debate, being waged in the wine press and trade, as to the benefits of cool-climate viticulture. The evidence tends to support that wine grapes tend to do best at the coolest limit of their range, perhaps because there, the grapes stay on the vine longer than in warmer climes. The longer a grape cluster stays connected to its roots, the more character it will be able to develop. At the same time, in a cooler climate grapes will achieve physiological ripeness more gradually, with lower sugar levels (and lower alcohols) the result.
In France, the examples of grapes achieving the summit of their complexity at the coldest edge of their range is well established. Chardonnay from Burgundy (a cooler climate) is considered better than that from the Languedoc (a warmer climate). Syrah from Hermitage (at the very Northern limit of the Rhone Valley) is considered better than Syrah from Chateauneuf du Pape (in the Southern Rhone). Sauvignon Blanc from Sancerre, in the Loire, is generally considered finer than that from Bordeaux, which is roughly 300 miles to the southwest.
The example has been broadly applied to California, which was for a long time considered unfit for earlier ripening varieties like Riesling, Sauvignon Blanc, Viognier, and (even) Pinot Noir. It's only relatively recently that maverick growers have been proving that it's possible to find the right climate to grow all of these grapes well in California.
Paso Robles is generally considered a hot climate, by California standards (certainly by California coastal standards). And, the measurement of degree days, which measure the number of days (or hours) over a minimum temperature lends support to this analysis. It is indisputable that during the heat of the day in summertime, Paso Robles is hot. 100 degrees is not uncommon. And yet, these same nights are cold... colder than in most other regions. Compared to Chateauneuf du Pape, our summer days are hotter (by about 10 degrees on average). Yet our nights are colder by 15 degrees on average. You would assume that this would lead most statistical measurements to conclude that our climate was cooler than that of Chateauneuf. You would be wrong. Degree days factor in the number of degrees and the number of hours that temperature surpasses an arbitrary value. How much the temperature drops below that value is not taken into account.
As you might expect, the grapevines at Tablas Creek ripen more gradually than those at Beaucastel. Yes, the sugar levels at Tablas (driven by the extra sun and daytime heat) accumulate more quickly than they do at Beaucastel. Yet, the acids at Tablas Creek, preserved by the nighttime cold, remain high significantly longer than in Chateauneuf, and the grapes achieve physiological maturity (shown by seeds and stems turning brown, or lignifying) on average about two weeks later in Paso Robles than in Chateauneuf du Pape.
Even better, the pre-fall-rain growing season in Paso Robles is significantly longer than that of anywhere in France. So, the cold nights and lengthened ripening cycle mean you can do the earlier-ripening varieties credibly here. And, if you want to grow a late ripener like Mourvedre, Cabernet, or Zinfandel, no problem. Just wait. In our example, we expected that the Paso Robles climate would allow us to focus on our southern Rhone varieties, particularly Mourvedre, Grenache, and Roussanne. What we didn't expect was that the cooler nights would allow grapes like Syrah and Viognier to thrive as well. We never expected to make a single-varietal Syrah, let alone something even earlier-ripening like our Antithesis Chardonnay. And, if we need to wait until November to get ripe Mourvedre, we can. It's a tremendous luxury.
So, the next time you hear a discussion about a cool-climate wine, or cool-climate region, remember to ask how they measure this. By speed of ripening, most of coastal California is cooler than most of France.