So, where does one start in determining the best grapes that grow locally?
Well, there are a few places.
One place to start is climate classification. The idea is to find other places in the world with a similar climate as yours locally, and see what grapes grow well there.
There are many ways to classify climates, but one of the most widely accepted is the Köppen–Geiger system, based on the concept that native vegetation is the best expression of climate. The classification uses a series of letters to denote types and subtypes of climate.
With a sufficient amount of geeking out, you can download the most updated Köppen–Geiger map as a Google Earth overlay. Taking a look at the local region around San Marcos, we see we are in a bright green area known as "Csa", which stands for Group "C" - Temperate/mesothermal climates, subtyped as "s" dry-summer tropical. Or much more easily known as "Mediterranean" (the areas in different shades of brown all belong to Group B - Dry climates).

If we expand out and take a look at the Western half of the United States, we see the well-known wine landmark of Napa is in a slightly different shade of green. This is "Csb", which differs from "Csa" in maximum summer temperature ("Csb" is cooler).

Our local region around San Marcos has a similar but different climate than Napa Valley and most of coastal California.
So, where in the world - specifically the Old World - is our area most similar to?
Well, the term "Mediterranean" gives a hint. If we turn to the Old World and highlight only those areas with the same climate as ours, we get the following map.

The results may seem surprising, but the local grape growing area around San Marcos is very similar to areas around the Mediterranean.
Turning first to France, the three major wine regions with the same climate are Languedoc - Roussillon, Southern Rhône, and Provence. A wide variety of grapes are grown in these regions, but the ones both generally considered of highest quality and significant production are Mourvedre, Grenache, Syrah, Cinsault, and Roussanne. Carignan is common, but usually used in more of a "filler" role.
Taking a look at Italy, the major wine areas are on the West coast, Sicily, and Sardinia. A wide variety of grapes are grown here too, but the most significant varietals are Greco di Tufo (red and white), Nero d'Avola, Fiano, and Vermentino.
Finally, in Spain, we have the regions of Castile-La Mancha, Andalucia, Extremadura, and Priorat. The most significant varieties are Tempranillo, Syrah, Grenache (Garnacha), Mourvedre (Monastrell), and Zalema.
From the above, I've excluded:
* Varietals that are primarily made as dessert wines.
* Any "international varieties" (Cab Sauv, Merlot, and Chardonnay) that are planted no matter where they excel or not.
* Rarer varietals from other Mediterranean areas (e.g. Greece) that are not currently grown at all locally (since I will be buying and not growing grapes).
Narrowing down all of the above to the most respected and also available locally, we have Syrah, Grenache, Mourvedre, Cinsaut, Tempranillo, and Roussanne.
That's the result based on climate classification. Another way to determine varietals is to ask the local winegrowers and winemakers.
Which I did, and their responses most often matched the above - Syrah mainly, followed by Grenache, Mourvedre and the rest. It is important to note that this doesn't always agree with what many of them produce the most of (which is often Cab Sauv).
Another way to judge is by awards. Syrah from Temecula is one of their most frequent award winners, which is especially notable given its much smaller production relative to many other varietals.
Finally, one can judge simply by taste - sampling the wines grown in the area. And here again, the results, in my opinion, fall in line with the same answers we've seen above.
Syrah. Grenache. Mourvedre.
(For now, I'll hold off on Tempranillo, Cinsaut, and Roussanne as I have not tasted many of them.)
So, why don't we see more Syrah, Grenache and Mourvedre produced locally? It unfortunately has to do with the recognition and familiarity the majority of wine buyers have with Cabernet Sauvignon, Merlot, Chardonnay, and now even Pinot Noir. Mediocre Cab Sauv can be a far easier sell than an outstanding Mourvedre, due to the simple name recognition and familiarity people have with it.
And when I say an "easier sell", I don't mean to imply that winemakers and growers in the area are looking to make a fast buck... but they do have to make
a buck to stay in business. And you can produce all the outstanding Grenache you want, but when consumers are buying the so-so Cab Sauv down the street instead, you just end up with a warehouse of wine going nowhere.
But ultimately, the area needs to start making more of the wines it can really excel in making. And this is where I am starting...