Appellations are by nature generalizations. You do your best in defining one to understand macro-level differences in soils, climate and rainfall (the three components which together give a place its signature). And, practically speaking, you won't ever be able to delineate an area that is absolutely uniform. Even within our vineyard at Tablas Creek, there are areas that are warmer and cooler, and have different amounts of topsoil above the high-calcium limestone-derived bedrock.
All this is a long-winded leadup to my point, which is that the Paso Robles AVA is essentially defined by climate. And, the appellation does have broad climatic similarities (probably the most important of the three variables) that make the AVA a meaningful designation. Still, there are over 40 different soil types within the AVA, and rainfall varies from an average of about 7 inches in the high plateaus east of town (think Creston, or Shandon) to nearly 40 inches in the extreme western reaches of the AVA. So, the proposal to subdivide the Paso Robles AVA has some good scientific reasoning behind it.
This last rainstorm that we received yesterday, which dumped 3.15 inches at Tablas Creek, is a good illustration of the diversity of rainfall amounts around the AVA. I've pasted in below the readings from the other weather stations in the region:
You can see how much more rain we get at Tablas than the other weather stations: nearly double what they get in the Templeton gap, and nearly triple the reading from the most easterly weather station in Shandon. And there's good logic for this; at Tablas, we're only about 10 miles from the Pacific, and in the rain shadow of the Santa Lucia mountains. Pacific storms get pushed up into cooler air by the mountain range, and drop their rainfall over us. By the time the storms get into town or further east, they've lost much of their moisture. This is why the Adelaida area was known until the middle of the 20th century as the breadbasket of the Paso Robles area. It's warm enough to ripen crops consistently, but wet enough to farm without irrigation.
And this is one of the characteristics we were looking for in founding Tablas Creek: enough rainfall to dry farm. At 28 inches average annual rainfall, we can do it most years. The Adelaida District area of Paso Robles is one of the only parts of California warm enough to ripen Rhone varietals and still with enough rainfall to farm without irrigation. No wonder this is becoming known as the center of California's Rhone movement!