“Climate is what you expect, weather is what you get.”
– Robert Heinlein, Time Enough for Love
When starting your search for a retreat location, concentrate on “dry land farming” regions, and of those, the ones that specialize in truck farming . Dry land farming regions are where crops can be grown with seasonal rains and are not dependent on electrically pumped irrigation water. Remember that when grid down , the areas in the West that were originally desert will revert to desert, in a hurry! Even an area that might otherwise look good for a retreat at present may be uninhabitable if and when the grid down  era begins.
Elevation and exposure are both critical factors. By concentrating on properties at low elevation and with a southern exposure, you will greatly extend your growing season. A growing season that is 30 or 40 days shorter might seem trivial now, but WTSHTF  it will be incredibly important. Do a detailed study of both the regional climate and the microclimates in the counties that you are considering for retreats. See City-Data.com  for detailed temperature, rainfall, and snowfall data for most locales with a population of 5,000 or more. By the way, there are lots of other interesting statistics there too, such as median age, education levels, and so forth.
In many parts of the country, the reverse side of a ridge (northern facing) can be snow-bound for an extra three months of each year! So be willing to pay a little more for a piece of land with an unobstructed southern exposure.
Environmental scientists can’t seem to agree whether or not the much-touted Global Warming is actually in progress. A minority of scientists have asserted that we might actually be in a cooling trend or perhaps even on the cusp of another “Little Ice Age.” If there is a large volcanic eruption or a comet or meteor strike, there could be some profound climate effects. This is a good reason to have at least two years of food storage. Even the best gardener in the world will not be able to feed their family if there are killing frosts in every month of the year for a couple of years. You might consider making preparations for the remote chance of sudden climate change. I even had one late friend who lived in the Philippines who had a large stock of cold weather gear!
For researching rainfall, population data, tax information, and so forth, a very useful resource is the Home Fair  web site.