Almond production might move elsewhere. Don't underestimate how incredibly efficient personal water use is in California and the Southwest, and how much room there still is for improvement.
You should change the title to the Western US. This is a problem effecting neighboring states too, notably Utah (Great Salt Lake), Nevada, and Arizona (Colorado River). Long-term, also the Great Plains moving east from the 100th meridian (Ogallala Aquifer). And then we might as well talk about the impact of rising sea levels on the South's Gulf coast and Florida.
This is a nationwide problem. Better question: what's left for climate refugees to flee to? The Great Lakes and New England? Too bad there's no jobs and/or it's too expensive.
The agriculture industries in California (especially the more water-intensive ones) can gradually move to the Midwest and the South, bringing the jobs with them.
I am personally not really sure exactly how intensive climate migration will be, but I can say this: its not very simple nor practical for the crops being farmed in California to just be moved elsewhere.
The reason that California is used so heavily for farming crops such as avocados, almonds, walnuts, grapes, etc. is due to its dry-Mediterranean climate. California is, in fact, the only region where this climate type can be found in the entire country.
This makes the region highly conductive to the creation of massive farms that create much of the US' (and in the case of almonds, the world's) crop supply in certain foodstuffs. Simply put, you can't replicate these kinda farms in other regions of the US, just as you can't create cornfields in Washington nor a cotton plantation in Maine. If California is facing an intense climate crisis in the form of a drought, worse than we've ever seen, the answer from farmers is not to move out east, but to downsize their farms.
This isn't as simple as moving a factory from Michigan to North Carolina. With climate-related disasters getting worse in California, the long-term effect will be a massive strain on certain aspects of the food supply. It's pretty easy to see almond production get downscaled to such a degree where its simply too expensive for casual buyers.
I'm well aware. California is a uniquely effective location to farm many crops and it will always have a significant agricultural industry. However, to the extent that water use must shrink, it will occur because of reduced agricultural water use in the state--not urban outmigration. Perhaps Central Valley almonds were a uniquely poor example because 1) the Central Valley is a more reasonable place to farm than the rest of California in the first place and 2) there aren't many domestic OR international places that almond production, should it not downscale, could relocate to. It is important to emphasize, however, that production moving overseas, rather than elsewhere in the United States, is absolutely viable.
Considering the nature of water infrastructure in the West, the Central Valley should be the last place to see a downturn in agriculture thanks to it 1) not being a desert and 2) having access to water from the Sacramento River. Far more vulnerable are places that pull from the Colorado River (so Maricopa County, areas towards Yuma, and the Imperial and Coachella Valleys.) For the most part, the agriculture in these areas isn't uniquely Mediterranean anyway and as Colorado River levels drop or demand from Utah, Arizona, and Nevada increases, this can be offset by reducing agriculture in the Colorado Basin rather than the Sacramento Basin. Also, as Southern California has access to both the Sacramento River and the Colorado River, it can lean more heavily on the Colorado at the expense of desert agriculture to free up water in the Central Valley.
TL;DR: California and the Southwest will be just fine and have a lot of spare capacity that can be leveraged by pushing desert (not Mediterranean) agriculture overseas.
