No, not all of Southern California is a desert. If that’s true then Italy is a desert too. There’s a reason it’s called a mediterranean climate and it’s not because it’s just like the Sahara.
And yes, I live here.
I have heard [feel free to tell me if it’s not true] that the Santa Ana Winds have the same properties that blow on the Sahara.
MOST of So Calif, not the High Sierras.