well, i always wonder how everyone calls southern california on the PCT the "desert." i mean, there is some desert involved (e.g. Mojave corner in Antelope Valley along the aquedect, edge of Anza-Borrego in Scissors, etc.) but most of it is chaparral and other arid ecosystems. not to split hairs, but i guess the point is that throughout all of that, you spend some time at high elevations where rain, snow, and bad weather are definitely possible. the lagunas, san jacintos, san bernardinos, san gabriels. in 2003, people got completely snowed off the san jacintos, i think.
anyway, i'd suggest having some warm clothes, some kind of rainshell/poncho, hat/gloves, etc. for the socal section. of course, most of the time you'll be very hot, so it's a balance of course. get by with the minimum that you'll be safe and (mostly) comfortable with.
have a great hike!