Did my JAA PPL when they were still doing them 3 years ago. Flew there in December and February. Hardly lost any days due to wx. In December a few days can be lost due to the Pacific storms. Smog/haze was there pretty much all the time - except after the storms. Occasionally had to be careful with the Santa Ana winds and x-country through the Banning pass. Over all I think I lost two days due to the storms in the four weeks I spent there.
When I went back to do my FAA IR at Santa Monica in March - much more chance of losing time due to the marine layer. Rolled in, rolled out half the runway would be covered the rest would be in clear blue sunshine.
PC