You would be surprised! We live in a colder/snowy state and absolutely freeeeeze at night when we are in California, especially in the winter months. I usually do not wear a coat when running errands in winter here unless temps get below 40-ish degrees. If the temp is going to go below 55 in Anaheim, I bring over all of our heavy stuff from the hotel otherwise we will be miserable. I think it is because we are used to dry cold versus the wet SoCal cold. It's much better if you are moving around, but it is different when sitting down/standing in place for fireworks/parade/WOC.
One night a few years ago when we were there in mid-December, they had generic black mittens some of the stores were selling that were flying off the shelves. It was insane lol We've also impulse-purchased a Christmas blanket and a few of the off-brand Disney snuggies to have when we were waiting on fireworks on night.
We're planning on packing our UGG boots, mittens, Columbia heavyweight parka, heavier socks, ear warmers, rain boots, and taking some of those camping heat packs for our hands. We've been through this rodeo enough times to come prepared lol
