While it feels cold to us natives, you'll probably be strutting around in a t-shirt and shorts! The only thing that really catches people off guard is the extreme difference in temperature between down here and in the actual desert. About the time you'll be here, it'll be in the mid to low 70's with some days high 60's, but out in the desert at night it drops to the mid 30's. Too many people don't consider this (it's the desert! Just how cold can it be?) and their camping trips turn miserable from not being properly prepared.
But overall, it's a very comfortable time of year for Northerners such as yourself that are used to December equating to 10 degrees. When my uncle came out here from Mass, he just couldn't believe how hot it was and I don't think the man wore a stitch of clothing besides shorts for the first 4 years he was here.
We do get some rain around that time of year, but it's usually sporadic and only lasts for an hour or two before it's bright and sunny again. A word of caution: the roads get EXTREMELY slippery when it rains from all the oil on them, so you might want to keep that in mind.
Shoot me a reminder PM before you head out, and I'll have a cold beer and a steak on the BBQ for you if you'd like.