If you're in NorCal you should be fine in Baja AZ! It's usually much dryer than NorCal, so even when it's cold it doesn't feel as cold (but the wind can be fierce). Also the sun is usually shining, which makes everything better IMO :-D
Layers are essential. Bring a daypack because you will be peeling off layers as the day warms up, then needing them back on again after about 4 p.m.
A heater is nice for those chilly mornings. The cool nights are great for sleeping, but you may need a hat to keep comfy in your bed.
So far it's been a cooler than usual December here on the southern border, but who knows what January will bring. One year it was in the 90s in January. Bring layers.
The Dire Wolfess