the weather in the winter is usually warm unless there is a cold front that passes through. just check the weather channel before you go. its usually in the 70's. maybe a little hotter, and colder if a front has passed. even if it is cold, it usually only lasts till the sun comes up, then it starts warming up. you can definitly wash you car all year round, so you can get all those bugs off that you accumulated on the way down.
