If you have a jacket or coat with a removable liner, that is a good choice. It's a good idea to bring a mix you can layer - lots of short sleeve shirts, maybe one long sleeve pullover or sweater, then a jacket. It can be anywhere from the 60's to 80's during the day and anywhere from the 40's to the low 70's at night. A lot depends on how soon winter arrives up north. Check the 10 day forecast on the Weather Channel's website the week before you are leaving to get a handle on how we're doing. Also, it's not always dry. I've been rained out of two
MVMCP's over the years (not just a sprinkle, but a torrential rain that lasted all evening). Unlike our summer showers that come and go, if it rains in December, it's usually because of a weather front, and can last several hours to all day. If no fronts are coming through, it should be dry and sunny. If we do have a hot or cold snap, it usually lasts a few days and then the temps moderate. Our coldest weather hits late January into February.