While I agree with others who have said it's extremely variable, I would disagree on the "pack everything" philosophy. Especially with weight limits on luggage these days that's not practical. Here's the thing on January - there is absolutely no telling what you'll have so check the forecast 3-5 days before you go and then pack accordingly. Drives my wife nuts when I say that because she wants to start packing 2 weeks out but any weather data beyond 7 days is bunk and because January is so variable you need to wait until you're in the 3-5 day range of arrival to have the best idea for the duration of your vacation.