,I often talk to people that are surprised that we are able to ride year round here in Central Washington. It's true that it gets pretty cold and we even have a fair amount of snow. What many people don't realize is that Washington has some pretty unique climates if you don't mind doing a little driving. Here in the Kittitas Valley we can typically ride the hills through October, some years even into November. Keep in mind that some areas, like Table Mountain, close early. So, always be sure to understand where you are going first, and check with either the USFS or DNR before you go. Both agencies have maps available online that you can download and print as well as paper copies.
By December it's time to start heading east and south. Our winter "go-to" place is the Saddle Mountain ORV area just outside of Mattawa. Most years we can ride there all winter; unless it's one of those really snowy years. Even then, it's possible to ride through the snow, just now nearly as much fun.
If you don't mind sand there is the Beverly Dunes, Moses Lake Dunes as well as Juniper Dunes (north of Pasco, WA). I personally don't have any great love for sand, but in the winter it can be allot of fun. The sand is pretty firm and there is no dust. Having a paddle tire can change your experience in the sand, but when the sand is frozen it really is not required.
The cold can be a bummer, but if you dress accordingly (don't put on your winter parka or you will be sorry) and stay moving it makes for a fun experience and best of all keeps you on your bike!