Riding in and around the greater Nevada City area yesterday (Sunday Dec 20th) proved to work out quit well. It looks like you could ride all the way up to Whitecloud Campground but would probably encounter snow further up the trail.
We did a combo of trails starting from town and riding as high as Conservancy Camp Rd. Trial conditions actually proved to be quit good with no snow and very little mud even on north facing trails... until it starter to rain towards the end of our ride.
I would assume that South Yuba, Round Mountain and Bullards Bar trails are all GTG (Good To Go) if you're looking to fill a void in your riding.