TRAVEL. It's a constant issue in roleplaying games. One thing we don't routinely think about (or at least I didn't for many years) is the heavy role that seasonal changes played on medieval and classical travel. Namely: snow and rain. Rains can make unpaved dirt roads impassable, as can winter snows. Ships did not sail in the late fall or winter for fear of the storms that wracked the seas. Travel was, to put it mildly, inhibited by seasonal changes.
I never really stopped to think about how modern convenience affects travel in the winter. After a big storm, the roads are impassable until they're cleared out a bit and cars being traveling down them regularly. Cars are huge things which clear out a lot of space as they go, our modern roads are excellent and smooth, and still we need to wait around for a gigantic plow attached to a truck to come and give us a head start.
Now, if the snow weren't moved, shoveled, and plowed it would stay in place until the weather warms up. That means that all winter, small flurries will build up on most of the cross-country roads (I would imagine cities would still be better off, as the need to get from building to building would necessitate some kind of public shoveling, or at least enough traffic to keep the snow melting) and a really big storm could make all roads impassable for a long time.
As recently as the civil war, rains could make it impossible for armies to march across the roads of Virginia. How crazy is that! So, when winter rolls around, your party might benefit from an extended stay in an urban environment, doing spying missions for nobles, politicking, and generally making a nuisance of themselves, particularly if there are heavy snows. Alternately, if they have enough money, they can just lounge for the winter, gorge themselves in an inn, and perhaps research spells until the roads are passable again.