It's not like it's a snowstorm throughout the whole winter. A bloody long winter, six months, btw. Especially since Frieren and Fern can levitate high up in the sky, how could they even get lost when a snowstorm isn't blocking the view? I feel like Fern didn't want to wander the wilderness in the winter cold and snow, it being too uncomfortable, Stark would be too scared to do it, and for Frieren mere six months means nothing.