If the panels are covered with snow, their output will be seriously diminished. Fortunately most of our annual sun hours occur during the summer so losing January is not going to have a big impact on your annual production. Also if it is cold enough the wind will often blow the snow off and if it is warm enough it can melt off. If you have a low roof it may be practical to clear the snow with a roof rake but it isn’t strictly necessary and most people don’t clear them.