Layers of snow on top of the panels will reduce the production of the panels, but it is much less than you may think. 5 years of local data was collected at NAIT and the results showed that snowfall caused a 3% energy loss. Read more here:
http://www.techlifetoday.ca/articles/2018/solar-shines-in-dead-of-winter-even-in-edmonton
Overall, system owners can expect to overproduce in the summer, while underproducing in the winter.