If you get a typical snowfall one day and there is 5 to 8 inches of snow piled up blocking your hive entrances you would leave it be? What about a week of sub-zero temps later when the snow has turned to ice?
I think if I left the screened bottom boards open for winter and the bees stayed in cluster due to low temps it would be OK to leave it blocked. But the first day above 50 degrees I'd have it open for them to fly. Fortunately we have those days down here.
Yes, I leave it.
The snow will melt back away from the hive from the hive's own warmth, and will almost always allow air penetration.
I've had situations where the hive was covered up to the second box. The bees came out the entrance, along the front of the hive and up to the top till they cleared the snow line, then flew around, and went back down. Never have I had a hive suffocate, no matter how deep the snow got.
And any day warm enough to have bees fly will be long enough for ice to melt back from any entrance. (As long as the hives are in sun, pointed south, etc. But that should be a given)
Last year, we had back to back huge snow storms that dumped 24 inched each time. I had a hive (three 5 frame boxes) fall off the stand between the storms. The second storm buried the entire hive under 24 inches of snow. 4 days later when I found it, all I saw was a small hump in the snow. I dug it out, and the bee cluster looked fine. The bees just don't need much air exchange. And the idea that any hive would suffocate is stuff that urban legends are made. Someone probably mentioned it once, and it has been repeated over and over since then. Many things in beekeeping are like this.
Show me one article, one research paper, one first hand experience of a hive that suffocated. There are none. Yet, this idea of hives suffocating comes up constantly. It just does NOT happen. The hive's own loss of heat and the rising or warm air, creates air flow and exchange.
I'm actually surprised that some beekeeper or company has not come out with some heated front entrance to make a buck off this erroneous idea. I bet it would sound reasonable to many beekeepers who worry too much about nothing. Hold that thought.....I might have something for you guys after I look into this. :-D