So to accumulate twenty feet of ice per year, that means we would need 100 feet of snow. That's 1200 inches a year - or 240 inches of rain equivalent in the winter. And that doesn't even compensate for seasonal melting.
Yes, I see.
But them damned glaciers sneak in from the north, sliding off the mountains. One day, open prairie. The next, Phloomf! 2 miles thick sheet of ice.