Such fine powder snow blows in through the eaves of our house and settles like a "dust layer" on the insulation in the crawl space. It's like this every year and it's obviously not good, but as long as you have good ventilation in the attic, nothing happens to the insulation other than it insulates worse. Better to have it this way than to seal the attic completely.
Of course, you can build a snow trap at the eaves - and that's in the plans for 2 - 3 years from now, when I will completely replace the roof with a standing seam metal roof.