sometimes when we get a blizzard, 1/4 inch of snow and the city goes into panic mode, I will see that fine powdery snow in my attic that the wind blows in sideways or something. This over the time could be an issue but to block this fine powder is a real problem.
An assuming that the powder is landing on the down louvers and sits there until a slight breeze blows it into the attic area.
Other than temp blocking the vents duringthis time, I cannot give you any suggestions. Use a paint that is slicker to be painted on the louvers to shed the snow, could be an idea. But nothing to solve your situation.