Iconic Image... Crater Lake
My experience is that the NPS does their helicopter survey of snow cover in early April to make plans and priorities for plowing in the western parks, with the start of plowing based on how much snow cover there is and where the snow has been heaviest. Lighter snow years mean plowing can start earlier. One year, back in 2006 IIRC, during the fly-over at Crater Lake the snow was so deep that lodge was not visible from the air! I was there in June of that year and found only about 1 mile of the road plowed, and at the end of the plowed section the snow on the road was a good 20 feet deep.