I don't think you can put a number to it like that. IMO historic is a combination of things that combine to create a very high impact event. For instance, that May frontal passage that spawned all of those tornadoes that devastated areas around here, that March storm two years ago that knocked the power out for a week here or 4/1/97 where I got 17" in less than 3 hours and hundreds of trees came down and the streets were blocked for 2 days so that I couldn't even get out of my dead end or more than 2 blocks in any direction after me and the neighbors cut the trees out of our street. We were super hungry by the time we could get out to get food.
I was walking home from school in Oceanside when that hit. Vis went from normal to 150 feet in seconds, the whole world went silent, cars started sliding off the road into front yards and it was a huge mess. If my memory is right it really only lasted about 20 minutes but we got a few inches. It was 15 years after that before I saw those kinds of snowfall rates again in Lake Tahoe where 10-12"/hr happened several times the winter I lived there and I spent 6 years in the Rockies before that and never saw more than 5-6"/hr.