I still remember the Sept 2000 storms, that basically shut down flying for 3 days, east of the Rockies. It took nearly 4 days to get back up to full ops, due to misplaced aircraft, misplaced and timed-out crews, etc. Line after line of bow echos, stretching from the Dakota's to the Gulf Coast, all moving eastward, with tops pushing 50,000. We sent one STL-RDU routing via GRB-YYZ-ALB to find a way through them. When they hit the East coast, it was 36 hours of chaos.
Or the 1999 Great Blizzard, where I spent 77 straight hours on the clock, dealing with all the cancels, diverts, and stranded's. At one point, my small airport had 24 diverts on the ground, most as non-destination cancels, including two Internationals. This at an airport with less than 1,000 pax a day average, and no customs. (I still haven't forgiven American for dumping that mess in my lap.)
Or 9/11, another nightmare that refused to end, when I was sending RJ's to FBO's at reliever airports, to get them on the ground.
Every two or three years we get a "Big One", that really screws things up for a few days, like a big Hurricane. But it rarely effects the whole country.