Climate change is doing us a favor with this one, the jet stream is like a wave that travels around the pole with the peak of the wave dipping south into the US. It used to be a pretty shallow wave that came down across the midwest and arced back up across the great lakes before passing over New England. Right now, the is a few hundred miles wide and stretches down to Texas. So, as it passes, Northerly winds will take the crap ion Ohio up into Canada, then southerly winds will carry the crap to the south. Basically, if it's unseasonably warm here we are fine, but in the 5 or so hours before a big temperature drop, if we get some snow, it might have some crap from Ohio in it.