Was looking back at some stuff from yesterday's wake low, and a number of places had a 10-12 mb pressure drop in only about 45 minutes. O'Hare dropped 12 mb between 12:51 pm and 1:35 pm. I dropped 6 mb in 20 minutes. You'll pretty much only see that much fluctuation in a tornado or hurricane (much greater pressure changes in those cases though) or some kind of mesoscale phenomenon like yesterday.