Since early the previous day, Case has been watching something called a “short-wave trough aloft” slide eastward from the Great Lakes. On satellite photos it looks like an S-curve in the line of clear dry air moving south from Canada. Cold air is denser than warm air, and huge, slow undulations develop along the boundary between them and roll eastward—on their side, as it were—much like an ocean swell. The undulation gets more and more pronounced until the “crest” gets separated from the rest of the warm front and just starts to spin around itself. This is called a cutoff low, or an occluded
...more
