Dramatic temp plunge to hit behind back-door cold front

Tuesday’s brief temperature surge is to end suddenly this afternoon. A back door cold front—one which “backs” into the Chicago area off Lake Michigan rather than approaching from the west—threatens a precipitous temperature drop and the arrival of gusty NE winds, not unlike the plunge two weeks ago when temperatures crashed as much as 22° in a single hour—ultimately to fall from 70° back to the 40s. Fronts like today’s pick up speed as they sail down the lake. That happens because sunshine heats the ground away from the lake which in turn transfers some heat to the air just above it which becomes buoyant and rises. Air pressures drop when this happens and the colder, denser air which hugs Lake Michigan’s chilly surface begins rushing inland with increasing speed. The process is a fixture of spring here and notorious for producing sharp temperature pullbacks. --By Tom Skilling, WGN-TV Chief Meteorologist