The development of an ice storm requires two primary ingredients: a shallow layer of cold air at the surface and a center of low pressure that sweeps warm, moist air above that colder layer. In this scenario, rain develops in the upper atmosphere and freezes within the cold zone (to produce sleet) or upon striking the frozen surface (freezing rain). Such conditions develop along the leading edge of a cold front, where frigid air is knifing in below warm, humid air.
A geographic belt of the Southern U.S., extending from the Carolinas to Oklahoma and North Texas, is especially prone to ice storms. Far enough south to avoid deep pockets of cold air and subject to incursions of warm, moist flow from the Gulf of Mexico, this region is regularly placed at risk as winter and early spring storms move eastward along the Gulf Coast. While most regions of the U.S. are subject to ice storms from time to time, this belt, caught between the warm Gulf waters to the south and the frigid snow belt to the north, gets more than its fair share of these deadly storms.
This weekend, the right conditions appear to be developing for another extensive ice storm across the Deep South. Time will tell where the clash zone sets up; further north, precipitation will fall as snow while, to the south, cold rain will develop. Either would be preferable to a crippling glaze of ice.