This week's winter storm has produced a swath of ice from Oklahoma to the Mid-Atlantic region; western Kentucky appears to be ground zero. Since ice accumulation brings down tree limbs and power lines, ice storms, often occurring over broad regions, are among the most destructive weather events on our Continent.
The development of an ice storm requires three ingredients: cold air at the surface, overriding warm, moist air and a system that is moving fast enough to keep that contrast in place. The cold surface air keeps the temperature of roads, buildings, trees and power lines below the freezing point while the storm's circulation sweeps warm, moist air up from the south, lifting it above the cold layer. This lifting generates precipitation in the upper atmosphere; if the cold, surface layer is thick, the precipitation will reach the ground as sleet or snow but, if the cold layer is thin, it will arrive as rain, freezing as it strikes the ground structures. Building up over time, the heavy ice brings down limbs and power lines.
If the storm is moving slowly, the southerly flow may warm the surface above freezing before the lift and precipitation occur. In this case, a mix of sleet and rain generally develops along the cold front, with snow to the north and rain to the south. Predicting an ice storm is thus difficult, since its extent (west to east) and width (north to south) will be determined by multiple factors, all of which remain in flux.
