As a winter storm approaches, the low pressure and its associated cold front lift warm, moist air that is pulled in from the south. Injected into the upper atmosphere, this moisture forms ice crystals which begin to fall. Along and near the front, where the lower atmosphere is warm, these crystals melt before they reach the ground, producing rain.
Further behind the front, where cold air is knifing in beneath warm, mid-level layers of the atmosphere, surface features such as tree limbs, power lines, cars, decks and bridges reach freezing temperatures and, as the rain hits them, it forms a glaze of ice, referred to as freezing rain; when the layer of ice exceeds 1/4 inch, damage to trees and power lines begins to develop. Still further behind the front, where the layer of cold air is thicker, the rain refreezes before hitting the ground, producing sleet. Finally, where the column of air in the lower atmosphere is entirely below the freezing point, the precipitation falls as snow.
On this winter solstice, the latest winter storm is centered over Arkansas this morning and will gradually move to the northeast. Heavy rain and thunderstorms are expected along the advancing cold front, with bands of freezing rain, sleet and snow (in order) arriving behind that liquid precipitation. Here in central Missouri, we are currently in the freezing rain band; while a glaze of ice coats the trees, wires and cars, the ground remains relatively warm due to our recent mild weather and, for now, road travel is unimpaired. On the other hand, the freezing rain will transition to snow overnight and 2-3 inches are expected to coat the landscape by tomorrow morning.
