This morning, an atmospheric dome of high pressure, centered over Minnesota, was bringing cold, Canadian air to most of the central and eastern U.S.; another high pressure dome sat over the Desert Southwest. Beneath the center of atmospheric domes, air sinks while, along their rims, air flows in a clockwise direction; the eastern dome lies within a broad atmospheric trough (a dip in the jet stream which allows cold air to plunge southward) while the southwestern dome is the heart of an atmospheric ridge (a northward curve in the jet which allows warm air to flow northward).
Northwesterly winds created by the cold, eastern dome were producing lake-effect snows in the Great Lakes region and sweeping frigid air as far south as the Mid-Atlantic States. The western edge of this dome had backed up against the Colorado Front Range, creating dense morning fog as cold air settled above relatively warm, moist ground.
As the Southwest dome expands eastward, warm conditions are returning to the Front Range and will spread across the Southern Plains, eventually pushing into the Heartland. In concert, the eastern dome will also drift eastward, bringing the coldest air of the winter to New England, the Great Lakes States and the Mid Atlantic region.