When polar air drops into the Northern Plains it is often rather shallow and its movement is frequently not governed by a fast-moving front. Under such conditions, the cold, dense air fans out across the Heartland, seeking lower elevations as water does on a tilted surface. As a result, this air mass tends to move primarily to the south and east, guided by the terrain of the region; the more frigid (more dense) air moves into the river valleys and lake basins.
If the layer of cold air is thick enough, some will move uphill (westward) across the High Plains, eventually reaching the Front Range of the Rockies. Like a giant dam, this lofty ridge prevents the polar air from spilling into the Intermountain West, producing remarkable weather variance on either side of the mountains. Furthermore, as the cold air "climbs" toward the Continental Divide, it cools further and drops its meager precipitation as light snow.
After a mild afternoon yesterday, a subtle wind shift (from the northeast) and a gradual drop in temperature indicated that the polar front had arrived in Denver. The upslope flow produced cloud cover along the Front Range and, by the overnight hours, light snow developed. Near 40 yesterday, we will remain in the teens today; meanwhile, west of the mountain barrier, mild conditions will persist.
