Wednesday, December 20, 2006

Upslope

Heavy snow and blizzard conditions are forecast for Metro Denver and other Front Range cities today. Contrary to public perception, this is not a common event during the winter months. Since winter storms usually arrive from the north or west, the storm dynamics produce only moderate snowfall in Denver. Western storms lose most of their precipitation as they cross the mountains and northern fronts are generally attached to low pressure areas that are north of the Front Range region. In the latter case, Denver receives "backside" snow as the primary storm moves eastward.

Today's event is more typical of spring, "upslope" storms. The center of circulation has moved across the southwest and is now in northeast New Mexico. Winds flow counter-clockwise around this area of low pressure, pulling in Gulf-humidified air from the Plains. As this moisture-laden air moves westward, it is forced to rise by the higher terrain and, eventually, by the Front Range of the Rockies. Rising air cools and, depending on regional temperatures, loses its moisture as rain or snow. Twenty inches of snow are expected in Denver.