As I packed my truck yesterday morning, it was sunny and mild in Littleton, Colorado. However, I noticed a swath of low clouds across the eastern horizon and the Front Range foothills had taken on a hazy appearance. Before I left the city, heading east, the gray clouds had spread westward, shrouding most of Metro Denver, and the temperature dropped in concert. East of the city, near Byers, I encountered mist and dense fog, which enveloped the highway until I crossed the Palmer Divide; beyond that topographic barrier, as the air was forced to decend, the fog disappeared and breaks appeared in the gray overcast.
The above account is typical of an upslope weather event, which occurs along the Front Range in the wake of a cold front; while the Continental Divide blocks the first band of clouds and precipitation as the front moves from west to east, northeast winds develop behind the front, pushing cool air and moisture toward Denver from the northern High Plains. As the air is forced to rise across increasingly higher terrain, low clouds form and precipitation develops; depending on both the availability of moisture over the Plains and the temperature of the air, this precipitation may be light or heavy and may fall as rain or snow. Once the front moves further east and the upslope flow resolves, the clouds and precipiation dissipate.
Yesterday's system was relatively starved of moisture and precipitation was minimal. I finally caught up with the front itself in western Kansas; while Goodland was raked with chilly, north winds under cloudy skies, Colby, just 30 miles to the east (but still ahead of the front) was sunny and warm, with strong, southwesterly winds. Though many regions of our country claim to experience rapidly changing weather, few compare to the Mountain West, where the varied topography has a dramatic effect on weather systems and where wind direction is often the primary factor when it comes to temperature and precipitation.
