A Chilly Fetch

Thursday afternoon, low pressure sat over north-central Kansas and its associated cold front stretched from the Great Lakes to West Texas. Ahead of the front, warm, humid air was moving up from the south while, north and west of this boundary, that cargo of moisture was swept counterclockwise through the chilly air.

Looking at the radar, one could see a swath of precipitation stretching from Minneapolis to Denver, rotating to the west and then southwest around the central low. In Minnesota, afternoon temperatures were in the fifties (F); as the air followed a broad curve toward Denver, it encoutered gradually rising terrain and cooled at is progressed. In South Dakota, temperatures were in the mid forties, falling to near forty in western Nebraska and into the upper thirties along the Front Range. Finally, one found that the rain turned to sleet and snow along the foothills of the Rockies and atop the Palmer Divide, south of Denver. On the south side of the latter divide, the air descended into Colorado Springs, which, just 70 miles south of Denver, enjoyed sunny skies, with temperatures in the upper fifties.

Such an upslope fetch of moisture is a common event across the variable terrain of the western U.S. and, depending upon the temperature of the entrenched air, rain or snow is produced; even in mid summer, upsloping moisture can blanket the higher mesas and mountains with snow. As the storm system moved further to the east, the upslope pattern broke down, high pressure took control and the Front Range returned to more seasonable conditions.