A Cap on the Plains

As I left Denver this morning, a weak upslope flow produced a layer of low, gray clouds above the city. Banners of virga dipped below the ceiling but only occasional showers reached the ground. Beyond the Palmer Divide, the sky cleared and sunny conditions prevailed across eastern Colorado.

From the Kansas line to western Missouri, white, "popcorn" cloud formations dotted the blue sky, increasing in number and size as I continued to the east; however, there were no signs of thunderstorm development. Such conditions are typical of a "capped atmosphere," when overlying high pressure and warm air aloft prevent the upward expansion of storm clouds. Since there was no front or upper level low to provide lift, the warm, humid air was trapped in the lower atmosphere and only a few showers appeared across the Plains.

Southeast of Kansas City, a few storms had clearly broken through the cap, as evidenced by the development of anvil clouds, produced by strong winds at higher altitudes. Once punctured, the cap begins to break down (at least locally) as lift is provided by outflow from these initial storms.