Shelf clouds ahead of derecho in Chicago area

Severe storms, called a derecho by meteorologists, moved through Chicago this morning. Ahead of the storm, people sighted a shelf cloud in Illinois.

Shelf clouds often form at the leading edge of a gust front or outflow boundary from a thunderstorm, or strong winds flowing down and outward from a storm. The outer part of a shelf cloud is often smoother with a notable rising motion exhibited by a tiered look. Underneath, a turbulent, unsettled appearance is often the case.

A shelf cloud should be seen as a harbinger of strong winds, in this case it was harbinger of derecho storm. A derecho is a widespread and long-lived wind storm that accompanies rapidly moving showers or thunderstorms.

Cool, sinking air from a storm cloud’s downdraft spreads out across the land surface, with the leading edge called a gust front. This outflow cuts under warm air being drawn into the storm’s updraft. As the lower cooler air lifts the warm moist air, its water condenses, creating a cloud which often rolls with the different winds above and below (wind shear).

A sharp, strong gust front will cause the lowest part of the leading edge of a shelf cloud to be ragged and lined with rising fractus clouds. Sometimes vortices can be seen along the edge, with twisting masses of scud that may reach to the ground or be accompanied by rising dust.

Source: AccuWeather