June 21 2012 Virginia Outflow Boundary

6/21/2012 outflow boundary, thunderstorm No Comments

Numerous and isolated single-cell thunderstorms are developing across parts of the Mid Atlantic this afternoon, and one of the storms in the mountains of southwestern Virginia just sent off a pretty nice outflow boundary. The storm near Hillsville, VA had a lifespan of about an hour and dissipated as quickly as it formed. The 3D radar image shows how the collapse of the storm's updraft, and subsequent collapse of the storm's elevated heavy precipitation, led to the development and propagation of the outflow boundary away from the storm towards the east. Click the image to enlarge.

An outflow boundary is a storm's "cold pool," or a pool of cold air that forms at the base of the storm due to rain-cooled air descending through the storm and piling up at the surface. As storms collapse, this cold pool radiates outward from the storm as a gust of wind followed by a marked downward spike in temperatures.

These outflow boundaries often serve as a focus for new storm development, and are a crucial component to the formation and sustenance of mesoscale convective systems (organized complexes of thunderstorms, usually in the form of a squall line).

Outflow boundaries can also create cool clouds known as "shelf clouds." As the cold air pushes out into the warmer, more unstable air, it acts as a mini front by lifting the warmer, unstable air up and over the cold pool, allowing it to condense and form a cloud that takes on the shape of a shelf. Below are two examples of a shelf cloud, both taken by me in May 2011 and June 2012, respectively.

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