The image above is from the National Weather Service's doppler weather radar at Old Hickory, Tennessee. The outflow boundary developed when strong to severe convection collapsed over western Kentucky earlier in the afternoon on May 27th 2010 sending out the thunderstorm-cooled air (outflow) that you see above.
Outflow boundaries are storm-scale or mesoscale boundaries separating thunderstorm-cooler air (outflow) from the surrounding air, similar to a cold front, with passage marked by a wind shift and usually a drop in temperature. Outflow boundaries may persist for 24 hours or more after the thunderstorms that generated them dissipate, and may travel hundreds of miles from their area of orgin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary such as a cold front or another outflow boundary. Although they can be seen at any time of the year they are more prevalent during the warmer months.
National Weather Service