July 28, 2015
On Saturday afternoon, a very interesting radar loop emerged from east-central Kansas. A complex of thunderstorms further north had produced an outflow boundary that sagged just south of the I-70 corridor. This boundary can be seen in the image below as the concentrated line of greens and yellows from just south of Junction City to near Lawrence. North of that boundary, the radar was clearly picking up gravity waves behind the outflow boundary – those appear as the fainter lines parallel to the actual outflow (east-west). Gravity waves form when cooler, denser air undercuts warmer, more humid air. The humid air rises to a point of neutral buoyancy and then oscillates vertically in a wavelike fashion, much like ripples on a pond. South of the outflow boundary you can faintly see what appear to be lines oriented north-south or perpendicular to the outflow boundary. These are called convective rolls that are in a sense an organization of the air oriented with the wind direction, in this case southerly.
The animated gif file clearly shows the convective rolls revealing the southerly winds abruptly meeting the outflow boundary. When air meets a boundary like this, it rises which can trigger thunderstorms. In this case, two severe thunderstorms producing moderately large hail and severe winds are shown on either end of the image.