Scattered thunderstorms developed in a highly unstable airmass near the south central Nebraska/north central Kansas border during the mid afternoon hours on Wednesday, June 24, 2009. Due to weak flow aloft, these were slow moving thunderstorms that dumped very heavy rainfall in a short time. In fact, the NWS Cooperative Observer 4 miles east of Superior, NE reported 3.13 inches of rainfall.
Of particular interest with these storms was the impressive outflow boundary that developed. The radar image below, valid at 442 PM, shows a well-defined circular outflow boundary, approximately 50 miles in diameter. Put simply, the thunderstorms acted as a breaking "water balloon": As heavy rain fell to the surface, strong downdraft winds were generated that spread rain-cooled air out in all directions. When these downdrafts first reached the ground near the storms, strong to severe wind gusts of 50-60 mph were reported. However, as this air pushed farther away these wind speeds gradually diminished with distance. However, as is often the case, this large outflow boundary acted as a trigger for new thunderstorm development, almost like a small-scale cold front. Although many thunderstorms generate outflow boundaries, few are as well-defined on weather radar as the one depicted below.
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Here is the definition of an outflow boundary from the National Weather Service glossary: A storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect 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 origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary (cold front, dry line, another outflow boundary, etc.; see triple point).
This page was composed by the staff at the National Weather Service in Hastings, Nebraska.