It is that time of year, when organized storm systems are few and far between. Therefore, we often have to rely on thunderstorms to help generate other storms from day to day. They accomplish this task by producing outflow boundaries. An outflow boundary is produced by the cool air flowing away from a thunderstorm, interacting with the warmer surrounding air. They act like mini-cold fronts, and can be very effective triggers for new thunderstorm development. Below is a classic example of a storm producing a substantial outflow boundary:
That narrow blue arc-line surrounding the storm is the outflow boundary. As it moves away from the storm, it could devleop additional storms. Many times when two outflow boundaries intersect or a mature storm intersects with an outflow boundary, intensification of existing storms can occur or other big storms can develop. The boundary can act as a mechanism to enhance low level shear, and cause an already severe storm to become even stronger and possibly become tornadic. It can also cause a storm that has tornadic characteristics but hasn't produced a tornado, to produce a tornado. Sometimes however, they won't do anything but produce a gust of wind when they pass through your area. So yes, sometimes they are pretty harmless. This is usually the case when moisture is lacking, and there is no fuel to get storms started. However this time of year, we have our eyes on the radar screen tracking these boundaries as they roam through Southern Colorado. They occur almost every afternoon until we get into the later part of the Summer and storm season winds down.
Chief Meteorologist Brian Bledsoe