Colliding Outflow Boundaries
On the evening of 16 June 2003, thunderstorms developed near the Missouri and James River Valleys in central South Dakota, and across northwestern Iowa. Both groups of thunderstorms produced convective outflow boundaries, seen here in Figure 1 (reflectivity/velocity from 16/2300 UTC). The western outflow boundary (#1) progressed east at approximately 25 mph while the eastern outflow boundary (#2) moved to the northeast at 20 mph.
As this reflectivity loop (16/2305 UTC to 17/0105 UTC) shows, these boundaries intersected in areas from north of Yankton to west of Sioux Falls. New thunderstorms developed along the boundaries from Yankton to Harrisburg near the time of the intersection. This occurs because the intersecting boundaries produced a narrow column of rising air, which helped the thunderstorms to develop.
In many cases, boundaries intersecting in this way will result in either both boundaries weakening or the stronger boundary continuing to propagate while the weaker boundary is eroded. However, in this case, both boundaries continued to progress to the east and west respectively (Figure 2 - reflectivity/velocity from 17/0035 UTC). The velocity data shows that winds remained from the southeast behind the "eastern" boundary (#2), while west winds remained behind the "western" boundary (#1 - difficult to see clearly due to proximity to the radar site). However, the interaction of the two boundaries resulted in the wind speeds behind both boundaries decreasing. Prior to the intersection, wind speeds behind both boundaries were 20-35 mph. However, once the boundaries intersected, wind speeds decreased to 10 to 20 mph.
Of further interest, is that as the boundaries moved away from each other, a large area of divergence resulted behind the boundaries. Divergence will result in air sinking toward the ground - the opposite of what occurred when the boundaries approached each other. As the divergence increased, the thunderstorms from northern Yankton County through Lincoln County quickly dissipated - in some cases in as little 15 minutes. Because these storms were short-lived and moved into an unfavorable environment, they did not become severe.
