Opposing Mesoscale Circulations: A Case Study

Abstract

Two mesoscale convective systems developed over the central plains in and near the OK PRE-STORM observing network on 23?24 June 1985. Mesoscale outflows from these two systems collided in central Kansas at 0430 UTC 24 June. Special soundings taken behind the outflow boundaries indicated a stable atmosphere. However, a sounding taken south of the area of outflow collision sampled a conditionally unstable atmosphere in which lifting of 90 mb was needed to release the instability. The OK PRE-STORM forecast team monitored this situation using conventional data and special surface mesonetwork and sounding data. They issued a forecast update at 0500 UTC 24 June that anticipated new convective development in the conditionally unstable atmosphere near the area of outflow collision. As the outflows moved southward, surface convergence increased near the area of outflow collision, yet no new development of deep convection occurred. Special OK PRE-STORM upper-air soundings are utilized to compute divergence and vertical velocity via the line integral method at various times and locations during the development and decay of the two mesoscale convective systems. Strong rising motion is found in the lower troposphere near the zone of outflow collision at 0430 UTC 24 June, suggesting that the outflows were significantly impacting storm development and system propagation. However, at 0900 UTC 24 June, while strong rising motion is found below 800 mb, there also is strong sinking motion between 800 and 400 mb. It is hypothesized that this mesoscale downdraft occurred between the two mesoscale convective systems as their upper-level outflows converged. This layer of sinking air may have helped maintain a capping inversion at the top of a deepening moist layer and apparently was sufficient to inhibit development of new convection even in the face of strong low-level forcing. The case illustrates the simultaneous development of mesoscale circulations that appear to be acting in opposition to each other, relative to the initiation of new storms. It further illustrates the complexity of the mesoscale and why it is often hard to anticipate the evolution of convective storm systems.