Mesoscale convective systems and floods: a review

There is a common tendency for deep convective cells to organize into larger convective systems having lifetimes longer than the individual convective components. As such, these systems (commonly referred to as mesoscale convective systems or MCSs) can produce considerable rainfall over widespread areas. The repeated development of MCSs over the same general area was instrumental in producing the Great Midwest Flood of 1993.

However, MCSs can also contribute to localized flash floods by producing conditions that favor repeated new cell growth over the same location. Most often, this is accomplished through the creation of cold outflows at the surface which provide a lifting mechanism for new cell growth. Flash floods often occur when cells move parallel or behind an outflow boundary, reinforcing it with their own cold outflows, and new cells form in locations previously occupied by the old ones. In this situation, individual cell motion and system propagation vectors are nearly opposite to each other, so that system motion is nearly zero. Many flash floods occurring in the mountain west, central, and eastern United States exhibit this behavior.

Flash floods associated with a nearly stationary thunderstorm outflow boundaries generated by prior convective activity have been identified as ``mesohigh-type floods" by Maddox et al. In a study of 151 U.S. floods from 1973-77, they found that mesohigh-type floods were the most common type, accounting for 34% of the total. In these instances, there is typically a low-level jet impinging on the south or southwest side of the cold pool where new cells develop. They found weak 500-mb short waves usually associated with these events, although the actual heavy rain area was near the large-scale ridge position.

Recent studies have noted that the organization and propagation characteristics of MCSs influence the amount of rain observed at a single point. Many MCSs contain trailing stratiform precipitation regions which extend the period of rainfall received at a specific location. A recent study of midlatitude MCSs using WSR-88D data by Parker and Johnson provides additional insight into the role of MCSs in flash floods. From the analysis of 88 cases, three basic modes of organization have been identified: convective lines with trailing (TS), leading (LS), and parallel (PS) stratiform rain. While the TS classification was the most common, accounting for about 60% of the cases, the LS and PS modes, which have not previously received much attention, accounted for about 20% each. Both the LS and PS modes were found to move more slowly and be shorter-lived than their TS counterparts, and their environmental flow structures displayed important differences from those of each other and of TS storms. Some of the slowest-moving LS systems produced flash floods and exhibited charcteristics of the mesohigh-type floods.

A variety of mechanisms exist for flash flood formation. The storm that produced the 1997 Fort Collins flash flood was not directly part of an MCS (it consisted of a series of convective cells forming, moving off, and then reforming over the same location), but was influenced by a nearby MCS. In particular, a bow echo to the south of the region modified the flow in the Fort Collins area, enhancing the upslope flow, and increasing the moisture flux into the storm.

This paper will review these and other processes associated with MCSs and flash floods.