The Effects of the Black Hills on Linear Mesoscale Convective Systems

The Black Hills are an isolated region of elevated topography that range from western South Dakota into far eastern Wyoming in the central continental United States. This region experiences multiple mesoscale convective systems (MCS) per year, some of which produce severe weather including large hail, winds in excess of 60 mph, flash flooding, and tornados. Despite continued advances in high-resolution numerical model guidance, these storms are difficult to operationally forecast. A particular challenge is predicting the changes in structure and intensity that are often observed as storms that form in eastern Wyoming cross the Black Hills into South Dakota. To aid in filling this knowledge gap, the goal of this project is to increase the scientific understanding of the interactions between lines of storms, of varying structures and intensities, and the Black Hills.

To complete this research goal, archived radar were analyzed for the months of May through September over a ten year span to identify cases of mesoscale convective systems interacting with the Black Hills. The criteria for the ~250 cases was, at some time during transition over the Black Hills, had to have a maximum reflectivity of at least 40 dbz, and had to have a width-to-length ratio of at least 1 to 3. As long as the line obtained these criteria at some point in its transition, it is allowed to decrease in intensity to a smaller/weaker system, or even dissipate completely, and still be considered as a case. Each case was then divided into three time periods: upstream from the hills, crossing the hills, and downstream from the hills. Within each of these partitioned zones, the cases were manually categorized based on a dominant convective modes as Single Cell, Multiple Cells, Width-to-length ratios of 1 to 2 or 1 to 3, and Long.

This presentation will detail the classification of these cases, and further quantify changes in intensity as MCSs encounter the Black Hills using plots of maximum reflectivity accumulated over time. Changes in intensity will also be compared with changes to the background environmental conditions with an aim toward isolating terrain effects from the MCS responding to environmental heterogeneity. Ultimately, through improved understanding of the interactions between MCSs and underlaying terrain, this research aims to further improve short-term forecasts of MCSs and their associated severe weather.