Severe Weather during the Lifetimes of MCSs that Affect a Limited Area of the Great Plains during the Morning Hours

During the summer months mesoscale convective systems (MCS) often move through the Great Plains overnight and frequently produce severe weather, mostly in the form of high winds and large hail. Most dissipate during the late morning hours; however, a few MCSs remain steady or increase in intensity during that period. The reason (or reasons) for the evolutionary behavior during the four hours or so before local noon is not well understood. Thus, MCS activity during this time of day creates a forecast problem. To address this problem, a research project is underway to investigate MCS activity that affects the county warning areas (CWA) of Norman, Oklahoma and Dodge City, Kansas during the morning hours. The project includes a climatological study and investigation of environmental influences on the evolution of the MCSs.

The climatological study included 182 systems during June, July, and August from 2001 to 2005. These systems affected one or both of the CWAs during the 09-17 UTC time period in the summer months. Each system lasted at least 3 hours and had at some point a size greater than or equal to 100 km in the longest dimension. System intensity of at least 40 dBz for at least one hour in the time period was also a requirement for inclusion in the study. Systems were tracked and plotted from initiation to dissipation. Most systems were initiated the previous afternoon or evening near terrain features in Colorado or New Mexico.

Severe weather reports were compiled for each system during the entire system lifetime. The histogram below (Figure 1) shows the number of severe reports by type during each hour in 2001-2004 (data not yet available for 2005) for a period within and surrounding the time range of interest (morning). The majority of the severe reports resulted from large hail and damaging wind. However there were a few tornadoes, mostly occurring early in the systems' lifecycles (during the previous afternoon or evening). Total severe reports maximize during the previous evening, and for those systems that survive through the late morning, there is a maximum of severe reports in the mid-afternoon. The distribution of severe reports by month will also be described in detail. Systems were classified into two evolutionary categories: a ‘decreasing' category includes 156 systems that dissipated or decreased in intensity during the 13-17 UTC time period and a ‘non-decreasing' category includes 26 systems that remained steady or increased in intensity during the 13-17 UTC time period. The amount and timing of severe reports associated with decreasing and non-decreasing systems will also be discussed in detail.