It is well known that mesoscale convective system (MCS) activity maximizes during nighttime hours over the central United States. It is also known by forecasters that this activity generally decreases in intensity or dissipates during the four hours or so before local noon. The reason or reasons for this evolutionary behavior in the late morning are not well understood; therefore, whether those systems maintain themselves or decrease in intensity through the late morning is a significant forecast problem.

The second author, as part of his masters thesis research, carried out a climatology that included 145 of these morning systems that affected the county warning areas of Norman, Oklahoma and Dodge City, Kansas over a period of five summers (1996-2000). From that work it was learned that a majority of the systems that affected this limited area were initiated during the previous afternoon or evening near terrain features in Colorado and New Mexico. It was also learned that about 60% of these systems either decreased in intensity or dissipated in the 13-17 UTC (7-11 AM) period, and another 12% dissipated between 09 and 13 UTC.

Additionally, as part of the thesis, an effort was begun to examine the influence of various environmental factors on the evolution of these systems in the hope of finding keys to aid in forecasting them. Because high temporal and spatial resolution data were needed, Rapid Update Cycle (RUC-2) model data were used to characterize the environment of 48 systems that occurred during 1999 and 2000 by deriving model soundings at hourly intervals 50 km in advance of each system as it moved through the area.

The current work of the project consists of examining combinations of environmental parameters in relation to the observed evolution of these 48 systems during the 13-17 UTC period. Two categories of evolution are defined: a ‘decreasing’ category includes 32 cases that either decreased or dissipated, while a ‘non-decreasing’ category includes the remaining 16 cases that either remained steady or increased in intensity during the period. Among the many environmental parameters that are being examined are stability measures (e.g., CAPE and lifted index), low-level and deep-level vertical wind shear in the plane of system motion, low-level horizontal flux of water vapor toward the system, and low-level warm advection.

As an example of one such evaluation, the value of CAPE was plotted against ‘shear offset’ in a scatter diagram for each of the 48 cases at 15 UTC. Shear offset is a measure of the 600-350 hPa vertical shear vector deviation to the left of the system motion vector. In one regime of the diagram 88% of the cases fall in the ‘decreasing’ evolution category, while in the other regime 75% fall in the ‘non-decreasing’ one (while ‘climatology’ would predict 67% to decrease in the overall sample). Other results will also be discussed based on combinations that include the parameters mentioned in the last paragraph (and others), as well as time changes in these quantities.