Mesoscale convective systems (MCSs) account for 40%-60% of warm-season precipitation over the central U.S. MCS activities outside of the warm seasons have received much less attention. To improve understanding of MCSs and their role in the regional water cycle of the U.S., we characterize the 3-D structure of MCSs and their associated large-scale environments by synthesizing 13 years (2004-2016) of three high-resolution observation datasets (geostationary satellite infrared brightness temperature, 3-D NEXRAD mosaic radar reflectivity, Stage IV precipitation estimates) and North American Regional Reanalysis. Robust MCSs are tracked throughout all seasons using a newly developed FLEXTRKR algorithm (Feng et al. 2018). Three key regions east of the Rocky Mountain frequented by MCSs are identified: Northern Great Plains, Southern Great Plains, and Southeast. Robust MCSs show clear seasonality across these regions, accounting for over 50% rainfall in the Great Plains during spring/summer and over 40% rainfall in the Southeast during winter. MCS convective features are both large (50-100 km diameter) and deep and they evolve over time and space. Genesis of MCSs occurs most frequently between sunset and midnight in the Great Plains throughout the year except winter, while a much weaker MCS diurnal cycle is observed in the Southeast except summer. MCSs over the Southeast and parts of the Midwest during winter/spring are dominated by synoptic forcing, where the largest MCSs and stratiform rain area are observed, while those over the Great Plains during summer are primarily driven by convective dynamics, as evidenced by the largest and deepest convective features in that region. Large-scale environmental patterns that support MCSs in these three key regions during various seasons will be examined.