Mesoscale convective systems (MCSs) are frequently observed over the U.S. Great Plains during boreal spring and summer. Based on self-organizing maps (SOM) analysis, we identified four types of favorable large-scale environments for spring MCSs and two types of favorable large-scale environments for summer MCSs. During spring, frontal systems and an enhanced Great Plains low-level jet (GPLLJ) are essential features accompanying spring MCSs initiation. The two large-scale favorable environments for summer MCSs also have frontal characteristics and GPLLJ respectively, but both are shifted northward compared to spring. We also identified two unfavorable large-scale environments for summer MCSs, each with an enhanced upper level ridges but with differing surface thermal condition and GPLLJ strength. A large-scale index constructed using pattern correlation between real-time large-scale environments and the favorable large-scale environments identified by SOM is found to be skillful for predicting MCS number, precipitation rate and area in spring, but its explanatory power decreases significantly in summer.

During summer, MCSs initiation tends to accompany eastward-propagating perturbations under both favorable and unfavorable large-scale environments. For the latter, subsynoptic disturbances can be identified as far upstream as northeastern Pacific Ocean and as early as ~2 days before the MCSs initiation, suggesting a potential precursor of summer MCSs. By comparing the MCS tracking and the tracking of mid-tropospheric perturbations (MPs), which are one kind of subsynoptic disturbances with a Rocky Mountain origin, during 2004-2017, it is found that ~30% of MPs are associated with MCSs initiation, but only 10% of MCSs are associated with MPs. MCSs associated with MPs tend to produce more rainfall with stronger convective intensity and larger convective areas. This result highlights the important contributions of MPs to more intense MCSs, while other subsynoptic disturbances contribute to frequent (~90%) initiation of weaker MCSs.