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Changes to the storm-scale physical processes of an eastern United States mesoscale convective system (MCS) on 14 May 2018 in response to global warming are quantified using the pseudo–global warming (PGW) numerical method. Climate perturbations in temperature DT and specific humidity DQ of different magnitudes are imposed separately and simultaneously. The mid-twenty-first century environment becomes increasingly unstable with larger DT, promoting more favorable MCS conditions. By the late twenty-first century, however, this warming, which maximizes in the mid-troposphere, results in increased convective inhibition (CIN) and decreased convective available potential energy (CAPE). Midlevel warming also reduces cold pool generation through the downward advection of the relatively warm midlevel air. Consequently, the MCS of interest is weak in the midcentury and propagates discretely over the Appalachian Mountains, while it fails to initiate in the late century. In contrast, projected increases in DQ support more intense MCSs in both the mid- and late twenty-first centuries. Moisture increases are maximized in lower troposphere, increasing CAPE and decreasing CIN. Additionally, the stronger convections generate deeper and denser cold pools. Therefore, storms remain robust as they move over the Appalachian Mountains. However, leeside isolated convective cells, which form due to lee waves in the more unstable environment, and their widespread cold pools reduce the leeside instability. This, in conjunction with the more intense MCS cold pools, leads to rapid MCS weakening in the lee. Experiments with both DT and DQ illustrate that larger magnitude increases in one thermodynamic variable may supersede increases in the other. 

This study employs a pseudo–global warming approach to investigate precipitation changes from a mesoscale convective system (MCS) on 14 May 2018 over the eastern United States. An Appalachian-Mountain-crossing MCS is simulated for historical, mid-twenty-first century (2045–54), and late-twenty-first century (2090–99) climate scenarios. For experiments using ensemble-mean perturbations in atmospheric, soil, and oceanic variables derived from 34 general circulation models, MCS precipitation diminishes by 25%in the midcentury and 65%in the late century. Experiments testing the sensitivity to these variables separately reveal that atmospheric variables primarily drive precipitation changes. Additional sensitivity experiments quantify MCS responses to temperature, moisture, and wind perturbations separately, with the magnitude of perturbations stratified as low, moderate, or high. Experiments highlight the dominant though contrasting roles of the thermodynamic variables. In midcentury, temperature increases lead to reductions in rainfall rates by up to 74.3%, while increased moisture raises rainfall rates by 75.1%. In the late century, the MCS fails to initiate for temperature perturbations of all magnitudes. Rainfall rate and precipitation area substantially increase with larger moisture perturbations, while the frequency of heavy (95th percentile) and extreme (99th percentile) precipitation increases more than 100%, with minimal changes in precipitation rate. Finally, ensemble-mean perturbations are added to all variables, except for temperature or moisture, to which either a low or high perturbation is added. MCSs are robust when low-temperature or high-moisture perturbations are included, though they fail to initiate for low-moisture and high-temperature perturbations, highlighting the challenges in projecting future MCS behavior.