Decreasing Warm-Season Extreme MCS Precipitation Over the Central US Under Global Warming: Associations with Low-Level Drying and Reduced Wind Shear

Mesoscale convective systems (MCSs) are organized thunderstorm clusters in mid-latitude and tropical regions that can produce extreme precipitation threatening human safety and property. Under global warming, extreme precipitation is expected to increase in intensity and frequency over most regions, primarily due to increased moisture availability and convective available potential energy (CAPE). However, using a recently developed 4 km, hourly MCS-tracking dataset (Feng, 2024), we find that extreme precipitation produced by MCS over the central US shows decreasing trends during the warm seasons (April–September) of 2004–2021.

Extreme hourly MCS precipitation events are defined using the 95th percentile threshold (63 mm h^-1). Over the study period, the number, mean rain rate, and total rainfall of extreme precipitation events all exhibit decreasing trends, with the decline in mean rain rate being statistically significant. Similar patterns are observed using the 90th (54.38 mm h^-1) and 98th (73.63 mm h^-1) percentiles, indicating robustness across thresholds.

To examine concurrent environmental changes, we extract sounding data from ERA5 and calculate thermodynamic and kinematic parameters. The results show that, despite increasing trends in instability (e.g., CAPE and lifting index) and downdraft CAPE (DCAPE), the decreasing MCS precipitation is associated with low-level drying and weakening of low-level vertical wind shear. Low-level relative humidity is found to decrease significantly, accompanied by rising lifted condensation level (LCL), indicating that the low-level atmosphere has become drier despite a slight (but not significant) increase in precipitable water. This suggests that warming has outpaced moistening. Meanwhile, both 0-1 km and 0-3 km vertical wind shear show consistent weakening trends.

Together, low-level drying and reduced vertical wind shear are likely to suppress organized MCSs capable of producing extreme precipitation under global warming. In addition, whether the overall extreme precipitation budget is compensated by isolated deep convection remains an open question for future research.