Recent Observed Trends and Interannual Variability of Summertime Extreme Convective Rainfall Events in the Central and Eastern United States

At many locations, extreme rainfall events (EREs) are increasing in frequency and magnitude with anthropogenic climate change, causing significant detrimental impacts on society. This study uses a statistical approach to determine if the frequency and intensity of EREs over the eastern two-thirds of the United States have changed over the past two decades. We also seek to determine how trends in EREs vary across the diurnal cycle and the relationship between variations in EREs and large-scale flow patterns. Our focus is on mesoscale convective events observed during June, July, and August (JJA) from 2003 to 2022). EREs were detected using 1-hourly Stage IV precipitation analyses through comparing 12-hour accumulations to 10-year recurrence interval thresholds from the NOAA Atlas 14 dataset. Approximately 60% of the EREs occurred during the JJA period. Automated quality control and event classification procedures were developed to filter out spurious events and those associated with tropical cyclones, stratiform precipitation, and highly localized air mass thunderstorms, leaving a dataset of EREs associated with non-localized convection, including mesoscale convective systems (MCSs).

Our analyses showed that rainfall amounts exceeding the “extreme” thresholds were often found to be over relatively localized zones or narrow swaths (Figure 1). A statistically significant increasing trend with time was also detected in the number of the filtered JJA EREs (Figure 2). This trend was generally associated with larger, longer-duration, and nocturnal mesoscale convective systems, especially over parts of the Ohio Valley and Midwest regions with more extreme nocturnal MCS events increasing east of the Great Plains. The peak hourly rainfall rates with the greatest significance in this increasing trend over this twenty-year period tended to occur during the late night and morning hours after the evening climatological peak. These findings have several implications for operational prediction of EREs since the accurate representation of nocturnal convection over continents is currently a major challenge for weather and climate models. In addition, the relatively small scale of the EREs also presents a major difficulty for using coarser-grid weather and climate models with parameterized convection to predict the occurrence of these EREs. This finding also highlights the benefit of using a high-resolution dataset based on both rain gauge and radar data due to the improved spatial coverage relative to a non-uniform rain gauge network.

Monthly averaged flow characteristics were also utilized to attempt to understand flow regimes more likely to result in a greater number of EREs. Significant relationships between event-counts and both moisture transport from the subtropics and positive height anomalies over the Gulf of Mexico were found through a correlation analysis. These findings indicate that the westward extension of the climatological North Atlantic subtropical high with enhanced northward moisture transport farther north (Figure 3) may impact the occurrence of EREs over the Midwest and surrounding regions. This result is generally consistent with several studies that suggest that the westward expansion of the North Atlantic subtropical high will intensify, tied to a poleward shift in the northern jet stream in a warming climate. This linkage between flow regimes and EREs may aid in seasonal forecasting and anticipating potential risk from EREs. In addition to these results, one or more case studies of recent high-impact EREs may also be added to the presentation.