Linking Flood Frequency With Mesoscale Convective Systems in the US

Mesoscale convective systems (MCSs) with larger rain areas and higher rainfall intensity than non-MCS events can produce severe flooding. Flooding occurrences associated with MCS and non-MCS rainfall in the US east of 110 degrees W are examined by linking a high-resolution MCS data set and reported floods in the warm season (April-August) between 2007 and 2017. MCSs account for the majority of slow-rising and hybrid floods, while non-MCS rainfall explains about half of flash floods in July and August as individual thunderstorms occur frequently in the Rocky Mountains and Appalachian Mountains. The event-total rainfall area of MCSs is the dominant factor of flood occurrences: MCSs with greater rainfall areas tend to produce more floods. While not related to flood frequency, propagating MCSs tend to produce flash floods with longer durations. These established links can improve our confidence in interpreting flood risks and their future changes due to changes in MCS characteristics with warming. Plain Language Summary Severe floods in the central US with significant socioeconomic impacts have been attributed to mesoscale convective systems (MCSs), a form of organized deep convection. However, the relationship between flood likelihood and MCSs is not well established. The goal of this study is to quantify the likelihood of floods in association with MCSs and how flooding occurrences can be affected by MCS characteristics. We found that the majority of floods in the warm season (April-August) in the central US are associated with MCSs, but flash floods in July and August are also commonly associated with non-MCS storms that occur frequently in mountainous areas and produce locally intense rainfall. More importantly, we found a predominant role of storm-total rainfall area on flood occurrence because flood-producing MCSs have significantly larger rainfall area than non-flood producing MCSs, and flood occurrences increase with MCS rainfall area. The identified critical role of MCS rainfall area on flood occurrences may improve projections of flood risk changes in response to warming-induced MCS changes in the future. Key Points Mesoscale convective systems account for most of the slow-rising and hybrid floods during the warm season in the central US Half of the flash floods in July and August are related to isolated convection occurring frequently and producing locally intense rainfall Mesoscale convective systems with larger rainfall area can produce more floods while propagation enhances the duration of flash floods