Varying importance of storm types and antecedent conditions for local and regional floods

Local and potentially more impactful regional floods are driven by a combination of precipitation-triggering storms and antecedent conditions. However, it is yet unclear how the importance of these flood drivers and their interplay differs between local and regional events. Therefore, we assess differences in the compounding drivers of local and regional floods in the United States using newly developed classification schemes for storm types and antecedent conditions. Our results show that the dominant storm type triggering floods is frontal events, in particular those related to mesoscale convective systems. The importance of different storm types varies by season, with frontal mesoscale convective systems being most important in summer, nonfrontal, and extratropical cyclone-related storms in winter and spring, and tropical cyclones in fall. Our comparison of the drivers of local and regional events shows that the relative importance of different storm types only weakly differs between local and regional floods, while antecedent conditions are clearly distinct. Regional events are in 75% of the cases related to wet antecedent conditions in some cases combined with snowmelt, while local events are more likely to also develop under dry conditions. Over all regions and seasons, regional events are most often the result of a frontal storm combined with wet antecedent conditions, which highlights the important role of compounding flood drivers. This finding suggests that regional flood risk and change assessments should account for the compounding nature of atmospheric and land-surface flood drivers. Plain Language Summary Floods with a regional extent may be more impactful than local events as they potentially affect more people and assets. Both types of floods are driven by a combination of precipitation-triggering storms and antecedent conditions such as snowmelt and soil moisture. However, it is yet unclear how the importance of these flood drivers and their interplay differs between local and regional events. Therefore, we assess differences in the compounding drivers of local and regional floods in the United States. Our results show that the dominant storm type triggering floods is frontal events and that the importance of different storm types varies by season. The relative importance of different storm types only weakly differs between local and regional floods, while antecedent conditions are clearly distinct. Regional events are in 75% of the cases related to wet antecedent conditions in some cases combined with snowmelt, while local events are more likely to also develop under dry conditions. Over all regions and seasons, regional events are most often the result of a frontal storm combined with wet antecedent conditions, which highlights the important role of compounding flood drivers that should be taken into account in flood risk and change assessments.