Comparing and Contrasting the Extreme Flood Index (EFI) Diagnostic Metric for Mid-Latitude Floods and Tropical Cyclones

The Extreme Flood Index (EFI) is a dynamic and thermodynamic metric designed to diagnose the frequency and intensity of extreme precipitation events associated with mid-latitude flow stagnation (i.e., Rex Blocks). EFI is calculated using a combination of flow stagnation (easterly wind anomalies) and thermodynamics (heat and moisture, θ_e). It focuses on detecting large plumes of warm moist air that are associated with cutoff upper-tropospheric cyclones persisting for durations of at least a day.

In recent years, several infamous billion-dollar flood and tropical cyclone (TC) disasters have been documented in the Northern Hemisphere (NH) (e.g., 2012 TC Sandy, 2013 Alberta Flood, 2016 Germany/France Flood, 2017 TC Harvey). However, there has not been much work investigating the importance of Rex Blocks in these events. Examining EFI, a subset of midlatitude and TC-related floods were examined. The flood associated with TC Harvey, which resulted in 60+ in. of rain in parts of Texas, was amplified by a persistent Rex Block that greatly increased the duration and amount of rainfall. The utility of the EFI in diagnosing and predicting this extreme event is explored.