The Extreme Precipitation Index (EPI): A Multivariate Metric to Diagnose Heavy Rainfall Associated with Atmospheric Flow Stagnation

The Extreme Precipitation Index (EPI) is a multivariate dynamic and thermodynamic metric designed to diagnose extreme precipitation events associated with mid-latitude flow stagnation (i.e., Rex Blocks). As the climate warms, Arctic Amplification may be helping to slow the mid-latitude westerly jet stream, resulting in increased upper-tropospheric flow stagnation. Furthermore, several recent catastrophic floods (e.g., 2013 Alberta and Colorado, 2016 Louisiana and Western Europe, 2017 Tropical Cyclone Harvey) were associated with Rex Blocks. During these extreme events, long-duration ascent associated with flow stagnation (dynamics) occurs in the presence of an anomalously warm and moist air mass (thermodynamics). The EPI is specifically designed to detect this potent combination of ingredients, offering better predictability than Quantitative Precipitation Forecasts due its complete dependence on mass fields. The dynamics component of the EPI is derived from operational ECMWF and NOAA atmospheric blocking criteria, adapted to better represent the relatively short duration of extreme precipitation events, while the thermodynamic component utilizes standardized anomalies of equivalent potential temperature (θ_e). First, the ability of the EPI to detect extreme precipitation events is evaluated for multiple reanalysis datasets over a long time period (1948–2016). Next, the primary EPI hotspots in the Northern Hemisphere are identified and discussed. Finally, the utility of the EPI in the diagnosis and forecasts of recent extreme floods is explored, including the record-breaking deluge in Texas associated with Tropical Cyclone Harvey.