Diagnosing high-impact precipitation events from mass fields: The Extreme Precipitation Index (EPI)

While forecast skill of mass fields (e.g., 500-hPa geopotential height) has improved greatly over recent decades, Quantitative Precipitation Forecasting (QPF) skill still lags behind, especially for extreme precipitation events. Therefore, as extreme precipitation events increase in frequency and intensity in a warming world, a greater understanding of the dynamics of extreme precipitation events is vitally important to QPF.

First, a novel, yet relatively simple and intuitive composite index, the Extreme Precipitation Index (EPI), is introduced. The EPI, which is designed to identify and predict extreme mid-latitude precipitation events, is based on an equation found in the original lecture notes of Professor Fred Sanders. This equation relates the precipitation rate (P) to the integrated ascent ω (assumed for the purpose of this work to be quasi-geostrophic [QG]) and to the incipient air mass (represented by drs/dp, where rs is the saturation mixing ratio). Since there is more QG ascent for a given QG forcing in the presence of weaker static stability, the equation suggests that the most ‘value for the dollar' is achieved with more ascent AND warmer, less stable air masses, represented by drs/dp.

Second, the EPI is evaluated by examining recent high-impact heavy precipitation events: 1) The record-setting Alberta flood event of June 2013, which resulted in widespread evacuations and damage, and 2) Extratropical Transition (ET) events such as Irene (2011) and Sandy (2012), in which the precipitation distribution (axis of heaviest precipitation) shifts from symmetric to left-of-center (LOC) as a tropical cyclone interacts strongly with a mid-latitude trough.