Observations of Extreme Short-Term Precipitation Associated with Supercells and Mesovortices

In several prominent extreme precipitation and flash flood events, radar and rain-gauge observations have suggested that the heaviest short-term rainfall accumulations (exceeding 75 mm/h) were associated with supercells or mesovortices embedded within larger convective systems (e.g., multicell clusters or MCSs). In this research, we aim to explore how often extreme short-term rain rates in the US are associated with mesovortices, with the ultimate goal being to better understand the dynamical processes supporting very heavy precipitation. This research involves building and analyzing an observationally based database of such events.

First, hourly precipitation accumulation METAR data was obtained through the Iowa Environmental Mesonet from 2013-2017 and filtered for precipitation accumulations over 75 mm. This data was then manually culled by regional radar analysis to remove gauge observations that are clearly reporting false data (e.g., no precipitation is visible on radar) and to filter out winter weather events. Local radar data, including equivalent reflectivity, radial velocity, and differential reflectivity (Z_dr), was then obtained for the remaining events for the hour leading up to the METAR observation. Each remaining case was subjectively screened for the presence of attendant or collocated rotation. Additionally, a similar method was performed on Stage-IV precipitation analysis data from 2013-2015 to supplement the dataset.

To supplement the subjective methods described above, mid and low-level rotation tracks from the Multi-Radar, Multi-Sensor (MRMS) were used to independently and objectively identify rotation in the METAR cases from August 2016 to the end of 2017.

The distribution of the valid METAR and Stage-IV short-term, locally extreme events shows the majority of the events located along the Atlantic and Gulf of Mexico coastlines with additional events occurring in the central plains and into the Midwest. While some of these events are associated with more traditional “training” precipitation cases, many others are associated with low-level rotation in high-precipitation supercells and/or mesoscale vortices embedded in more organized storm modes. Between 40% and 50% of these extreme short-term rainfall accumulations from each dataset were associated with subjectively identified rotation. The environmental, geographic, and storm characteristics of this observationally based dataset are discussed, and connections will be drawn to recent research on concurrent, collocated tornado and flash flood events.