Differentiating Plowable Hailstorms from Heavy Rain Events in the Colorado Springs Area

Over the past several years, the Front Range corridor of Colorado has been affected by a number of heavy accumulation hail events. These events are brief and highly localized, and as such are a challenge for forecasters to predict. In addition, they occur almost exclusively between the months of May and September, a time when snowplows are not ready for immediate action. The objective of this research is to distinguish atmospheric conditions present during the hours leading up to hail-producing storms from those present during heavy rainfall events.

This research will compare atmospheric conditions from two hail events which took place on May 7, 2015 and July 28, 2016, to two rain events that took place on July 29, 1997 and June 15, 2015. I will use data from the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) to determine the areal extent and depth of precipitation. Archival radar data from the National Centers for Environmental Information (NCEI) will be used to supplement data from CoCoRaHS. Atmospheric profiles from sounding data and synoptic scale flow from archived upper air data will provide insight for moisture content, general wind fields, and, crucially, melting layers.

I will start by distinguishing between areal extent and liquid water equivalent (LWE) during each individual precipitation event. The total estimated precipitation volume within the areas will be compared with the precipitable water content. All precipitation will likely have started from ice crystals in the upper troposphere, so this study aims to prove that the melting layer was lower and/or cloud top heights were higher (colder) during the hail events. Hydrometeor classification data from NCEI will be used to achieve this goal. Mesoscale wind circulations will depict driving updraft mechanisms; hail events are expected to have strong, localized updrafts relative to the rain events.

Beyond the mesoscale and microscale circulations directly responsible for the storm, synoptic scale forcing mechanisms need to be considered. Archived GEMPAK data will facilitate the tracking of moisture sources for the precipitation. Since the events take place during three different months it is possible that the synoptic scale forcing mechanisms will not be similar. However, correlation between the individual hail events will be sought by examining NCEP/NCAR reanalysis data from the Earth System Research Laboratory (ESRL).

Plowable hail events in Colorado typically take place during the summer with the deepest accumulations occurring in narrow swaths. Because of their size they are difficult to forecast. The goal of this study is to provide a description of atmospheric differences between accumulating hail events and heavy rain events, primarily on the local scale but also to a lesser extent synoptically. Kalica et. al. (2016) provides an excellent description of characteristics of plowable hail events in the Denver metro area but does not compare them to similarly heavy rainfall events. Schlatter, Schlatter and Knight (2008) study a specific case of plowable hail in Boulder but do not fully investigate synoptic scale features. I have yet to uncover any research that addresses cases south of the Palmer Divide. I hope that the data uncovered in this study will aid in the forecasting of plowable hail events in the Colorado Springs area.