Environmental Controls on Banded Versus Cellular Organization of Mesoscale Snow Squalls in Western South Dakota

Snow Squalls are defined as mesoscale bursts of heavy snow characterized by gusty winds, and visibilities below a quarter of a mile. They are fast-moving systems that generally impact an area for less than an hour but can cause hazardous driving conditions and have proven difficult to forecast. Past research has focused on the mesoscale and synoptic-scale environmental conditions associated with these systems and have identified both banded (more common) and cellular snow squall organizations. To date, however, there has been little study of what governs the organizational mode of these features. The primary aim of this project is to address this knowledge gap by investigating the organizational structure and development of snow squalls in Western South Dakota.

Forty snow squall cases were identified between 2012 and 2017 using surface weather observations from Rapid City, Philip, Spearfish and Custer, South Dakota. WSR-88D radar data were used to determine the mesoscale structure of the snow squalls. Banded cases were defined as having lengths longer than the average width of any given echo, and cellular cases had a length to width ratio close to one. This produced 22 banded, 11 cellular, and 7 mixed cases that had characteristics of both banded and cellular systems. Thirty of the cases occurred between 16z and 00z, suggesting a connection to diurnal heating. Banded cases were more likely to occur during colder months (November – February), while cellular cases occurred during warmer months (February – May). Rapid Refresh model analysis (RAP) data were used to determine synoptic patterns, forcing mechanisms, vertical shear, and static stability associated with each type of snow squall. By the end of this project we expect to better understand the environmental parameters that control banded versus cellular structure and have a better understanding of the meso-gamma scale and meso-beta scale structures within snow squalls.

This presentation will focus mainly on the environmental characteristics related to snow squall development and organization. The initial classification into banded and cellular cases will be defined, environmental parameters, and a radar analysis of reflectivity and velocity patterns will be presented. With a better understanding of snow squalls, we hope to provide aid for forecasting snow squall events that can cause significant transportation impacts.