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 visibility 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 meso- 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 (ASOS) 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 21 banded, 11 cellular, and 8 “mixed” cases. The mixed category consisted of any case that didn’t fit into the banded or cellular categories or systems that had both present at the same time. 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.

Most of the banded cases occurred during colder months; November through February. These cases were characterized by elevated layers of weak potential instability (equivalent potential temperature decreasing with height) and comparatively strong (relative to the cellular cases) vertical wind shear in the lower troposphere. Most banded cases (72%) formed in the vicinity of a cold front, consistent with past studies. Banded cases had less available moisture to produce snow but deeper dendritic growth zones, likely a result of overall colder temperature profiles. Cellular cases occurred during warmer months; February through May. Vertical equivalent potential temperature profiles and convective available potential energy (CAPE) revealed larger potential instability, however lower vertical wind shear was observed compared to the banded cases. Unlike banded cases, cellular snow squalls generally did not form in the vicinity of a cold front. More moisture was present with cellular cases, but they typically had a shallower dendritic growth zone.

In terms of impacts, on days when snow squalls were observed, approximately 31% of vehicular crashes in the region could be attributed to a passing snow squall. When crash data were compared between the two modes, more vehicle crashes were recorded with banded cases than cellular cases, presumably owing to their larger areal extent. This presentation will discuss the results of detailed radar and environmental analysis from this study and provide some implications for forecasting snow squalls in the future.