Case study of an anomalous, long-lived convective snowstorm

On 23 March 1966, thundersnow was reported over a period of 9 hours (non-consecutive) at Eau Claire, Wisconsin. This event constitutes the longest period of thundersnow at a single station from a surface observation dataset of 226 stations spanning the years 1961--990. Herein, the dynamic characteristics of this long-lived convective snowstorm are thus examined. Using objective analysis of rawinsonde data and model simulation output from the Workstation-Eta (WS-Eta), it was determined that the thermodynamic characteristics of the thundersnow event did not evolve appreciably with the evolution of the cyclone. For the duration of the event, Eau Claire was north-northeast of the surface cyclone, with ample moisture, and forcing for ascent. Indeed, the theta e pattern at 700 mb indicated a trough of warm air aloft (TROWAL) upstream at 0000 UTC and then coinciding with Eau Claire at 1200 UTC. Yet, elevated convective (potential) instability failed to develop. Negative equivalent potential vorticity (EPV) was present in cross-section analyses, suggesting the presence of conditional symmetric instability (CSI). The WS-Eta further showed that this snow event resulted largely from the prolonged presence of frontogenesis in the presence of CSI or weak symmetric stability. This scenario was created and maintained by the presence of a TROWAL airstream over the EAU region for an extended period.