A Case Study of Severe Winter Convection in the Midwest

Abstract

Between 2100 UTC 11 February 2003 and 0200 UTC 12 February 2003, a line of thunderstorms passed swiftly through parts of eastern Iowa and into north-central Illinois. Although this storm somewhat resembled a warm season, line-type mesoscale convective system, it was unique in that the thunderstorm winds exceeded the severe criterion (50 kt; 25.7 m s 21 ) during a snowburst. While the parent snowband deposited only 4 cm of snow, it did so in a short period and created a treacherous driving situation because of the ensuing near-whiteout conditions caused by strong winds that reached the National Weather Service severe criteria, as the line moved across central Illinois. Such storms in the cold season rarely occur and are largely undocumented; the present work seeks to fill this void in the existing literature.While this system superficially resembled a more traditional warm season squall line, deeper inspection revealed a precipitation band that failed to conform to that paradigm. Radar analysis failed to resolve any of the necessary warm season signatures, as maximum reflectivities of only 40-45 dBZ reached no higher than 3.7 km above ground level. The result was low-topped convection in a highly sheared environment. Moreover, winds in excess of 50 kt (25.7 m s 21 ) occurred earlier in the day without thunderstorm activity, upstream of the eventual severe thundersnow location. Perhaps of greatest importance is the fact that the winds in excess of the severe criterion were more the result of boundary layer mixing, and largely coincident with the parent convective line. This event was a case of forced convection, dynamically linked to its parent cold front via persistent frontogenesis and the convective instability associated with it; winds sufficient for a severe thunderstorm warning, while influenced by convection, resulted from high momentum mixing downward through a dry-adiabatic layer.

A Diagnostic Examination of the Eastern Ontario and Western Quebec Wintertime Convection Event of 28 January 2010

“…The total measured snow accumulation for the day at CYOW was 3.6 cm, very similar to the precipitation accumulation in Illinois in the event studied by Pettegrew et al (2009). The total measured snow accumulation for the day at CYOW was 3.6 cm, very similar to the precipitation accumulation in Illinois in the event studied by Pettegrew et al (2009).…”

“…4e). Pettegrew et al (2009) found substantial low-level (925 hPa) frontogenesis associated with the snow burst in Illinois. By t 5 13 h (Fig.…”

“…However, this paper differs from all previous work other than Pettegrew et al (2009) andLundstedt (1993) in that we are analyzing the dynamics and thermodynamics associated with a low-quantitative precipitation forecast (QPF), high-impact precipitation event in a region not prone to lake-effect snow. However, this paper differs from all previous work other than Pettegrew et al (2009) andLundstedt (1993) in that we are analyzing the dynamics and thermodynamics associated with a low-quantitative precipitation forecast (QPF), high-impact precipitation event in a region not prone to lake-effect snow.…”

“…DeVoir (2004) pointed out that events ''characterized by extremely heavy but short-lived snow bursts or squalls'' fit into such a category. So as not to confuse the reader with ''snow squalls'' that would typically be associated with lake-effect precipitation regions, we will subsequently refer to these events as ''snow bursts,'' following Pettegrew et al (2009). Additionally, aviation can often be affected, as winter precipitation and low visibility can greatly impact airport operations, albeit for relatively short periods of time during events such as the one discussed here (i.e., compared to a winter storm or blizzard).…”

“…To our knowledge, there exists little in the way of published literature on the topic of cold-season, highimpact, limited-moisture events (Pettegrew et al 2009). Pettegrew et al (2009) is a recent study that included a synoptic-dynamic analysis on an event similar to the one in this paper, which created near-whiteout conditions in Iowa and Illinois in 2003, while only depositing 4 cm of snow.…”