A case study of a rare severe thunderstorm with snowfall

At 0000 UTC 12 February 2003, a line of thunderstorms passed swiftly through parts of eastern Iowa and into north central Illinois. This storm resembled a warm-season, line-type mesoscale convective system rather than a winter storm. In addition to the severe storm wind reports of over 50 knots in Illinois, this storm also dropped up to 4 inches of snow on the ground. Synoptic analysis of the system reveals a typical wintertime, northwest flow, clipper system with a well-defined frontal structure at the surface, a trough axis aloft, and the presence of a jet streak. This case was analyzed on the mesoscale to determine the origins of convection and high wind meeting the severe criterion. Such strong surface winds appear to be a case of planetary boundary layer mixing, with observed winds at the top of the layer near 50 knots. Surface observations from around the area and rawinsonde upper-air observations were compared to the 20-km Rapid Update Cycle (RUC) model initial fields. The model fields correctly diagnosed the mesoscale situation leading to the triggering of convection and the boundary layer profile that contributed to the high wind event.