An Intense Small-Scale Wintertime Vortex in the Midwest United States

Abstract

An intense small-scale low pressure system that moved across portions of the midwest United States is examined. The system produced a continuous band of significant snowfall, typically only 50 km wide but extending over 1500 km in length. The system traveled across the Iowa Department of Transportation surface mesonetwork, allowing high-resolution surface analyses that show a closed circulation and intense pressure gradients around the mesolow, comparable to those occurring in warm season MCS events. Radar and satellite images also revealed the small-scale low-level circulation, which apparently was confined below about 800 mb. Although the strong vorticity advection aloft and baroclinicity at lower levels present in this system are typical of baroclinic cyclones, the unusually small scale and short lifetime of the surface system are more reminiscent of polar lows. Mesoscale simulations of the system using the Pennsylvania State University?National Center for Atmospheric Research Mesoscale Model Version 5 with 20-km horizontal grid spacing and initialized with standard synoptic-scale data were unable to capture the closed circulation and significantly underestimated the strength of the mesolow. The inclusion of mesonet surface data in an initialization significantly improved the initial pressure field but did not significantly change the simulation. The simulation was also not strongly sensitive to variations in horizontal and vertical resolution, surface characteristics, convective parameterizations, and the use of nudging toward observations. However, an adjustment of upper-level fields to support the surface mesoscale low did result in a significantly improved simulation of the event, apparently due to better simulation of forcing from warm advection in low levels. A simulation neglecting latent heating produced a surface low that was at least 1 mb weaker than the full-physics run and had much weaker and disorganized upward vertical motion. The mesoscale low was apparently the result of upper-tropospheric forcing, which eliminated a capping inversion in a small region, permitting precipitating convection and latent heat release.