An intense small-scale low pressure system that moved across portions
of the midwest United States is examined. The system produced a contin
uous band of significant snowfall, typically only 50 km wide but exten
ding over 1500 km in length. The system traveled across the Iowa Depar
tment of Transportation surface mesonetwork, allowing high-resolution
surface analyses that show a closed circulation and intense pressure g
radients around the mesolow, comparable to those occurring in warm sea
son MCS events. Radar and satellite images also revealed the small-sca
le low-level circulation, which apparently was confined below about 80
0 mb. Although the strong vorticity advection aloft and baroclinicity
at lower levels present in this system are typical of baroclinic cyclo
nes, the unusually small scale and short lifetime of the surface syste
m are more reminiscent of polar lows. Mesoscale simulations of the sys
tem using the Pennsylvania State University-National Center for Atmosp
heric Research Mesoscale Model Version 5 with 20-km horizontal grid sp
acing and initialized with standard synoptic-scale data were unable to
capture the closed circulation and significantly underestimated the s
trength of the mesolow. The inclusion of mesonet surface data in an in
itialization 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 o
f nudging toward observations. However, an adjustment of upper-level f
ields to support the surface mesoscale low did result in a significant
ly improved simulation of the event, apparently due to better simulati
on of forcing from warm advection in low levels. A simulation neglecti
ng 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.