On 29-30 January 2001, a heavy snow band occurred across portions of eastern South Dakota. Over 50 cm (20 in) of snow fell within 24 hours across the James River Valley of South Dakota, with one location setting an all-time record for snowfall. This snowfall was associated with the development of an intense frontal band within a TROWAL (trough of warm air aloft) and remained nearly stationary for 12 hours.

The trowal developed as an isolated potential vorticity (PV) anomaly moved from southern Texas into Iowa. As the storm occluded and the PV anomaly moved northeast, quasi-geostrophic forcing rotated the isentropes parallel to a deformation zone across eastern South Dakota. Within 6 hours, intense frontogenesis developed across the James River Valley of South Dakota. In addition, the transport of moist and unstable air allowed for a stronger response as the frontogenetical forcing increased. This allowed moderate to heavy snow to persist across the James Valley for over 12 hours. As the PV anomaly moved to the northeast, the frontogenesis weakened and the circulation diminished.

This case is unique from other documented cases in that the distribution of PV aloft favored the development of the cold frontal circulation instead of along the warm front. This resulted in the heavier snowfall along the southern edge of the trowel. This paper will examine how the large scale potential vorticity distribution determined where the trowal would develop and favored development of an intense cold front aloft. This cold frontogenesis resulted in a strong frontal circulation and produced heavy snowfall on the mesoscale.