The northwest quadrant of wintertime extratropical cyclones is commonly associated with heavy snow and blizzard conditions. On 10-11 December 1997, a cyclone of moderate strength swept through the Ohio valley. A swath of heavy snowfall, with 15-30 cm accumulations, was produced by a mesoscale precipitation band northwest of the cyclone center in southern Michigan and northern Indiana. The structure and dynamics of this band will be presented in a companion paper in this conference (Grim et. al.). In this paper, a simulation of this band with the NCAR-PSU MM5 model will be employed to analyze the frontal structure, dynamic forcing, and associated secondary circulations which account for the rising motion that produced the band and associated heavy snowfall.

A detailed analysis of the forcing for vertical motion within the snowband will be presented through diagnosis of the model output using the geostrophic momentum form of the Sawyer-Eliassen equation. Ageostrophic circulations associated with both frontogenetic and frontolytic forcing in the vicinity of the warm front and the trough of warm air aloft will be shown to have led to the persistent band of rising motion that produced the heavy snow. The effects of the shearing and stretching deformation will be discussed. It will be shown that the importance of these terms varied along the length of the band, with shearing deformation dominating on the north side of the cyclone vortex center and stretching deformation dominating well east of the cyclone center. The implication of the forcing on the distribution and intensity of the snowfall will be discussed.