Band on the Run - Chasing the Physical Processes Associated with Heavy Snowfall
AUGUST 2003 | n the afternoon of 10 November 1998, much of southeastern South Dakota and the surrounding area was digging itself out from under a foot of snow. The snowfall was spread over several states; however, the heaviest totals were found in pockets around Sioux Falls, South Dakota (FSD; Fig. 1). The impressive snowfall near Sioux Falls was accompanied by nearly five hours of thunder and lightning during the most intense period of snowfall (1100–1600 UTC). The extratropical cyclone (ETC) responsible for creating the heavy snow and blizzard conditions in parts of South Dakota and Minnesota was of historic magnitude with a central mean sea level pressure (MSLP) of 964 hPa at its peak over Duluth International Airport at 2035 UTC on 10 November [National Climatic Data Center (NCDC) 1998]. The clarity of the features in this early season snowstorm makes it a good candidate for illustrating the common characteristics of synoptic-scale processes often found with heavy banded snowfall in the central United States. To examine the main physical processes attending this deep system, both observed surface data and initialized upper-air data from the Rapid Update Cycle (RUC) II model are used. The major synoptic features at the time of most intense snowfall (1500 UTC) are the focus of this study; however, some analyses from nine hours earlier (0600 UTC) are shown to illustrate the dynamic development of this exceptionally strong ETC. The system initially formed to the lee side of the Rocky Mountains and moved eastward across Colorado and Kansas early on 9 November. It then turned northeastward and intensified rapidly as it moved through eastern Nebraska into Minnesota. At 0600 UTC 10 November, the ETC was located in eastern Nebraska and had a central MSLP of 984 hPa (Fig. 2a). The surface analysis reveals a warm front extending eastward ahead of the system through Missouri and two cold fronts pinwheeling southward from the center of low pressure. The dual cold-front structure at this time is supported by the presence of two wind shift lines (not shown) and two distinct thermal gradients, as illustrated by the packing of the isentropes in Fig. 2a. By 1500 UTC the system had moved into southern Minnesota and the central MSLP had dropped to 967 hPa (Fig. 2b); most of this 17-hPa pressure fall occurred in the last six hours. It was during this rapid intensification phase that snowfall rates became sigTHE MAP ROOM