A large, negatively-tilted trough dominated the upper-level synoptic flow over most of the north-central United States on the days leading up to the December event. By 0000 UTC (1900 Local Standard Time) on December 9th, a 500 mb trough axis stretched from the central Northwest Territories in Canada, south-southeastward into the upper midwest of the United States. A large section of the Great Lakes region -- including Lake Huron, Lake Superior and the entire state of Michigan -- was under strong upper-level diffluence. Embedded shortwaves were extremely intense. At the surface, a low pressure system was moving eastward from northern Minnesota, and artic air was wrapping around behind it. Ahead of the low, the low-level flow was from the southeast. For Sault Saint Marie, Michigan (Y62), this meant the upwind fetch was across the long axis of Lake Huron.
