The Midwest Snow Storm of 8–11 December 1977

Abstract

Conventional surface and upper-air aerological data are combined with radar and aircraft measurements to give a description of a major winter storm that deposited over 69 cm of new snow at Muskegon, Michigan, between 8 and 11 December 1977. It is shown that most of this snow occurred during four distinct episodes, three of which were related to air-water temperature contrasts as Arctic air flowed over the relatively warm surface of Lake Michigan. These four episodes have been identified as 1) pre-cold frontal, 2) post-cold frontal, 3) secondary trough, and 4) mesolow phases. During the pre-cold front phase heavy snow was associated with an advancing synoptic-scale trough. Little direct effect of the lakes was detected. Following passage of the cold front, strong northwesterly winds across Lake Michigan resulted in strong air-mass transformation with light steady snow along the downwind shoreline. The evidence suggests the presence of horizontal-roll convection as the dominant organizational mode of convection over the lake during this phase. The third phase of this storm was associated with the movement of a secondary trough which itself was a direct result of air-mass transformation over the upper-lakes region. In its initial development this trough was oriented approximately east?west, with the warmest and most moist air to the north. This warm air drifted slowly southward and collided with the eastward moving Arctic air. This produced a frontal-like mesostructure with a line of clouds and snow. At Muskegon the heaviest snow of the entire storm occurred during a five-hour period when this mesostructure was overhead. During the final phase of this storm at Muskegon, a closed mesolow pressure center developed over Lake Michigan as a result of intense air-mass transformation. This set up easterly winds along the Michigan shoreline and focused the convection into a narrow band, parallel to the downwind shore. Very heavy snow occurred in this band, both over the lake and over land at the southern end of the line where it curved eastward in response to winds around the bottom of the secondary trough. This positive feedback nature of shore-parallel bands, i.e., a mesolow resulting from air?mass transformation which in turn focuses the convection into a line producing the maximum residence time for the air over the lake, can explain the protracted periods of heavy snow which often occur along the downwind shores of the Great Lakes.