Lake-effect snow storms are important parts of the climate of the U.S. Upper Midwest, doubling average wintertime snowfall amounts in some locations close to the Great Lakes. The economic and societal impacts of these storms depend critically on the timing of their occurrence relative to activities of the 35 million people living in the Great Lakes basin. This study utilizes hourly precipitation data collected in the snow belt regions of Lakes Superior and Michigan to determine the typical diurnal behavior of lake-effect precipitation intensity.

The diurnal evolution of precipitation on lake-effect days was determined through examination of hourly data collected during 1988-1993. Lake-effect dates used for this investigation were identified using visible satellite imagery by Kristovich and Steve (1995, J. Applied Meteor.). Hourly precipitation observations were taken by the NOAA Cooperative Observation Network (TD 3240). Precipitation observation sites used for this investigation were chosen based on the completeness of data, their spatial distribution within the snow belt regions of Lakes Superior and Michigan, and their proximity to locations of hourly observations of other atmospheric conditions. In total, precipitation data from seven stations were analyzed south of Lake Superior and east of Lake Michigan.

It was found that the frequency with which precipitation was observed on lake-effect days was considerably greater during the pre-dawn hours than during the afternoon hours at all locations. The largest morning-to-afternoon differences were in locations known to have the most frequent lake-effect storms (i.e., Muskegon and Marquette, MI), where precipitation occurred more than four times as frequently at 06 LST than at 20 LST. Comparable analyses for non-lake-effect days showed no such diurnal variation. Interestingly, this pattern is in opposition to that found by previous studies of autumn lake-effect rainstorms. Potential factors influencing the diurnal evolution of lake-effect precipitation (such as the temporal evolution of lake surface fluxes) will be explored.