LAUSR

Using a temporal synoptic index, synoptic-scale atmospheric patterns suitable for lake-effect snow downwind of Lakes Erie and Ontario are identified and analyzed from 1950 to 2009. In response to prior research noting a trend reversal of snowfall in this region, changes in the inherent meteorological characteristics and winter-season frequencies of lake-effect synoptic weather types are evaluated as possible forcing mechanisms. Four atmospheric patterns are identified during the December–February winter season as lake-effect synoptic types. Changes in inherent meteorological characteristics and winter frequencies of these types are attributed to between 88% and 95% of the observed snowfall changes during the study period. Decreasing air temperatures and surface pressures, increasing boundary layer instability, and changes toward stronger zonal flow are noted for multiple lake-effect synoptic types from 1950 to 1979 as likely forcing mechanisms of observed snowfall increases, on the order of nearly 1.0 cm yr−1 downwind of Lake Ontario per individual synoptic type. Similarly, the significant increases in the winter frequency of multiple lake-effect synoptic types also are attributed to some of the increases in snowfall. From 1980 to 2009, however, the lake-effect synoptic types remained relatively unchanged or decreased in frequency, as did snowfall totals. The results of this study indicate that changes in the synoptic-scale environment are a viable mechanism forcing snowfall trends, in addition to the more commonly considered seasonal temperature and lake-ice considerations, and should be incorporated into future discussions of lake-effect snowfall projections.