Climatology of November Snow Days in the Lake Michigan Region: Is the Decrease in Lake Michigan November Snowfall Frequency Mostly Due to Lake-Effect Snowfall, or Synoptic Systems?

Previous assessment of Lake Michigan basin snowfall characteristics since 1950 revealed a marked decrease in November snowfall frequency in recent decades, which has been driven by a decreased frequency of cold Novembers during the period. The change has been greatest in the eastern and southeastern sub-regions, implying a diminution in the frequency of lake-effect snowfall. However, the proportional decrease is similar in the western sub-regions, and the adjudication of lake-effect and synoptic system contributions to snowfall is a challenge. Here, based on the snowfall from forty-seven locations and visual inspection of maps from NCEP/NCAR Reanalysis and the historical Daily Weather Map series, 937 snow days are classified as primarily synoptic, lake-effect, both, insignificant, remnant or unclear. Results show that roughly half of the November snowfall in the eastern sub-regions is due to lake-effect snowfall, but a precise adjudication is vexed by cases with lake-enhanced system snow and other instances of synoptic and lake-effect snowfall occurring within the same day. In terms of the decreasing November snowfall frequency, both synoptic and lake-effect snow days have decreased in recent decades. While the change in lake-effect snowfall dominates east of the lake, the regional proportional decrease of snow days with synoptic and lake-effect classifications is similar. This suggests that warmer Novembers reduce both the likelihood of snowfall as mid-latitude cyclones traverse the region and the frequency and/or longevity of lake-effect snowfall during the cold air outbreaks which follow.