Seasonally and Diurnally Varying Cold Front Effects along the Minnesotan North Shore of Lake Superior

Cold fronts are typically associated with cooling, drying and a strengthening wind that shifts to have a northerly component. Cold front effects at a particular point, however, are dependent upon pre-existing air mass characteristics. Here, we examine 634 passages of synoptic-scale cold fronts in northeastern Minnesota from 2010 to 2018. While these fronts are associated with the expected effects in some areas, they are often associated with warming and enhanced drying in the region directly influenced by an air mass from Lake Superior (coastal sites). Coastal sites experience warming during more than half of cold frontal passages, in contrast to proximate inland sites out of the influence of the lake. This warming, combined with a removal of the moist lake air mass, often leads to a sharp post-front decrease in relative humidity. These relatively unusual local effects indicate a need to carefully consider characteristics of the lake air mass and likely changes during cold frontal passage when forecasting regional temperature and fire weather conditions.