Zones of Origin for Great Lakes Cyclones in North America, 1899–1996

Abstract

The zones of origin for all cyclones that traversed the Great Lakes region from 1899 to 1996 are analyzed using a digital daily record of central pressure and location for individual cyclones. Plots of latitude of formation show that Great Lakes cyclones form (or reform) east of the Rocky Mountains at all latitudes between 25° and 65°N. In winter, about the same number of cyclones originate to the northwest as to the southwest of the Great Lakes region. In spring, the southwest zone is dominant. The number of summertime cyclones is greatly reduced, with the west zone of origin most active, while the fall plot displays a transition between the summer and winter distributions. The proportion of strong Great Lakes cyclones that originate in the southwest zone is greater than for all cyclones; however, the seasonal shifts in the latitudinal distributions of origin in the two datasets are similar. An analysis of differences in frequencies by zone of origin for Great Lakes cyclones during months characterized by positive and negative Pacific?North American (PNA) index patterns reveals a statistically significant relationship between the midtropospheric flow pattern and cyclogenesis. The results indicate that the number of cyclones per month for the positive (PNA index > 0.5) and negative (PNA index < ?0.5) categories are approximately equal and that the combined frequencies for positive and negative PNA pattern categories for the northwest, west, and southwest zones of origin are similar. The study supports the intuitive assertion that more Great Lakes cyclones originate from the northwest during months characterized by positive PNA index values than the negative pattern while more cyclones from the west and southwest are associated with the negative PNA index pattern than the positive one. Approximately 20% of the cyclones that traversed the Great Lakes from 1899 to 1996 originated in the region. The most noteworthy and puzzling finding of the study is that cyclogenesis over the lakes as a proportion of cyclone presence in the region is highest in the summer months. This result corresponds with the finding that cyclones traversing the Great Lakes region in May?July accelerate as they approach the region and increase their rates of deepening over the lakes.