Lately our fair city has been the recipient of continuous dustings of light snow evoking images of a Brueghel painting. In fact, Thursday was the ninth consecutive day with measurable snow, tying a record set only one other time (Feb. 23-March 3, 2007) in Madison’s history.
One might wonder if it is unusual to have such a parade of frequent light snowfalls. As it turns out, the large-scale setup that has brought our current snow streak is quite normal for the winter around here. Nearly every winter the western Great Lakes states experience extended periods in which the airflow in the middle part of the troposphere (about two to three miles above the surface) is from the northwest, with origins in arctic Canada.
Embedded within this northwesterly flow are barely perceptible vortices that are carried rapidly southeast. Such vortices are enhanced by passage over the Canadian Rockies in Alberta. These mid-level vortices are capable of creating rising air which, in turn, produces clouds and precipitation.
Since the air that is forced to rise is cold and quite dry, there is usually not sufficient water available to these so-called Alberta Clippers to create big snowfalls — just lots of small ones. It has long been our sense that the total seasonal snowfall in Madison (approximately 40 inches each year) is largely the result of many small snowfalls and very few more substantial ones. This is consistent with the prevalence of cold northwesterly flow that characterizes our winters.