A Spectacular Tropical Storm-Like Meso-Low Over Lake Superior

A small-scale "meso-low" formed in the cold Arctic air flowing over the relatively warm waters of Lake Superior on Sunday, and had a remarkably spectacular tropical storm-like appearance on radar and satellite imagery. As the meso-low moved south over Michigan's Upper Peninsula, it brought sustained winds of 20 - 25 mph and bands of moderate snow that dumped 4 - 7 inches of snow across the region. The snow was dry and fluffy, with a ration of 25:1 between the depth of the snow and depth of the equivalent melted water (a 10:1 ratio is more common in major snowstorms.) The winds in Marquette, Michigan increased from 10 mph to 23 mph with gusts to 32 mph as the meso-low moved over at 4 pm. In nearby Munising, the pressure increased from 1005 mb to 1015 mb in four hours after the low moved through. As documented in a 1984 paper by Dr. Greg Forbes, "Mesoscale Vorticies over the Great Lakes in Wintertime", these type of lows are not uncommon over the Great Lakes in wintertime, with an average of three appearing each winter. They arise in response to the difference in heating between the land and the lake when there is a strong contrast in temperature, and do not occur when the lakes are ice-covered.


Figure 1. From the NWS Marquette Facebook page: "In this image a meso-low can be seen (shown in the blue circle), which is a smaller scale low pressure system that commonly forms on the Great Lake during the winter time. This is primarily what has been responsible for the strong gusty winds and heavy snow bands that have been pushing into areas along the Marquette and Alger county shorelines. The system even has sort of an awesome eye near the center of rotation. This image was take around 1:30 pm EST."


Figure 2. Radar image of the meso-low after it moved ashore over Michigan's Upper Peninsula.


Figure 3. The pressure trace (2nd graph) from Munising, Michigan showed a sharp rise in pressure from 1005 mb to 1015 mb in four hours after a meso-low moved through.

Links
A radar loop of the meso-low as it approached land. Thanks go to wunderground member StAugustineFL for saving this radar loop.

The University of Wisconsin CIMSS Satellite Blog has an in-depth discussion of the meso-low.

Jeff Masters