As arctic air masses begin to visit the Midwest more often over the coming winter months, the chance for lake-effect snow increases for residents along the Great Lakes. Lake-effect snow can turn a bright and sunny day into a near white-out conditions in an instant, dumping snow on the order of feet. But how does lake-effect snow form, and why are some events so intense?
The first thing needed for any lake-effect event is well…..a lake! Throughout the summer, the Great Lakes will absorb the increased energy from the sun, raising their temperatures into the late summer months. The lakes will retain their heat and slowly cool then into the winter. During the late fall and early winter months, the surface temperature of the lakes can be much warmer than the air over top of them. That temperature difference will come into play later.
As arctic air masses move into the region, cold air moves in behind a cold front. The cold front is a source of rising motion, coupled with the moisture of the lakes, generates unstable conditions. With instability comes clouds and showers, which cools temperatures aloft. Cold air moving over the lakes then absorbs moisture from the warmer surface. This allows the air to condense and instability to increase.
While the word fetch is more synonymous with the movie Mean Girls, it also doubles as a meteorological term. A fetch is the length that air must travel over to reach from one side of the lake to another. The longer the fetch, the more time air flowing over the lakes will have to absorb more moisture before reaching the other side.
With moisture-rich and moderately unstable air now to the other side of the lake, one last ingredient is needed. Friction. Air will naturally slow down due to the friction over land, causing the air to “pile up” and rise, creating snow squalls downwind from the lake. As winds stay in a constant direction, this process amplifies with time and snowfall rates can quickly rise.
Lake effect events can last one the order of hours to days depending on how persistent the winds are. Most events will end when the flow over the lakes changes direction, cutting off the moisture feed and decreasing the instability. Make sure to watch out for Winter Storm Warnings issued by the National Weather Service, and to avoid driving into lake effect bands, as any can become dangerous and difficult when a heavy band sets up over a region.