Behind the Forecast: Lake-effect snow

LOUISVILLE, Ky. (WAVE) - Lake-effect snow is commonplace during the late fall and winter across the Great Lakes.

Lake-effect snow occurs when cold air travels over the relatively warm waters of the Great Lakes. The air rises, forming narrow bands of clouds that can produce two to three inches of snow or more.

The wind direction and the geography of the land and water are vital to what locations receive lake-effect snow. The bands of clouds are so narrow that the sun may be shining in one area while another just a mile or two away may be dealing with blinding, heavy snow.

For lake-effect snow to occur, an unfrozen bay or lake about 62 miles wide or larger is needed. Air has to travel over a greater distance across a long lake, enhancing the convection. If a lake is frozen, the air passing overhead would be unable to pick up moisture from it. There must also be a temperature difference of at least 23°F between the lake and the nearby land; the greater the difference, the more moisture the air can absorb. Light winds are also necessary; a wind that is too strong (over 30 MPH) is limited in the amount of moisture it can evaporate from the lake’s surface.

Let’s break down step by step how lake-effect snow forms. Below-freezing air must drift over a warm lake or bay, evaporating water into the cold air. As it picks up moisture, the cold air warms, becomes more humid (and less dense), and rises. As the air rises, it cools and the moisture condenses, forming clouds. Eventually, the air reaches the land, and increased friction forces it to slow down and pile up, leaving to more air rising in the atmosphere. That air cools, even more, creating more clouds, and dumping more snow on land.

Usually, the water picked up from a lake doesn’t travel more than 25 miles before falling as snow. However, it can sometimes travel up to 100 miles away!

Lake-effect snow is most common from November through February over the Great Lakes when an area of low-pressure passes by, allowing for arctic air to rush out of Canada.

Lake-effect snow can be difficult to forecast because slight changes in wind direction can lead to significant changes in which locations get the most snow.

Lake-effect snow is different from lake-enhanced snow. Lake-enhanced snow occurs when a passing weather system’s snow or rain is enhanced by Great Lakes’ moisture. So existing precipitation moves over a lake and is enhanced like what we saw on Tuesday.