A snow plow contractor starts his day in Toledo, Ohio, with a sunny sky. This seems like a quiet day until a call comes in from Cleveland saying they are getting 2 inches per hour snowfall rates and blizzard-like conditions. What could cause this extreme, abrupt change in such a short distance? The short answer is a phenomenon called lake-effect snow.

As the winter season kicks into high gear, lake-effect snow begins to take shape across the Great Lakes region of the United States. Snowplow contractors that work around the Great Lakes know the variability and challenges lake-effect snow produces in just a short time period. However, even if you live away from the Great Lakes, it is still worth understanding how lake-effect snow works as it can impact areas well downwind of the shore, up to 500 miles away from the lake.

While most in the U.S. associate lake-effect snow with the Great Lakes, this process can occur with any larger body of water (especially with any lake that has a fetch of 50 miles or greater). Other bodies of water that experience the similar phenomenon include the Kamchatka Peninsula in Russia and areas near the Great Salt Lake, Black Sea and the Caspian Sea.

Meteorologists look for signs of lake-effect snow when there are two main factors taken into account. These are the temperature difference between the lake surface and the air mass above it and wind direction.

1. Temperature difference

One of the key properties of any body of water is the high latent heat capacity (meaning it takes longer for the temperatures to change). Therefore, as the air temperature turns colder heading into the winter season, the water temperature still remains relatively mild. This temperature difference between the air above the ground surface and water helps set the stage for the lake-effect machine during the late fall months.

When a cold, Arctic airmass passes over, the warmer water conducts and convects heat upward, warming the air just above the lake. The temperatures gradient (difference) between the air right above the water and the temperature of the air mass around 5,000 feet above lake will then determine if conditions are favorable for lake-effect snow and how much. The general rule of thumb is a 13-degree Celsius temperature difference between the air and the water. The greater the temperature difference, the higher potential there is for a major lake-effect snow event.

2. Wind and the lake-effect snowbelt

The orientation of where a snow band will ultimately set up is determined by the direction of the wind and the distance of the water in which the wind travels over (also known as the “fetch”). As a general rule of thumb, a fetch greater or equal to 62 miles over open water typically leads to the most significant lake-effect snow events. The two most common fetches are short and long. A long fetch is defined as wind that travels over water over a long duration of time, and a short fetch is the wind travelling a short duration of time over water.

The most common wind direction across the Great Lakes is from the northwest. This often results in the southern and eastern portion of the Great Lakes regions receiving the brunt of the lake-effect snow. Lakes Superior, Huron and Michigan climatologically experience a shorter fetch, with the longer fetch and greatest potential for substantial lake-effect snow originating from Lake Erie and Ontario.

Wind speed also plays an important part in the development of lake-effect snow. With lighter winds, the snow maximum will be closer to the lakeshore. The opposite happens when there are stronger winds. This allows the lake-effect snows to travel much further inland — as much as 250 to 500 miles away.

Make it stop!

As we have talked about earlier, lake-effect snows are most intense through the late fall and early winter seasons (November through February). Once we get into the heart of the winter season, lake-effect snow can diminish completely. When plowing snow around the area, you can look for some particular signs to watch out for that may signal the end of lake-effect snows for the winter season.

The main signal is when the majority of each lake freezes over. This shuts off the ever-important conduction and convection process because the necessary moisture and instability is no longer available for lake-effect snow formation. Each lake is different as the depth of the lake plays a factor as well. Lake Ontario may experience little to no freezing during an entire winter season. However, most times, just the cooling of the lake water by February is enough to drop the temperatures difference to less than 13 degrees Celsius.

In the U.S., most do not ever have to worry about lake effect snow. But for those who do, it is the wildcard of the winter with the ability to change conditions quickly over such a short area.

Knowing how it forms and ways to track it will help you keep operations running smoothly and efficiently through the heart of the winter.