Lake-effect snow squalls are a fairly important meteorological phenomenon in the fall and winter for those who live near large bodies of open water, such as the Great Lakes in Ontario or Lake Manitoba and Lake Winnipeg in Manitoba. It can bring heavy snow (potentially feet of the white stuff) and near-zero visibilities to those who experience it.
So What Is It?
Lake-effect snow is a very particular form of snowfall that is generated through a combination of cold temperatures and large bodies of open, unfrozen water with a wind that blows over it for a long distance. The result is narrow, but intense, bands of snow downwind of the body of water that can dump snow at rates in excess of 5 cm/h with near-zero visibilities or whiteout conditions.
Ingredient 1: Instability
As I’ve mentioned before, instability occurs when air near the surface is lighter than air above it, which causes it to rise. This is very important in the creation of lake-effect snow. Temperatures 1.5 km off the surface need to be about 13°C colder (or more) than the temperature of the water. This difference ensures that there will be ample instability for the snow to form.
Ingredient 2: Fetch
Fetch is the distance that an airmass will travel over a body of water. Since lakes come in all sizes and shapes, the direction that the airmass is traveling can drastically alter it’s fetch.
Typically, a meteorologist will look for a fetch of at least 100 km or more when trying to diagnose lake-effect snow. A longer distance is preferred because it gives more time for the lower levels of the atmosphere to become saturated with water vapor from the open water it’s traveling over, which increases the amount of snowfall generation.
Ingredient 3: Wind Shear (…or Lack Of)
Wind shear is a measure of how the wind changes with height and is made up of two things: the direction of the wind and speed of the wind. The greater change in direction or speed, the greater the wind shear. For lake-effect snow, weak directional shear is favored for intense squalls; this means that it’s preferred that the wind be from the same direction as you move from the ground to higher in the atmosphere. If the wind changes 30° or more from the surface to about 3 km above, development of lake-effect snow can be drastically hindered. If the wind changes 60° or more, heavy lake-effect snow will not happen.
Speed shear is important, but not as important as directional shear. Ultimately, as long as the wind speed difference between the surface and about 3 km above is less than 40 kt (or ~ 75 km/h), lake-effect snow generation will be unhindered.
Some Other Factors to Formation…
Large-scale forcing can enhance the development of lake-effect snow, through large-scale lift (by vorticity advection) and the advection of cold air, such as behind a cold front. Any upsloping terrain that the air moves over once it exits the body of water can also enhance snowfall. Also, any moisture added to the air before it enters the body of water in question will also be beneficial, as it will require less moisture input to create snow if it is already moist.
When Does It End?
As a lake gradually freezes over, the amount of lake-effect snow that it is able to produce is reduced. Ice diminishes the production by two factors: it reduces the amount of fetch over open water, and as water cools, it reduces the amount of energy available to produce snow.
And The Risks…
The biggest effects from lake-effect snows are often felt by the transportation sector. The large quantities of snow can make roads impassable, and the intensity of the snow can often produce whiteout conditions making driving incredibly hazardous.
Wasaga Beach after 60 cm of snow fell in 12 hours.
Lake-effect snow squalls can drop feet of snow in very localized areas and can last anywhere from a few hours to days on end. Buffalo, NY once recieved over 2 meters of snow over the course of 4 days due to these squalls. They are primarily a late fall and early winter phenomenon with the end-date to the season set by how soon waters begin to ice up.
