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.
Filed under: Uncategorized
One of the most frequent reports of damage that results from severe thunderstorms is hail damage. Chunks of ice falling from the sky, hail has the ability to damage homes, vehicles, plants, and anything else in it’s way. Reaching sizes bigger than softballs or grapefruits, hail is a definite danger in severe thunderstorms.
Formation
Hail is formed when three things happen:
- A thunderstorm grows tall enough that a significant portion of it’s clouds are below freezing.
- There is enough shear (change in wind speed) in the atmosphere to “tilt” the storm slightly off vertical.
- The storm develops sufficiently strong updrafts.
The first reason is fairly obvious: hail is made of ice, and to make ice, water has to freeze. The more of the storm that is below freezing, the more likely it is for water to freeze. The second reason is that when storms become titled, the updrafts, or winds moving upwards through the storm, can last much, much longer than if there was no tilt to the storm. This ensures that there will be an updraft long enough to develop hail. The last reason is simple physics: hail weighs a lot. Raindrops can split apart and break, hail does not. So as the size of the hail grows, the updrafts need to be sufficiently strong to keep it in the air. Fortunately (depending on your view), updrafts in some storms can exceed 100 km/h, which is strong enough to grow hail to damaging sizes.
Structure
Hail is often white in appearance, very hard, and has a concentric structure.
Common Damages
The most common reports of damages from hail are property damage (roofs, windows, cars) and crop damages (large hail can completely decimate a farmer’s crop). Although infrequent, there have been reports of personal injury due to being struck by large hail while outside. I cannot stress enough that in some severe thunderstorms, hail can grow to the size of a grapefruit. This is traveling extrmely fast (over 150 km/h) when it reaches the ground. You do not want to be hit by large hail. Indirect damages also occur from hail, one of the most common in cities being storm drains that are blocked by a large quantity of hail which cause localized flooding.
Not every thunderstorm produces hail. The number that produce hail bigger than a dime is even less. And the number that produce hail bigger than a quarter is even less. And the number that produce hail bigger than a golf ball is even less.
Most hail is around the size of frozen peas. However, with the capability to grow to enormous sizes and produce significant damage, hail is a important weather phenomenon to be aware of when severe thunderstorms strike.
