Weather Words: Snownado

While most associate tornadoes or dust devils with spring, warmth and humidity, tornadoes can also occur during winter, even with snow and temperatures near freezing.

These snow tornadoes are referred to as “snownadoes” and can take on several different forms.

The first way a snownado can form is a “snow-devil,” the snow equivalent of a dust devil. These snow devils can form under mostly clear conditions and are the result of converging winds with different speeds and directions.

This convergence of winds leads to a localized area of increased spin, or vorticity, near the surface. This area of spin can then pick up snow, spinning it into a weak vertical column of spinning air.

Snow devils are relatively common and do not pose a major threat to people or property.

The second way a snownado can form is as a waterspout, with a very cold air mass moving over a warm body of water. Because the warm water heats the air directly above it, this air becomes buoyant and rises through the cold air above it. When this region of rising air encounters a region of localized higher vorticity or spin, a water spout can form, even with sub-freezing temperatures and snow.

These waterspouts can then move over land, briefly becoming de facto tornadoes, in this case snownadoes. The mechanism by which snownadoes form is common during lake-effect snow events, and has been observed around the Great Lakes, particularly during heavy lake-effect snow events.

During winter, these landspouts often occur with snow squalls, or the winter equivalent of thunderstorms (this topic will be discussed more in depth in the next section). Once a landspout forms over snow, it often remains weak but does reach the technical definition of a snownado.

The final, and most rare circumstance by which a snownado can form is in a similar mechanism to a traditional tornado.

During winter, subfreezing temperatures make thunderstorms rare, but when they do happen, they are referred to as snow squalls. Snow squalls occur when there is a temperature imbalance between the land and a very cold air mass above it.

When the land or air at the surface is relatively warm, compared to the pocket of cold air above it, snow squalls or convective snow showers can form. These snow squalls are the winter equivalent of summer thunderstorms and can even have thunder.

In exceptionally rare circumstances, these snow squalls can overlap with favorable changes of wind speed and direction with height or wind shear. When this occurs, a snow squall can develop into the winter-time equivalent of a supercell, even having a weak hook echo and other features similar to a supercell.

Once these criteria are met, a brief or weak tornado can touch down from the snow squall. These tornadoes can be wrapped in snow and graupel, thus meeting the official definition of a snownado. An example of this occurred in Southern Ontario in November 1983. Check out this article by NOAA for more details.

Hayden Marshall is a meteorologist intern and First-Year-Master’s Student at the Georgia Institute of Technology. He has been following weather content over the past three years as a Storm Spotter and weather enthusiast. He can be found on Instagram and Linkedin.