What caused rare snow mesovortexes across Lake Michigan?

Just under three feet of snow blanketed cities in northern Indiana last week thanks to an impressive system of snowy whirlwinds that formed over Lake Michigan.

Residents of the Great Lakes region are no strangers to heavy snow in January — but Friday’s system wasn’t a run-of-the-mill snowstorm.

That’s because the snow came from a rare band of low-pressure systems called mesovortices, also known as mesovortexes, that formed over Lake Michigan. More than a dozen arranged themselves into a tight band with a braided appearance, ultimately dumping a massive amount of lake-effect snow.

So, what exactly happened on Friday, and why were meteorologists so amazed?

Lake-effect snow is no rare thing in the regions surrounding the Great Lakes. It falls when cold air from Canada moves across the Great Lakes, forming clouds that can produce two to three inches of snow per hour.

The severity of the lake effect is tied to how warm and ice-free the lakes are, as well as the difference between the Canadian cold air and the lakes themselves, according to research from Tom di Liberto with the US National Oceanic and Atmospheric Administration.

While lake-effect snow is par for the course, Friday’s system wasn’t your average weather event.

On Friday, more than a dozen mesovortices formed in a braided band over the lake.

Two complimentary weather conditions caused this intense string of lake effect snow, said meteorologist Kyle Brown from the National Weather Service in Northern Indiana.

Those factors were winds from both the northwest and northeast that pushed the systems into a single band, as well as a just-right wind speed that was fast enough to further propel the whirlwinds but slow enough not to break them apart.

“Therefore, we were able to create this kind of delicate environment where these mesovortices were able to spin up in much like a similar process to how a thunderstorm would kind of bubble up over a hot sunny day on the plains, for example,” Mr Brown told The Independent.

These winds caused the band to tighten, said TJ Turnage, science and operations officer for the National Weather Service in Grand Rapids, Michigan.

“The key point is that you had winds blowing in different directions, really somewhat opposite directions, on either side of the band,” Mr Turnage told The Independent.

This led to a “roll-up” effect on the mesovortices.

“Imagine if you had a rag between your hands, and you run your hands back-and-forth in opposite directions,” he continued. “That rag could roll up — and that’s what happened on Friday. But instead of one, it was a series of them.”

Just-right temperatures also helped fuel the intensity of the mesovortices — the lake was just above 0 degrees Celsius, while the air above the lake was near –20C.

“That’s a really large temperature difference over a short layer of the atmosphere,” Mr Brown said.

Friday’s system was not unprecedented but quite rare, according to Mr Turnage.

What made the phenomenon special was the delicate factors that came together.

“Yes, we can get lake effect snow but the ingredients really have to come together just right to get as strong of a lake effect snow band as we had for this event,” Mr Brown said.

“The reason they don’t always show up is because you really do need a pretty specific set of circumstances with the way the winds are,” Mr Turnage added.

Friday’s system was more dangerous for drivers than the average lake-effect snowstorm because it was made up of many small mesovortices, rather than a single dominant system.

With clusters like this, snowfall rates can change drastically over a short time and distance, Mr Turnage said, making conditions unpredictable on the roads.

“The upshot is: It is rare, and what we saw is something that a lot of people haven’t noticed very often,” Mr Turnage said.