This weather phenomenon on Lake Michigan stunned meteorologists Friday

This weather phenomenon on Lake Michigan stunned meteorologists Friday

An extremely localized but intense band of lake-effect snow slammed northeast Illinois and northwest Indiana on Friday, dropping nearly three feet of snow in a narrow swath only a few miles wide. Travel was halted as roadways became buried, with near whiteout conditions. But there was something a little special with this particular snow band — it was made up of more than a dozen wintry whirlwinds.

These whirlwinds are known as mesovortexes (spelled mesovortices in academic literature). They are small eddies about 10 or 20 miles wide and akin to tiny low pressure systems. Each one locally intensified snowfall and was separated by a small moat or cutoff.

On radar and satellite imagery, the conga line of snow showers traced repeated loop-de-loops. When they appeared, meteorologists and weather enthusiasts expressed their amazement on X, formerly Twitter:

• “I have never seen such a great example of mesoscale vortices down Lake Michigan as this one here. WILD!!” wrote Tom Niziol, winter weather expert for FOX Weather.
• “Lake-effect mesovortices are always fascinating, but I can’t remember another sequence of them as amazing as this one,” wrote Stu Ostro, a meteorologist for the Weather Channel.
• “These LES [lake-effect snow] mesovortices can’t get any more sexy,” wrote Victor G., a storm chaser in northern Illinois.

They weren’t just impressive visually — they also dumped a lot of snow. Michigan City, Ind., picked up 35 inches of snow, which fell just 34 miles west of South Bend, Ind., which reported only 6 inches.

Snowfall rates of 2 inches or more per hour at times which hit some of the same areas repeatedly.

The radar animation below, posted to X, showing the evolution of the curled snow squalls and their serpentine shape is mesmerizing:

They key to their formation was convergence, or a gathering of winds near the ground.

All lake-effect snow forms via the same basic premise — cold air blows over a relatively warmer body of water. The water heats the air from below and adds moisture. Those heated plumes of humid air billow upward into the colder atmosphere, releasing moisture in the form of snow. The greater the temperature difference between the air aloft and waters below, the heavier the resulting snows.

But when winds converge, the air is forced up faster and can be sculpted into the mesovortexes that formed Friday.

“These convergent winds caused a single, long, north-south oriented snow band,” Robin Tanamachi, an associate professor of atmospheric sciences at Purdue University, said in an email. “The converging winds were also angled, behaving like two hands rubbing a rope of dough.”

Because air streams were converging at slightly different speeds and from different angles, they “chaffed” against each other, spinning up the series of whirls.

“Imagine bringing your hands together around a glob of dough, and then moving them in opposite directions,” Tanamachi said. “You end up with a vertically oriented dough ‘rope’ rotating between your hands. The converging winds did something very similar all along the length of the snow band, creating the meso-vortices.”

Configurations like this in the atmosphere are far from unprecedented. They’re quite common in places that experience von Karmen vortex shedding, a process which produces alternating mesovortexes induced by wind curling back on itself downwind of an obstacle or high terrain.

An example of von Karman vortexes, downwind of Tristan da Cunha, a remote volcanic island in the South Atlantic, in 2017. (NASA)

More localized convergence processes have produced rotating snow squalls over Lake Tahoe and given rise to waterspouts in significant lake-effect snow bands over Lake Erie off Buffalo.

In the springtime, convergence bands can give rise to “landspouts” — tornado-like whirlwinds that form because invisible eddies of surface wind, beneath towering clouds feeding off gathering air.