Then they joined.
“45 years of combined meteorology experience on this shift and … yep … this is a new one,” tweeted the National Weather Service in Kansas City. Some Twitter users joked at the meteorological rendezvous, one writing “did [the storms] just kiss?” Another wrote that the storms were clearly not social distancing.
A wacky setup
The atmospheric intimacy is as spectacular dynamically as it is compelling to watch on satellite. And while two colliding thunderstorm clusters is atypical, ’tis the season for irregularly-moving thunderstorms.
That’s because it’s June, and large thunderstorm complexes, known as mesoscale convective complexes, run rampant. With weaker upper-level winds than during April and May, thunderstorms that develop over the Plains oftentimes lack the organization needed to attain rotating supercell structure. But toasty June temperatures and building humidity provide the fuel to sustain evening storm growth and expansion.
Moreover, when the sun sets, winds at the mid-levels of the atmosphere tick upward a bit — allowing thunderstorms to “vent” their spent air and continue booming during the night. Since steering currents are weak, however, mesoscale convective complexes tend to have motions that are difficult to predict.
While the majority have some eastward component to their motion, not all do. Others propagate along boundaries in search for the greatest instability, or fuel. And once in a while, an environment can support two massive storm clusters moving in different directions.
That’s what happened Wednesday night over Kansas City. While the forward-moving storms from the west were behaving so-to-speak, storms from the east expanded backward. And it was all thanks to a front parked right over the Kansas City area.
“We were looking at this convergent boundary draped across the area, and that kind of allowed the two to connect and continue that upward growth,” explained Sarah Atkins, a meteorologist at the National Weather Service in Pleasant Hill, Mo. “They actually merged around two thirty in the morning, and that’s when we ended up getting a couple brief tornadoes.”
So far, two have been confirmed — one on the southeast side of the Kansas City metro area, and the other farther south in Cass County, Mo. Both were rated EF0s.
Tornadoes require a change of wind speed and direction with height. When the low-level cool, wet air from the back-building storm cluster swept westward, it brought in some easterly winds near the surface. That enhanced those varying winds with height, known as wind shear, along the leading edge of the forward-moving storm complex. That in turn allowed a few areas of rotation to develop.
Around the same time, a disturbance at the mid-levels of the atmosphere provided additional support for storm sustenance.
“We had a shortwave trough moving through,” said Atkins. “It made itself known all the way down to the Missouri-Arkansas border.”
She couldn’t remember having forecast an event like this before, but did note that once in a while, oddities like this can happen.
Forecasting the unusual episode
As one can probably imagine, predicting episodes like this before they occur is equally challenging, if not impossible. Atkins described the effort as “a tough one.”
“We did have a bit more confidence with the complex we thought would come out of Nebraska,” she noted. But one model — the NAM NEST — hinted that something a little extra may sneak in from the east.
“The NAM NEST was showing this yesterday, and we were talking about this … we said ‘that wouldn’t be fun.’ At the end of the day yesterday, I started noticing the [high resolution model] was converging toward that solution as well.”
A similar event happened on June 4, 2018, over Central Texas.
Merging storm complexes aren’t rare everywhere
Steve Nesbitt, a professor of atmospheric sciences at the University of Illinois, explained that these sorts of occasions are unusual in the United States — but occur more frequently farther south.
“This is something that happens quite commonly in South America,” explained Nesbitt. “We actually see this quite commonly in the lee of mountain ranges, especially the Andes.”
A major contributing factor? The undular bore, or a type of wave that ripples through the atmosphere in a direction that counters the prevailing flow.
“You can observe that sometimes in front of ships,” said Nesbitt. “[Wednesday] night if you looked at the radar, you could see a couple of boundaries in a line. [These events] can be responsible for heavy precipitation events. … It dumped six inches of rain in north central Missouri.”
The overarching atmospheric setup which helped fuel these explosive storm complexes is sometimes referred to as “a ring of fire.” It refers to a zone of the jet stream that curls around a dome of hot high pressure to its south. This zone divides hot air from much cooler air to the north and is the breeding ground for vigorous thunderstorm complexes.
In addition to the storm complexes that erupted in the Midwest, this volatile zone has also triggered violent clusters of storms in the Great Lakes and Northeast, notably the derecho which blasted through the Philadelphia area on Wednesday.
Jason Samenow contributed to this report.