The number of tornadoes and severe storms in the U.S. so far this year has been well above average.
There were nearly 6,300 severe storms reported in May, many concentrated in the Midwest and Great Plains, according to preliminary data from the National Weather Service’s Storm Prediction Center. That includes more than 570 tornadoes, the most recorded in recent years. That number continues to climb as the center updates its data.
It was the second-most active May for severe storms since 2004. The most active remains May 2011, the month a massive tornado tore through Joplin, Missouri.
The storms have resulted in 39 deaths this year, according to the Storm Prediction Center. Tornadoes in Oklahoma have killed eight people and demolished homes and businesses. In Iowa, a large tornado crumbled five massive wind turbines and killed five.
A main reason for the activity was weather conditions that made it more likely for tornadoes to form, said Patrick Marsh, the chief of science and support at the Storm Prediction Center in Norman, Oklahoma.
Throughout the spring, a pocket of cold air called a “trough” put winds from the Southwest across the middle of the country, Marsh said. Winds from the Southeast carried warm, moist air from the Gulf of Mexico closer to the ground.
The combination created a good environment for tornado-producing storms known as supercell thunderstorms.
“We’ve had ingredients in place for supercell thunderstorms over the Plains for pretty much the entire month of May,” Marsh said. “And when you have that many of these kinds of thunderstorms, inevitably you will have an increase in tornadoes as well.”
Three tornadoes this year ranked EF4, which is the second-highest ranking on the Enhanced Fujita Scale. This spring produced higher-ranking tornadoes than in recent years, Marsh said. But, this system is also based on the amount of damage caused, rather than just the size and strength of a tornado.
“If a tornado producing wind speeds equivalent to an EF5 – so 205 miles per hour or stronger – if it never hits a structure that has an engineer rating to withstand 205 mile per hour winds, we can't assign the rating of an EF5,” Marsh said.
The increased number of tornado reports could also be due in part to more available data.
An advancement in radar technology more than a decade ago allows weather trackers to better detect weak tornadoes by radar. Plus, there are more potential eyewitnesses with cameras, thanks to smartphones, and more structures that can be in the path of a tornado.
These are all factors to consider when comparing current and historical data, said Zack Leasor, the Missouri state climatologist.
“It's really hard to disentangle, is this a greater number of tornadoes themselves? Or are there just more buildings that can be damaged by those tornadoes? Are there more people out that can see those tornadoes, report those tornadoes with their smartphones?” Leasor said. “All of these kinds of characteristics go into those trends as well.”
Quincy Vagell started storm chasing across the Midwest and Great Plains in 2014. He said he often sends reports of what he sees to the National Weather Service.
“Sometimes storm chasers may be the first people on the scene, especially if it's in a very rural area where there's not a lot of people,” he said.
The activity this season allows him to choose which storms to chase. He said he tries to stay closer to Oklahoma, where he’s based.
“I've only seen, actually, two or three tornadoes this year, but I've had an active season,” Vagell said. “I've seen plenty of supercells, great storms and all that. So it's definitely been a busy season for me.”
A more active spring season for tornadoes is difficult to link directly to climate change, according to Marsh. However, warm and moist air in the wintertime could cause more tornadoes in off-peak times, he said.
Warmer weather earlier this year has opened the door to a longer season for severe storms in Missouri, Leasor said. There have been 184 tornado warnings in the state this year so far. This is the fourth highest number of warnings for that timeframe since records began in 1986.
Marsh expects storm activity to be calmer in early June. Beyond that, it’s more uncertain, he said.
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest. It reports on food systems, agriculture and rural issues.
