Maryland's tornado risk: Conditions are riper, but rarity complicates outlook

Jun. 17—As the world reached 12 consecutive months of record global heat, the nine tornadoes that touched down earlier this month in Maryland raised the question of whether the state's residents can expect to experience the threat more often in the face of climate change.

Maryland set a single-day record June 5 when the National Weather Service rated six of the nine at EF1, the most ever of at least that rating, which measures wind speed and damage to trees and buildings.

Determining how Maryland's changing climate influences tornado formation is more complex than simply counting storms, but some researchers have sought answers by studying the components of a tornado. In much of Maryland, the number of days a year when the atmosphere is unstable enough for thunderstorms — a precursor to severe thunderstorms and therefore tornadoes — has increased by 20 to 39 over the past 40 years, according to research from the nonprofit Climate Central.

Additionally, the number of tornado-favorable days for stronger tornadoes has gone up by one day per decade since 1979 in much of Maryland, though not in the Baltimore region, according to Climate Central.

The findings are from national research that shows that while the frequency of tornadoes hasn't changed, the area in the Great Plains known as "tornado alley" has shifted along and east of the Mississippi River, according to Climate Central. The number of favorable days for stronger tornadoes has increased over the past four decades centered on parts of Arkansas, Mississippi and Tennessee. Parts of Kansas, Oklahoma and Texas have seen decreases.

The nature of tornadoes makes them hard to predict, even minutes before they form. Some of the same fundamentals can also limit long-term outlooks, said Climate Central meteorologist Lauren Casey.

"There's several reasons for that, mainly because tornadoes happen on such a small spatial scale, that our climate models can't get down to that granular of a level," Casey said. "Also, the period of record is very limited."

The National Centers for Environmental Information Storm Events Database includes tornadoes going back to 1950, but apparent historical trends can be misleading. That's especially true when it comes to the lowest-rated, or EF0 tornadoes, which appear to not have been recorded as frequently before the early 1990s, said Harold Brooks, a senior research scientist for the National Severe Storms Laboratory.

Map: Where tornadoes touched down in Maryland on June 5

The Enhanced Fujita Scale rates tornadoes from EF0 up to EF5. On June 5, six EF1 tornadoes peaked from 95 mph to 110 mph in Arbutus, Columbia, Eldersburg, Middle River and Montgomery County, two EF0 tornadoes peaked from 70 mph to 80 mph in Southeast Baltimore's Canton neighborhood and in Carroll County, and a tornado with unknown wind speeds touched down in Boonsboro in Washington County, the weather service said.

"The way we collect tornado data now is very different than it was 80 years ago, and so your trends can be hard to detect when you don't do things with the same label all the time," Brooks said. "You're always worried that anything you see any trends in the data may be a result of ... changes in practice, not changes in meteorology."

When you remove these lowest-rated tornadoes from the data in Maryland, trends still aren't easy to spot. The rarity of tornadoes in the state can make even small changes seem significant.

"I don't think there's much we can say in the mid-Atlantic about [tornado] trends at all. That doesn't say there aren't things changing," Brooks said.

Data for EF1 or above tornadoes shows that some of Maryland's largest spikes were in the 1990s, with a slight decline in more recent decades (not including tornadoes confirmed so far in 2024). Maryland's highest number of EF1 or above tornadoes on record in one year was 13 in 1996, according to the Storm Events Database.

Nationally, a region experiencing a spate of tornadoes in a single day, as Central Maryland did June 5, has become more common, according to a 2022 Climate Central study "Changing Thunderstorm Potential."

Previously, the most EF1 or higher tornadoes in Maryland in one day since the 1950s was five, which happened in 1979, 1995, 1996 and 2002. Maryland received four or more tornadoes in a day once in each the '70s and '80s and three times in each the '90s, '00s and '10s. June 5, 2024, marked the first time that six EF1 tornadoes touched down in Maryland on the same day since the beginning of the period of record in the 1950s.

Altogether, the United States records around 1,200 tornadoes a year. Larger datasets quantify the components that can lead to a tornado. Many variables contribute to tornadoes' formation, but two of the main ones are instability, when the air is moist and warm near the ground and dry and cooler above, and wind shear, when the wind's speed or direction is significantly different lower down than it is higher up. Both variables can create strong updrafts, and wind shear promotes rotation.

With climate change warming the planet, there are more days of high instability, especially in the eastern United States, Casey said.

Climate Central's thunderstorm potential study shows how Convective Available Potential Energy (CAPE), an ingredient in severe weather and tornadoes, has changed amid global warming. CAPE is a measurement of atmospheric instability, so the higher the CAPE, the more fuel is available for thunderstorms. Thunderstorms often have CAPE values around 1,000 J/kg (joules per kilogram). Above 4,000 J/kg represents "extreme atmospheric instability," according to the study.

It found much of Maryland has experienced a significant increase in the number of days per year in which CAPE is 1,000 J/kg or higher, with areas recording 20 to 39 more such days in 2021 than they did in 1979. The analysis also indicated parts of the Northeastern U.S., including Maryland, have seen "10-15 more days of high CAPE values during both spring and summer — prime time for thunderstorms."

What exactly this all means for the state, country and planet requires more research into how CAPE affects weather and into other weather components, Brooks and Casey indicated.

The rarity of tornadoes plays a large role in the complexity of the data, but also in public perception of the issue. The state Department of Emergency Management has various methods of notifying Marylanders of tornado warnings, including through social media and a text alert system in English or Spanish. Many in impacted areas June 5 also received wireless emergency alerts, courtesy of the weather service, that blared loudly on their phones.

While the fifth most densely populated state, Maryland faces a popular misconception that tornadoes don't occur in cities. Tornadoes' relatively small footprint — commonly around 50 yards wide with a damage path of 1 to 2 miles, compared with the broad swath of hurricanes — is what makes them a rare occurrence in more populated areas, rather than anything to do with whether an area is urban, suburban or rural.

Casey underscored the importance of people in the mid-Atlantic and Northeast knowing tornado safety protocols. The Maryland Department of Emergency Management outlines safety protocols on its website, including keeping windows closed, going to a small interior room (or a hall on the lowest floor of an apartment building), and watching out for debris.

"Even a weak tornado is capable; the winds are 100 plus miles per hour. So that's capable of doing real damage, taking down trees, and creating debris, throwing debris aloft," Casey said.