Every day that passes, the calendar draws nearer to spring — peak tornado season for much of the central and eastern United States. And atmospheric scientists have cast their predictions for tornado season 2020, calling for a near to slightly more active season than average. There are also signs that peak periods of storminess could ramp up earlier in the year than during some previous seasons.
Victor Gensini, a professor of atmospheric sciences at Northern Illinois University, said he gently leans toward an above-average tornado season.
Genisini rose to prominence in the field of seasonal forecasts for severe weather in recent years, most recently predicting 2019′s record tornado onslaught nearly four weeks in advance. He specializes in determining how “teleconnections,” or the presence of distant and seemingly tangential atmospheric features, can have impacts on weather closer to home.
An average to above-average season
This year, he’s forecasting bouts of severe weather to begin increasing in frequency and intensity during March and April. May looks to be a busy month as well.
“If the current pattern doesn’t change significantly, we could be up for … periods of intense activity, followed by shutdowns,” he explained. Predicting exactly when those busy spurts will line up is a bit more challenging. “It’s going to coincide with where we are in the sub-seasonal cycle.”
Gensini alludes to smaller-scale, shorter-duration atmospheric disturbances that can bolster or inhibit storminess over the Lower 48. In past years, there have been larger, more obvious ingredients in place. But not so much this year. The atmosphere will be a bit more fickle.
“Last year it was very obvious when the stars would align,” Gensini said. “I think usually by March 1, things become pretty clear. Are there any sort of ‘big players?’ I kind of think of [the atmosphere] as a big orchestra … all the instruments playing together in an ensemble. Some years, you have some instruments playing louder than the others and controlling the narrative and the tune.”
John Allen, a professor of meteorology at Central Michigan University, says the “big player” is usually from El Niño or La Niña.
“If you think about how these systems are connected, we’re thinking here about a cascade of scales,” said Allen, discussing the link between large-scale climate systems and the comparatively fleeting nature of tornadoes and severe thunderstorms. “El Niño interacts on a scale of several thousands of [miles], with interactions going through a middle man.”
That middle man? The jet stream.
The jet stream’s role
In order for supercells — rotating thunderstorms — and tornadoes to form, two ingredients must be present in the atmosphere: instability, and wind shear.
An unstable atmosphere, caused by vertical temperature contrasts, results in lifting motion that can give rise to thunderstorms. To get those storms to spin, you need wind shear — a change in wind speed and direction with height. And when the jet stream is nearby, there’s usually plenty of wind shear present. The jet stream is a swift current of rapidly moving air in the upper atmosphere. Jet stream speeds can occasionally top 150 mph.
As winds near the ground generally don’t approach speeds of that magnitude, there’s a significant change of wind speed with height in the vicinity of the jet stream generating wind shear.
Disturbances embedded within the jet can trigger bouts of severe weather, since jet stream meanderings usually carry cold air aloft. That’s a key factor in thunderstorms. The wind shear imparted by the jet, meanwhile, can encourage storms to rotate.
Connection to El Niño and La Niña
Long-range severe weather forecasts don’t start in Tornado Alley. In fact, they don’t even start in the United States. The first place to look? The Pacific Ocean.
Water temperatures in the tropical Pacific Ocean can have enormous bearings on our weather. Water temperatures there fluctuate in response to the El Niño Southern Oscillation, or ENSO. During El Niño, water temperatures over the eastern equatorial Pacific run anomalously warm; those same seas are atypically cool during La Niña.
ENSO, sort of like a pendulum, can swing irregularly between the two states every few years. Knowing a bit about the status of El Niño or La Niña generally offers insight as to how the jet stream will behave.
The warm East Pacific waters that occur with El Niño help steer moisture from the subtropics across the Baja Peninsula, and eventually over the northern Gulf of Mexico. That can increase low cloud cover and lower temperatures. Meanwhile, the energized polar jet stream is diverted far to the north, where it’s far removed from the key zones for severe weather. Without the polar jet’s added wind shear, hail and tornado chances on the Great Plains decline. Allen and his team showed this in 2015.
La Niñas, on the flip side, bring warmer and drier conditions to the southern United States, and colder weather to the north. That temperature clash can brew severe storms. And a polar jet lurking nearby can help spin up tornadoes — especially in the eastern Plains.
This year, however, ENSO looks to be “close to neutral,” according to Allen.
“ENSO is a dominant mode for most seasonal forecasts,” Allen said. “[This season] you don’t have the main predictor that’s pulling the strings.”
More subtle weather features may dominate this season’s forecast
With no dominant ENSO influence, the focus shifts to other features.
Those more subtle features are a lot more finicky and tougher to predict. Plus, they’re much more difficult to spot from a distance. Gensini, however, already has his eyes on something.
“There’s a lot of convection anchored near the equatorial international date line,” said Gensini, referencing a batch of showers and thunderstorms in the central Pacific. It’s tied to a region of above-average water temperatures. “If that warm pool continues into the spring, I’d have reason to consider an average to an above-average season.”
The warm pool could influence the position and strength of the jet stream enough to shape the Lower 48′s severe weather season.
“The positioning of the jet stream across the U.S. is vital for springtime severe weather frequency,” Gensini said.
Gensini and Allen alike hope these predictions will help emergency managers better plan and prepare for dangerous bouts of severe weather.
“FEMA could benefit in moving around assets or resources,” said Gensini, on the benefits of seasonal tornado forecasts. “Insurance companies could model potential losses. These forecasts are targeted toward educators, administrators, [emergency managers] — the folks that are sort of spreading the message and creating a weather-ready nation.”