The month of March is known for its extremely changeable weather as warm spring conditions compete to replace the cold of winter. When and where the collision between the two converges, severe weather and tornadoes will emerge.
This common March collision is on track Tuesday through Thursday in the southern US, with a particularly dangerous outbreak of severe weather on Wednesday and Wednesday nights from Arkansas east to Alabama. There is a threat of numerous tornadoes, with a few large and powerful tornadoes possible.
Wednesday morning, the NOAA Storm Forecast Center issued a high-risk warning – 5 out of 5 on the alert scale – stating, “Long-track, intense tornadoes expected along with damaging winds and large hail” are expected to begin this afternoon. parts of Louisiana and Arkansas, heading east to Mississippi and Alabama at night.
Since 2012, there has been no ‘high risk’ warning in March. This is about as dangerous as it gets, with tornadoes likely to be widespread, destructive and potentially deadly.
It’s no surprise to forecasters that that harsh spring season is kicking off with a bang. That’s because there is still a formidable La Niña event in the tropical Pacific Ocean, which is usually associated with a more extreme, wavy jet stream and thus an increased tornado threat.
Tornadoes are also increasingly common in March. Last weekend, like historic snowfall plastered Denver and Cheyenne, the the same storm 19 tornadoes wound up in the Texas Panhandle. In a 2015 paper, researchers found that the start of the tornado season has shifted 13 days since the late 1970s.
And the tornado threat isn’t just shifting in time, it’s also shifting location. A 2018 study found there is a downward trend of tornadoes in what is traditionally known as Tornado Alley in the states of the Plains, but a increasing trend of tornadoes occurring in the mid-south – the same general area expected to face this week’s outbreak in dire straits.
The danger may already be increasing in this region. Two of the deadliest tornado outbreaks of the past two years occurred in the southeast in early March. In 2019 41 tornadoes concentrated in Alabama and Georgia 23 people killed. In 2020 there were 25 dead as 15 tornadoes tore through Tennessee.
This week’s threat
The main features of this storm are both the pool of cold air in the sky and the fact that it spins like a top. The cold air upwards causes warm air on the surface to rise faster, causing larger thunderstorms. The storm’s twisting motion helps to give twisting motion to the individual thunderstorms and any tornadoes that may spawn them.
Typically these spinning storms rise northeast in the Mississippi Valley area, but this storm can’t because of an atmospheric block in the north. Instead, it will continue to move eastward Wednesday and Thursday, bringing strong momentum across the Southeast, posing the threat of a widespread outbreak and a particularly dangerous situation.
The main event occurs as the cold-spinning storm slides eastward and collides with warm, moist air pouring out of the Gulf of Mexico.
Beyond the “high risk” bullseye, the Storm Prediction Center has exposed a large area for an “increased risk” of severe weather – a 3 out of 5 – from the far south of Missouri southeast to the Florida Panhandle.
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Burial in that larger enhanced area is an even greater risk, as seen in red on the map. This is technically called a “moderate risk” and it is an abnormally large risk area – but the name is misleading. A moderate risk is a 4 out of 5 on the severe weather scale and indicates the potential for very dangerous severe storms. This includes cities like Little Rock, Memphis, Tupelo and Birmingham.
If you live in one of these risk areas, pay close attention to local warnings and remember that storms become even more dangerous as night falls and the tornadoes become harder to spot. Computer models do indicate that the threat of major tornadoes will be maximized on Wednesday evening and night.
CBS News
Nocturnal tornadoes are more common in the mid-south. That’s because, unlike the drier states of the plains that rely on the heat of daylight to provide energy for thunderstorms, the region bordering the Gulf of Mexico, due to extra humidity, becomes a more energetic and unstable much later in the night. atmosphere.
The threat of severe weather will continue until Thursday as the system shifts to Georgia and the Carolinas, where tornadoes, damaging winds, hail and lightning are likely to be once again.
CBS News
Southeast more vulnerable to tornado damage
There are several reasons why tornadoes are more dangerous and destructive in the Southeast, in addition to the greater tendency for them to occur at night. This greater vulnerability is why trends in more and stronger severe storms in this region are worrying.
Because these storms form in a humid environment, they tend to be shrouded in rain and are harder to see. Additionally, unlike the states of the Plains, where the terrain is often wide open allowing people to spot tornadoes from miles away, the southeast is covered with more hills and trees.
The region also has a higher population density and much weaker houses, increasing the risk of damage and death. Research indicates that this danger will continue to increase as the building continues to grow and the distribution of tornadoes changes, with a potential tripling of disaster potential by the end of the century.
While scientists are still trying to figure out why tornado patterns are changing, there is plenty of research showing that dangerous storms, and the environments in which they form, are increasing, at least in part due to human-made climate change
A 2016 study found that the frequency of tornado outbreaks has been increasing since the 1960s, and the most extreme of these outbreaks are increasing the fastest. And they are outbreaks, unlike individual tornadoes, which tend to be the most destructive, with 79% of tornado fatalities between 1972 and 2010 as outbreaks.
Probably the most revealing paper on tornado trends was published in 2018 by Dr. Victor Gensini and Dr. Harold Brooks. Their research shows a marked upward trend in tornadoes in the Mid-South and lower Mississippi Valley – an area that, as noted above, is particularly vulnerable. The biggest increase is near towns like Little Rock, Arkansas; Memphis, Tennessee; Tupelo, Mississippi and Tuscaloosa, Alabama. That corresponds to the largest threat area on Wednesday.
CBS News spoke with Gensini, an extreme weather researcher at Northern Illinois University (NIU). He says his research shows that in the mid-south an area the size of a city can expect an additional 4 tornadoes per decade on average than 40 years ago.
In the image below, taken from Gensini’s paper and adapted by NIU, the red shaded area indicates where tornadoes are increasing – almost exactly ground zero for Wednesday’s expected outbreak.
Gensini, Brooks 2018
In the blue shaded area, tornado numbers are declining in Texas and the lower plains. While that area – commonly referred to as Tornado Alley – still has the highest density of tornadoes, the upward trend is weighted eastward.
While Gensini cannot say with certainty why the eastward trend is occurring, he says it is consistent with what we would expect from climate changeThat’s because as the climate warms, the arid desert regions of the southwest are expanding eastward. This limits instability above the plains, and thus tornado formation, and pushes the convergence zone between dry and moist air to the east.
In addition, the waters of the Gulf of Mexico and adjacent land areas are warming as a result of man-made climate change. This influx of more heat and moisture provides more energy to fuel more extreme tornado outbreaks in the Southeast.
And as humans continue to warm the climate, most scientists agree that the environment causing these dangerous tornadoes will become increasingly unstable and dangerous.