ROANOKE, Va. – A lot of us think of spring as the time of year when flowers bloom. It’s also the time when we’re most at risk for severe thunderstorms. This can range from flooding to damaging wind and from hail to tornadoes.
We’ll make sure you’re aware well ahead of time with the issuance of Weather Authority Alert Days.
Tornado Alley vs. Dixie Alley
Picture The Wizard of Oz. Where did Dorothy live? Kansas - the heart of Tornado Alley.
Lately, however, the deadliest tornadoes aren’t occurring there. They’re happening farther east in states like Alabama, Tennessee Arkansas and Mississippi. It’s a region called “Dixie Alley.”
“So we are seeing an increase in tornadoes in the Southeast part of the United States and those numbers are more than in Tornado Alley.”
Why is Tornado Alley moving?
Phil Hysell is the Warning Coordination Meteorologist with the National Weather Service in Blacksburg. He says the cause of this shift is unknown. However, he does have a few thoughts as to why this is happening.”
“I think you can look at it from three different angles. Is this something that naturally occurs decades over time? We’re going to need additional decades to see if this fluctuates naturally. Is it caused by a warmer climate? There are some non-meteorological factors as well. This part of the United States - the Southeast - is the fastest growing in terms of population, so there are more people to spot tornadoes. I think the truth is somewhere with all three of them.”
Therein lies the problem. Tornadoes in Dixie Alley can be more destructive due to the high population density and the high amount of mobile homes (which aren’t as sturdy as traditional homes). While the tornadoes in Dixie Alley are often smaller and a bit weaker, the sheer amount of people can result in more injuries and/or fatalities.
Topography also plays a role. The Great Plains, in Tornado Alley, are generally flat. This makes it easier to spot a tornado. In comparison, the Southeast has plenty of trees and hills, making them harder to see. At times, they’re rain-wrapped too.
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Tornadoes in Tornado Alley can travel longer distances and move a little faster, while those farther east tend to move slower which can lead to more damage.
We asked Hysell to elaborate a little more on climate change and what part of it is impacting this shift in tornadoes the most.
“I think it’s the increased moisture. As you warm the atmosphere it can hold more moisture so that’s one of the ingredients need for thunderstorm development. I think that’s the biggest contributor.”
Certainly, the Southeast is known to have very high levels of humidity, which in turn can lead to high levels of instability in the atmosphere.
What allows us to stay out of Dixie Alley?
“Temperatures. just our higher elevations allow those temperatures to be a few degrees cooler and allow our relative humidity to be a little bit drier. Even though it may be by only a few degrees, that makes all the difference in the world.”
While we do not, thankfully, see all that many tornadoes around here, Hysell lays a gentle reminder.
“Again, it takes just one event to create a disaster. Be prepared.”
Researching the Terrain’s Effect on Supercells
“Tornadoes can happen anywhere, any time, any month of the year, any state and over any type of terrain.”
That was Kathryn Prociv, a Virginia Tech alum we interview in 2019 when exploring the effects of terrain on tornadoes.
Five years later, more research is being down at UNC-Charlotte.
Casey Davenport, Roger Riggin and Logan Twohey are three scientists leading the charge, researching how storms react to our complex terrain.
Davenport says, “There are times when mountains are actually going to enhance the storms instead of breaking them apart. Part of our research has shown that.”
Riggin echoes, “Under those certain conditions when they’re actually favorable, you need to have that clear communication and understanding – not depend on that prior notion.”
In the last 70 years, there have been a handful of storms that have produced tornadoes in our mountainous counties. Once you jump the Blue Ridge, however, that number spikes.
“Think about a slinky. When you go down a certain mountain, that slinky stretches out and so that can enhance rotation even as you’re getting more downward motion.”
Their research, therefore, delves into the storms that cross mountains versus those that ride parallel to them. Riggin says that ones that cross the mountains tend to move more quickly.
“The primary discriminator between a crossing and a non-crossing storm was looking at your low level shear. Generally in your crossing storms, you’d see a lot more speed shear.”
Where do those come from?
Twohey explains, “The ones that come from the northwest have tended to cross more often than ones from the southwest.”
One extreme example of a crossing storm would be the 2012 derecho. Another, from the southwest, however, is the April 2019 tornado that hit Franklin County.
That, according to the team’s research, is when we most often see storms crossing the terrain.
“They’re more likely than in other months to be crossing in April.”
Does that mean, then, that storms that move across the mountains are more dangerous than ones that ride parallel to them?
“The ones that are going more parallel tend to have more intensification, but that really depends on the more localized effects of the terrain.”
In recent memory, storms like the Bedford County tornado in 2022, the Botetourt tornado in 2020, the Appomattox tornado in 2016 and the Super Outbreak of 2011 show storms that were enhanced when riding parallel to the mountains.
It’s not just tornadoes, though. Supercells are prone to producing wind damage and hail.
Now that there’s a better understanding of how these storms form, the team at UNC-Charlotte hopes this will help with how quickly you’re warned.
“Then you can more finely tune what the warnings look like. In terms of longer term expectations, we’re also coordinating with the Storm Prediction Center. Is a further severe weather watch warranted or a tornado watch or whatever it might be”>
How a Tornado Forms
Tornado Alley is an area of the country where multiple air masses collide.
Cold dry air from polar regions to the north funnel down. Warm dry air from the west flows in. And warm moist air from the Gulf of Mexico is lifted into the region.
The combination of these three different air masses and different wind directions makes tornado alley a unique location unlike anywhere else on earth.
States included in tornado alley extend from Texas all the way up to South Dakota.
Another area that gets its fair share of severe weather and tornados is Dixie Alley. This includes states from Louisiana to Georgia.
Now, most states across the U.S. see tornados each and every year, but there are a few states that stick out. Texas, Kansas, and Oklahoma take the top three spots with over 60 tornados annually.
Virginia falls in the bottom half of the table where only 6-20 tornados are seen on average each year. That is not to say we don’t see any impact from severe weather.
Supercell structures are the primary storms that produce tornados and severe weather. Across the globe, less than 10% of storms are supercells. And within the population of supercells, less than 10% of them produce a tornado. In other words, a rare breed.
Structurally, there are three important components that define supercells.
The rear flank downdraft, the forward flank downdraft, and the updraft. The forward flank downdraft is responsible for providing rain cooled air. It is often downwind and produces heavy rain and hail. The updraft is the region where the tornado forms.
You may also see a wall cloud associated with this area as the supercell gets stronger and healthier. The rear flank downdraft is very important. As warm air is pushed upward into the tornado, it is then sent down on the backside of the storm. This same air is recycled into the storm. This cycle continues and is important to how long a supercell lasts.
So, how does a tornado form in a supercell?
It starts with the combination of differing air masses that makes a conducive environment. Then, winds at various levels are at different speeds. At the lower level, they are slower. At the upper level, they are faster. This causes vorticity, or spin.
On one side of the storm, the air is sinking, and on the other, the air is rising. Because of this, we now have a rotating horizontal column of air. The updraft starts to form, and then we get areas of spin at different levels in the atmosphere.
Eventually, the entire column of air is flipped upward and turns vertical. This is where the wall cloud begins to form, and in some instances, a tornado.
If the rotating column of air doesn’t reach the ground, then what we see is often classified as a funnel cloud.
Alerts are issued ahead of these storms, but it can be confusing to know which one is which.
Let’s fix that for you.
A tornado watch is issued when the environment is favorable for tornado development. A tornado warning is issued when an ACTIVE tornado is on the ground or has been radar indicated. A tornado emergency is a rare situation, but is considered life threatening.
On days when there are severe weather threats, we will be issuing weather authority alert days.
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Recent History of Severe Weather in Our Area
Looking to the past can give us perspective on what to expect from a new year of severe weather.
While 2023 was a down year for tornado warnings there is still plenty of information we can gather. The Blacksburg National Weather Service office issued 8 tornado warnings in 2023. Only three of those were in counties in our region: Wythe, Bland, and Greenbrier.
The tornadoes were limited to late spring into Summer. The first warning came out on May 16th with the latest on August 15th. Those eight warnings are less than a quarter of what 2022 brought.
2023 turned out to be the calmest stretch for tornadoes in the last five years.
While tornados were below average we had a relatively standard number of severe thunderstorm warnings.
Severe Thunderstorms are much more common since they often take less energy. We had one tornado warning for every 28 severe thunderstorms last year.
To be severe a storm needs wind gusts at 58 mph or hail that grows to an inch across. Any storm that produces a tornado counts as well and usually has the other two pieces.
Most of the time a stark difference in temperature through the atmosphere allows gusts to gain strength and hail to grow larger.
High surface temperatures help storms develop which is why most of our warnings came in the summer. Fifty-nine storms reached those criteria in the spring with only seven in the winter and none in fall.
Days in the summer when it was close to 90 degrees were great for developing severe storms. 70% of our storms were in the months when our temperatures soared highest.
While all our 90-degree days were in the summer some of our tornado warnings were on days with highs in the 80s.
That’s an important thing to remember as severe weather season approaches.
It only takes a strong difference in temperatures for a storm to turn severe.
If surface temperatures are in the 70s or even 60s colder air higher up can still lead to high winds.