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It has been a rough few days in the Plains states, with over 100 tornado reports, focused on Nebraska and Iowa on Friday, then Oklahoma overnight Saturday, and continuing this week. Many have been big, destructive, highly visible tornadoes, though the death toll has been thankfully low.
The story in the Plains states is not so much about something extraordinary having happened but rather a sharp return to something that has been fairly frequent historically but seemingly lacking for a few years.
Tornado Alley, at least presently, is being Tornado Alley again. (Literally doing it in Kansas as I type these words on Tuesday evening.)
The storm system that caused the weekend round only brought some mild thunderstorms and light showers to Virginia on Tuesday. The next storm system may bring more rain and storms to our region by the weekend, but likely nothing close to the severe storms and tornadoes the Plains are experiencing with it.
We could get into a whole discussion here about how traditional Tornado Alley may be expanding or shifting eastward, perhaps guided by climate change. That could and should mean we need to be more vigilant about the potential for tornadoes in Virginia.
For a while in the 2010s it seemed we were seeing more strong, long-track tornadoes in Virginia than in the past. However, WSET (Channel 13) meteorologist George Flickinger, a former Plains storm chaser himself, examined the trends closely and found no evidence for an increase in stronger tornadoes in Virginia over the past 50 years, at least.
Virginia simply isn’t a tornado alley, even with lowercased letters.
Virginia does have tornadoes — the Old Dominion averages about 18 per year. While the vast majority are brief, fairly weak and not photogenic “Wizard of Oz” or “Twister” classic funnels, we have just enough destructive and deadly tornadoes that we can’t ignore them, but not enough that they are part of the fabric of our annual experience.
If a rural farmhouse has a cellar here, it’s presumed to be for storing canned food items or perhaps wine, not principally as a tornado shelter, as was the case on my mother’s farm in northern Arkansas in the 1940s. (They also stored canned food in their storm cellar.)
A few communities may have sirens that can sound during tornado warnings, but almost nowhere would they be called “tornado sirens” as I grew up with, awakening me on many stormy nights, in northeast Arkansas.
It takes something extraordinary, usually related to an inland tropical system like Hurricane Ivan in 2004 (a record 38 tornadoes) or being pulled into a frenetic tornado spree centered elsewhere like in 1974 or 2011, for there to be anything resembling an “outbreak” of tornadoes in Virginia.
Tornadoes are a tragic oddity in Virginia, but not a core part of our culture the way they would be in, say, Oklahoma or Kansas, where a May day can be marked by three meals: Breakfast, lunch and tornado warning.
Why is that? Or rather, why is Virginia not really core tornado country?
Here are some meteorological reasons.
Lack of an elevated mixed layer
This is a crucial element for large severe storms outbreaks in the central U.S. as a layer of dry air aloft from the Desert Southwest and/or the Rocky Mountains overspreads warm, moist air near the surface.
The bottled-up warm, moist air at the surface can erupt into the drier air, which cools rapidly as it moistens, creating steep lapse rates or decrease of temperatures with height that leads to explosive growth of storms and giant hail. And those westerly winds aloft are often overspreading winds that are southeasterly near the surface, southerly a little higher up, and southwesterly just above that, giving storms a nice spin that often manifests in the tight updrafts that can spawn tornadoes.
We rarely see the elevated mixed layer develop over us to a similar degree as what you’d see over the Plains states during the spring tornado outbreaks, largely because we are simply farther away from deserts and really tall mountains that help produce that flow of air. “Weak lapse rates” are a common phrase in forecast discussions about severe weather threats here. And even if the layer is there to some extent, getting the same conditions underneath that the central U.S. often has can be more challenging.
Cold-air damming/wedging
Extensive research has shown that rotation within supercell thunderstorms doesn’t decrease, but can actually increase, while crossing mountains, as the rotating column is stretched — IF, that is, instability and shear are similar on both sides of a ridge.
But many times, those conditions aren’t similar on both sides of our Appalachian ridges.
Cooler and/or drier air from the northeast is often trapped east of the Appalachians and can put a damper on thunderstorms trying to cross. This can be a full-fledged “cold-air damming” or “wedge” effect where temperatures are 10-20 degrees cooler on this side of the mountains, but the effect can still exist even if it is much more subtle.
On March 2-3, 2012, supercells spawning a series of deadly and destructive tornadoes in the Ohio Valley were headed directly for western Virginia. But in West Virginia, they hit the western side of a colder, more stable air mass trapped against the mountains. The mighty tornadic storms died suddenly like a flock of birds slamming into a brick wall.
The caveat to this is that sometimes near the southern or eastern edge of the cooler-air wedge, there can be a narrow zone of heightened potential for supercell thunderstorms and tornadoes, as horizontal rotation develops along the temperature boundary that can be pulled vertically by storm updrafts. A tornado tore through the eastern side of the Charlotte, N.C., metro area early on March 3 as we were shielded in 2012. A year earlier, in April 2011, our region’s own Pulaski was hit by a highly damaging EF-2 tornado that formed where a cool air wedge keeping Roanoke in the 50s bumped against warm, humid air pushing toward 80.
Deep shear, strong instability rarely coincide
We have several days a year when the speed and direction of winds shifting with height would be capable of supporting rotating thunderstorms and even tornadoes. But these days more often happen in cooler months, with strong low-pressure systems riding deep dips in the jet stream, and not in warmer months, when those shearing winds could be coupled with stronger instability needed for severe thunderstorms.
It’s difficult, though not unheard of, to get the deep shear and strong instability to coincide over our region. By the time it gets warmer and sticky, the jet stream has often retreated to the north and northwest, more apt to dip south toward the middle of the country rather than cutting against high pressure that often forms over the warm waters of the Gulf of Mexico or western Atlantic Ocean.
Rare low-level shear strength
It is difficult to get weather patterns here in which stronger low-pressure systems move in a way that would enhance easterly or southeasterly surface winds for a prolonged period of time, providing the “backing winds” at ground level that often help feed low-level circulations that can lead to tornadoes. Lows are more apt to move up the East Coast, giving us cooler northeast winds, or else cut northward well to our west or northwest, carrying the stronger wind dynamics through the Tennessee and/or Ohio valleys toward the Great Lakes.
This is one reason tropical systems coming from the Gulf of Mexico often spawn tornadoes in Virginia — they can provide that strong easterly to southeasterly surface wind flow.
Orientation of Appalachians vs. Rockies
Besides the fact that the Rockies are so much taller and have deserts west of them to deposit dry air aloft, the north-south alignment of the Rockies from Canada to Mexico is ideally set up to allow both warm, moist air from the Gulf of Mexico and cold, dry air from the tundra to move almost unimpeded by terrain over the Plains, forming boundaries along which severe thunderstorms and tornadoes can form.
The southwest-to-northeast angled Appalachians as we’ve noted, can trap cooler air from the northeast, or else, trap densely moist air from the Gulf or Atlantic with weak lapse rates more likely to squeeze out copious downpours rather than spawn tornadoes.
Be tornado-ready
This is only a partial list of why tornadoes don’t frequent Virginia. But rather than making us complacent, this should make us all the more vigilant in those atypical cases when conditions do exist for potential tornado development, often marked by a tornado watch. It is advisable for each family to have a tornado plan, to get in a central part of the lowest floor without windows, and cover heads, if a tornado warning is issued or it is otherwise suspected a tornado may be close by. And take school and work tornado plans and drills seriously.
We’re not a tornado alley, and most of our potential tornado outbreak setups roll into the gutter, but there may at any time be that occasional lucky strike.
Journalist Kevin Myatt has been writing about weather for 20 years. His weekly column, appearing on Wednesdays, is sponsored by Oakey’s, a family-run, locally owned funeral home with locations throughout the Roanoke Valley.