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Unread 02-17-2012, 10:10 PM
 
Location: Athens, GA (via Pittsburgh, PA)
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Exclamation Everything you know about "Tornado Alley" is wrong

Ask anybody with a rudimentary knowledge of weather patterns in the United States where "Tornado Alley" is, and they'll all mention Kansas, Oklahoma and Texas, with many including Nebraska, Iowa and Missouri as well. This definition of Tornado Alley never seems to be questioned, but now there's evidence that it should be.

We've all seen professional storm chasers documenting the big "wedge" tornadoes that rumble across the Great Plains every spring, and it is true that prevailing weather patterns in the Great Plains during the spring are very favorable for tornado outbreaks. But what about the the rest of the country and the rest of the year? And what about the strongest tornadoes?

Using a combination of strength, density, track length and fatalities as the criteria, it appears that Tornado Alley might have been misplaced. Defining strong tornadoes as anything rated F3 to F5, and long-track tornadoes as anything that stays on the ground for at least 20 miles, the most vulnerable region is actually south and east of where we've learned it to be. According to this study by Michael Frates of the University of Akron, the frequency of strong tornadoes between 1950 and 2006 was highest in "Dixie Alley," which is centered around the lower Mississippi River Valley rather than the Great Plains.

Strong tornado density during that period of time was at least as high in northeastern Arkansas, western Tennessee, northern and central Mississippi and northern Alabama as it was in eastern Nebraska, central and eastern Kansas and central Oklahoma, if not higher. Not only that, but the highest density of killer tornadoes between 1950 and 2004 seems to be centered on that same area as well, as this map illustrates:




Also illustrated is the percentage of nocturnal tornadoes by state, with Tennessee, Arkansas and Mississippi having three of the four highest percentages. In each of those states, at least 40 percent of all tornadoes between 1950 and 2005 occurred at night, which makes them even more dangerous than they already are.

As we all know, the Great Plains has a very high frequency of tornadoes during the spring. What we forget, though, is that the winter, summer and fall tend to be quiet there. Conversely, the lower Mississippi River Valley has a moderate to high frequency of tornadoes during the winter, spring and fall, with the summer being the only reprieve. Tornado outbreaks in December are not unheard of in this part of the country, and the only two F5 tornadoes ever observed in the United States in December occurred here as well.

Even more sobering is that none of the information I've presented includes anything that happened in 2008 or 2011, two disastrous years for tornadoes in the southern United States. As I saw somebody on the Alabama message board comment after last month's tornadoes near Birmingham, "it's like we're living in the barrel of a gun." In short, it appears that dangerous weather is more likely to impact I-55 than I-35 during the course of the year.
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Unread 02-18-2012, 04:57 AM
 
Location: Carrboro and Concord, NC
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Interesting studies. I think the definition came about due to the actual per capita number of tornadoes, with the highest concentration in OK. Add to that that tornado strength is determined by damage cause, so a "rating" is assigned after the fact, and a lot of very, very large tornadoes on the High Plains are traversing very sparsely populated territory, don't hit anything other than agricultural fields, and this are rated as EF0-1-2, even though their actual windspeed strength is likely EF-3-4-5.

One thing that IS notable is that the East - East of the Mississippi River - seems to be verifiably slightly more prone to outbreaks that cover vast amounts of territory - there are numerous outbreaks - Feb 19 1884 is one of the first well-documented, April 3-4 1974, November 1989, November 21-23 1992 are some older examples - where you see an outbreak stretch from the Gulf Coast up into Canada (March 3-4, 1974), Mississippi to Virginia and north to Ohio (Feb 19 1884), the Deep South to Upstate New York (November 1989 - the "Huntsville" outbreak, as that's where the most severe storm struck), or Texas to Maryland (November 1992).

Two odditites as well - outbreaks that - unusually - track directly west-to-east:
The June 1952 Flint/Worcester Outbreak, which (first) struck the N Mississippi Valley (Iowa was hit VERY hard), (second) struck the lower Great Lakes (Michigan and Ohio were hit very hard, with 1 F5 and multiple F4's), and (third) New England (several F2, at least 1 F3, and the infamous Worcester tornado, which is officially an F4 but is somewhat widely thought to have likely done F5 damage).

More recently, a late April 2002 outbreak followed a similar track, but slightly to the south, with tornadoes over 2 days from Kansas and Nebraska straight east to Virginia and Maryland, and - perhaps unusually - the most destructive of those storms hitting various locations in VA and MD right before it all blew out into the Atlantic. The series of tornadoes that struck La Plata, Maryland devastated the town, and were later photographed (amid some very heavy hail) as a spectacular triple-funnel event (likely one weaking tornado, one strengthening and the third a satellite to one or the other) as tracked over Calvert Cliffs before then making a 10-mile crossing of the Chesapeake Bay.

The 2nd of the big 2011 outbreaks underscored something also seen in the May 31 1985 Pennsylvania outbreak and the April 3-4 "Super Outbreak" - large, violent tornadoes can and do occasionally strike mountainous areas. The same 2011 outbreak the produced the Tuscaloosa/Birmingham tornado later spawned a long-track EF-4 over mostly wilderness (and VERY rugged terrain) in Great Smoky Mountains National Park. A 1989 F4 in Wyoming crossed the continental divide at ~11,000 feet, forming NE of Jackson Hole and tracking through the Teton Wilderness immediately SE of Yellowstone NP. During the 74 Super Outbreak, both mountainous SW North Carolina, southwest Virginia, and the southern part of West Virginia were struck by some F3-F4 storms that aggain crossed rugged terrain. And in the May 31 1985 outbreak, one supercell tracked from near Youngstown, Ohio to just southwest of Wilkes-Barre, Pennsylvania, producing serial tornadoes along about 70% of that supercell track, including the F5 that struck Niles, OH and Wheatland, PA and the infamous, later 2.5-mile-wide storm that cut through the Moshannon State Forest north of I-80 and was picked up on seismographs at Penn State (20 miles south of the storm track), as it tracked for more than 60 miles across mostly wilderness in the Alleghenies. Researcher Thomas Grazulis put that last (Moshannon) tornado on his short list of the largest (physically) tornadoes to have ever been observed, and it - even in wilderness - did F4 damage nearly continually for 60 miles.
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Unread 02-18-2012, 06:01 AM
 
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Can't say that's reassuring. I always knew we had a ton of tornados. I just always figured they had as many or more. So its safe in tornado alley, but still dangerous in the South. Eh it is more dramatic to film them there because they have flat, treeless land so I guess they get more press.

One other thing not really related is that south Alabama, Mississippi, and Georgia catch a lot of tornados spawned by hurricanes that make landfall on the Gulf Coast.
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Unread 02-18-2012, 08:49 AM
 
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Very interesting article. I thought tornado alley was traditionally drawn a little further east anyway. When I stidied this years ago, tornado alley was NE Texas, and did not include the panhandle. Then it cut across extreme east OK, and included NW Arkansas. And this study has shifted it further east.
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Unread 02-18-2012, 09:44 AM
 
Location: Athens, GA (via Pittsburgh, PA)
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Quote:
Originally Posted by davidals View Post
Interesting studies. I think the definition came about due to the actual per capita number of tornadoes, with the highest concentration in OK. Add to that that tornado strength is determined by damage cause, so a "rating" is assigned after the fact, and a lot of very, very large tornadoes on the High Plains are traversing very sparsely populated territory, don't hit anything other than agricultural fields, and this are rated as EF0-1-2, even though their actual windspeed strength is likely EF-3-4-5.
The Great Plains gets the most tornadoes per square mile, but there is some doubt about whether it gets the most strong tornadoes per square mile. And while it is possible that some tornadoes might be underrated if they don't hit anything, it's still just speculation, so it wouldn't be of much value in meteorological analysis.


Quote:
Originally Posted by davidals View Post
One thing that IS notable is that the East - East of the Mississippi River - seems to be verifiably slightly more prone to outbreaks that cover vast amounts of territory - there are numerous outbreaks - Feb 19 1884 is one of the first well-documented, April 3-4 1974, November 1989, November 21-23 1992 are some older examples - where you see an outbreak stretch from the Gulf Coast up into Canada (March 3-4, 1974), Mississippi to Virginia and north to Ohio (Feb 19 1884), the Deep South to Upstate New York (November 1989 - the "Huntsville" outbreak, as that's where the most severe storm struck), or Texas to Maryland (November 1992).
That has everything to do with the Gulf of Mexico. You don't really get the juicy air until you get into eastern Texas. The tornado outbreaks in the Great Plains tend to cover less area simply because there's less moisture to work with. That's probably a good thing, because if the Gulf of Mexico was south of the Great Plains, then tornado development during the peak season would be high enough to risk making the Great Plains uninhabitable simply for safety reasons. It's already a tough environment in the first place.


Quote:
Originally Posted by davidals View Post
The 2nd of the big 2011 outbreaks underscored something also seen in the May 31 1985 Pennsylvania outbreak and the April 3-4 "Super Outbreak" - large, violent tornadoes can and do occasionally strike mountainous areas. The same 2011 outbreak the produced the Tuscaloosa/Birmingham tornado later spawned a long-track EF-4 over mostly wilderness (and VERY rugged terrain) in Great Smoky Mountains National Park. A 1989 F4 in Wyoming crossed the continental divide at ~11,000 feet, forming NE of Jackson Hole and tracking through the Teton Wilderness immediately SE of Yellowstone NP. During the 74 Super Outbreak, both mountainous SW North Carolina, southwest Virginia, and the southern part of West Virginia were struck by some F3-F4 storms that aggain crossed rugged terrain. And in the May 31 1985 outbreak, one supercell tracked from near Youngstown, Ohio to just southwest of Wilkes-Barre, Pennsylvania, producing serial tornadoes along about 70% of that supercell track, including the F5 that struck Niles, OH and Wheatland, PA and the infamous, later 2.5-mile-wide storm that cut through the Moshannon State Forest north of I-80 and was picked up on seismographs at Penn State (20 miles south of the storm track), as it tracked for more than 60 miles across mostly wilderness in the Alleghenies. Researcher Thomas Grazulis put that last (Moshannon) tornado on his short list of the largest (physically) tornadoes to have ever been observed, and it - even in wilderness - did F4 damage nearly continually for 60 miles.
May 31, 1985 laid in ruins the myth that tornadoes don't strike mountainous areas. It also laid in ruins the myth that strong tornadoes don't strike in Pennsylvania, even though there was already plenty of evidence to the contrary. Western Pennsylvania in particular had seen plenty of strong tornadoes before 1985.

The Appalachian Outbreak of 1944 spawned two F4's in Pennsylvania, one near Canonsburg and the other near McKeesport. This would be the southern fringe of the Pittsburgh urbanized area today. In 1963, Glassport was hit by an F3 that mortally wounded the town's economy by destroying the glass factory. In 1980, the "Alle-Kiski" area From Natrona Heights over to near Apollo got hit by an F4 that seems to be strangely forgotten in Pennsylvania meteorological history.

The F3 that hit Beaver Falls and Evans City in 1985 would be passing across the northern fringe of the Pittsburgh urbanized area today. The F3 that hit Somerset County in 1998 passed within a few miles of Mount Davis, the highest point in Pennsylvania. And even though the tornado that hit Pittsburgh that same day was only rated an F1, it stayed on the ground for 26 miles, officially making it a long-track tornado.

Pennsylvania is easily the most tornadic state in the Northeast. Oddly, it seems like its history of strong tornadoes (F3+) is longer than Virginia's, and similar to North Carolina's. And speaking of mountains, the area of Georgia that's been the most susceptible to tornadoes has been northwest Georgia in the "ridge and valley" Appalachians around Rome.


Quote:
Originally Posted by AuburnAL View Post
Can't say that's reassuring. I always knew we had a ton of tornados. I just always figured they had as many or more. So its safe in tornado alley, but still dangerous in the South. Eh it is more dramatic to film them there because they have flat, treeless land so I guess they get more press.
I think there are three reasons why tornadoes seem to be deadlier in the South than they are in the Great Plains:

1. The visibility of tornadoes in the South is less than it is in the Great Plains. Sight lines are more limited, and many tornadoes are either wrapped in rain or occur at night.

2. Storm shelters are inadequate in the South, if not nonexistent. Building codes in the Great Plains are more strict, and many places mandate the construction of storm shelters.

3. A lot of people outside the Great Plains have the "it won't happen here" mentality, so there's a general apathy and lack of preparedness for such an emergency.

I'm glad to see the governor of Alabama talking about updating building codes to account for violent weather, especially after last year. I say it's about time that one of the states in the South took the lead in this regard.


Quote:
Originally Posted by AuburnAL View Post
One other thing not really related is that south Alabama, Mississippi, and Georgia catch a lot of tornados spawned by hurricanes that make landfall on the Gulf Coast.
That's true, but tornadoes spawned from a land-falling hurricane are relatively weak. They're mostly F0's and F1's, with the occasional F2. I've never heard of anything stronger than an F2 coming from a hurricane, though. Because of the relative weakness of those tornadoes, they wouldn't matter in a study examining the density of strong (F3+) tornadoes.

Last edited by Gnutella; 02-18-2012 at 10:17 AM..
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Unread 02-18-2012, 10:39 AM
 
Location: Western Massachusetts
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And Massachusetts has as many violent tornadoes per square mile as Tennessee and Missouri

http://www.washingtonpost.com/rf/image_606w/WashingtonPost/Content/Blogs/capital-weather-gang/201106/images/deadly-per-10k-tornado.gif (broken link)
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Unread 02-18-2012, 11:59 AM
 
Location: Paris
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Quote:
Originally Posted by Gnutella View Post
That has everything to do with the Gulf of Mexico. You don't really get the juicy air until you get into eastern Texas. The tornado outbreaks in the Great Plains tend to cover less area simply because there's less moisture to work with. That's probably a good thing, because if the Gulf of Mexico was south of the Great Plains, then tornado development during the peak season would be high enough to risk making the Great Plains uninhabitable simply for safety reasons. It's already a tough environment in the first place.
What is the parameter that makes the Great Plains as prone to tornadoes as the lower Mississippi valley, in spite of less Gulf moisture? Is it related to the bigger proximity to the Rockies?
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Unread 02-18-2012, 03:21 PM
 
Location: Bothell, Washington
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The term tornado alley doesnt have anything to do with tornado strength, it is strictly about the sheer number of tornadoes. The tornado alley we all know about gets the most tornadoes- both overall and per capita- than anywhere else in the country or world.
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Unread 02-18-2012, 03:40 PM
 
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Even then it depends on what is in its path and how long it sdtays on ground. that has alot to do with tornado alley and why the chasers go there.They could spend all season eher and maybe see one that touches down for a few minute but in Okalhoma where I was stationed I saw tweo and they were wider than naything I ever saw plus stayed down for a long times. Luckiloy nothing but fields to plow with both.
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Unread 02-18-2012, 03:58 PM
 
Location: Two Rivers, Wisconsin
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Interesting reading, thanks! Living close to Lake Michigan most of my life, I've found people get comfortable with the "not this close to the lake" theory.

I've never bought into that one, especially because I was in a mobile home park for 30 yrs. before I moved to NE Wisconsin. Had some very close calls over the years but consider myself lucky. Now I have a house and a basement!

I've always thought of Oklahoma, Kansas, Nebraska as Tornado Alley, but things have changed.
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