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Posted at 11:00 AM ET, 04/26/2010

Tornado Alley: What's that???

By Steve Tracton

* Cooling off before late week warm-up: Full Forecast *

Graphic courtesy Wikipedia.

Notwithstanding a tardy beginning to the severe weather season this spring, the atmosphere has made up for lost time with destructive and deadly outbreaks of tornado generating thunderstorms for the past few days over much of the south and southeast U.S (here).

We generally expect a maximum in the occurrence of major tornado activity within a swath of the central U.S. colloquially referred to as "tornado alley" . Tornado alley is really just a nickname for the area of the relatively flat land in the Great Plains where warm moist air from the Gulf of Mexico clashes with cold air from Canada to generate conditions ripe for severe weather. But a new study suggests the greatest concentration of intense activity fall outside what's been traditionally considered tornado alley.

Keep reading for more discussion of tornado alley and actual storm activity...

There are several differing and sometimes inconsistent depictions of the geographical boundaries of tornado alley (here) depending upon how tornado occurrence is measured and differing databases over different time periods. (Note: there is no official National Weather Service (NWS) definition of the area delineating tornado alley)

An outstanding question is whether there is even justification for describing any single contiguous region as tornado alley (here)?

The latest contribution to this question (controversy?) is by Michael Frates of the University of Akron (Ohio) who sought to verify the existence of the "colloquial tornado alley" and to delineate other zones in the United States with high tornado activity. As reported by an article in Science News, Frates explored the distribution of F3 to F5 tornadoes with tracks 20 miles or longer over the southern and eastern U. S. from 1950 to 2006. A key distinguishing aspect of this study was its high spatial resolution and rigorous analysis techniques (quadrat analysis , Choropleth maps ).

Frequency distribution of F3-F5 tornadoes. Image from

Results from this analysis indicate the so-called "Tornado Alley" fails to represent the areas of most intense tornado activity in the United States. Surprisingly, the maximum number of intense tornadoes was found in what is referred to as "Dixie Alley", centered over Mississippi and northern Alabama. Frates also notes that tornadoes tend to strike Dixie Alley all year round, unlike the peak season in Tornado Alley being only four months long. Runner ups to these two top-prone areas in frequency of tornadoes occurred in Hoosier Alley (Indiana) and Carolina Alley.

Saturday's deadly twister in Mississippi is clear demonstration of the well-known fact that violent and killer tornadoes occur outside Tornado Alley every year. The study described here is an important contribution in defining regions subject to maximum danger and calling specific attention to the vulnerability in Dixie.

Beware, however, that dangerous tornadoes can occur almost anywhere in the U.S. including west of the Rockies and east of the Appalachians, and of course, the D.C. metro region. So, just in case, be prepared (caveat: if confronting a tornado while in a car, see, "Confusion on What to Do If Car Meets Tornado")

By Steve Tracton  | April 26, 2010; 11:00 AM ET
Categories:  Science, Thunderstorms, Tracton  
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Thanks for the interesting article. In some future article could you discuss the difference between a tornado and what is called, I think, a microburst? One night when I was a teenager in Houston there was a severe thunderstorm and I heard that classic sound of a train. I pulled the covers over my head just before my bedroom window shattered. The insurance co thought it was not a tornado but a microburst. (A couple of nearby houses had similar experiences.) Anyway, a weather topic for another day.

Posted by: erbele | April 26, 2010 9:04 AM | Report abuse


A microburst (downburst) is the outflow from a thunderstorm downdraft where it strikes the surface. Think of pointing a concentrated spray from a garden hose straight down to the pavement. The water has to go someplace when striking the surface, so it spreads out with intense, straight-line spray in all directions. If the hose is not quite vertical the spray diverges from its source in the direction the hose is pointing.

A tornado is a counterclockwise (occasionally clockwise) rotating column of air in contact with the ground and extending upward into a thunderstorm. Surrounding air from near the surface is sucked into the thunderstorm via the tornado funnel.

Consequently, the signature on the ground with a tornado is damage having indications of a circular-like counterclockwise pattern, while with microbursts the damage is more straight line diverging in all or some preferred direction. For a good example, see:

Posted by: SteveT-CapitalWeatherGang | April 26, 2010 11:49 AM | Report abuse

You can also add our own "Chesapeake Alley" [Southern Maryland, the District and Northern Virginia] but the map doesn't extend this far north. Suffice it to say that we've had two big tornadoes in the past 150 years in LaPlata, MD, plus many smaller ones in the area extending from College Park, MD to Petersburg, VA. and from the Blue Ridge to the Bay. It's said that the mix of conflicting wind patterns involving the Bay and the mountains around here can enhance tornado formation when the conditions are right.

We're also just coming out of a modest El Nino. During and just after El Nino years, tornado formation is suppressed in the "classic" Tornado Alley, but seems to be enhanced in "Dixie Alley" and possibly also in "Hoosier Alley" as well.

In determining the pattern of wind damage on the ground, it's quite easy to distinguish a tornado from a microburst. Tornado damage leaves a "rotational" or rotary fingerprint while the usual pattern of damage from a microburst is a star-shaped pattern with the fallen trees or other damage radiating outward from a central point [where the microburst first strikes the ground]. Such patterns are easily visible on an aerial or satellite photo of the damage.

Posted by: Bombo47jea | April 26, 2010 1:24 PM | Report abuse

BTW, damage from a falling meteoroid or asteroid [e.g. Tunguska, 1907] will look just like that from a microburst--a large enough meteoroid creates its own microburst upon contacting the atmosphere.

Posted by: Bombo47jea | April 26, 2010 1:32 PM | Report abuse

Thanks all! I love learning new things! The analogy is perfect and the link is a great visual.

Posted by: erbele | April 26, 2010 3:47 PM | Report abuse

I think things have moved east. Just as well, but it really means we need homes that will stand up under this weather. More expensive? Of course. But can we afford what we have now?

I hope not.

Posted by: GaryEMasters | April 26, 2010 5:55 PM | Report abuse

There is nothing surprising at all about these "Tornado Alley" maps.

The reason that the Western Great Plains got the reputation of "Tornado Alley" goes back to before the era of high-tech forecasting/detection.....when most tornadoes were seen snd reported visually. That's because, like in large parts of interior Argentina and Australia (other area with a lot of tornadoes) the semi-arid climate and low population density means high-base thunderstorms with easily-seen wall clouds, a lack of blocking obstacles, and also easily-seen funnel clouds.

Farther east, and especially east of the Mississippi, the higher-moisture content and dew points, together with more cities, more trees, and a larger population density, meant that many tornadoes amd wall clouds were not as easily seen as farther west. The storms had lower bases, larger rain areas, and poorer visibility.

People usully think of tornadoes and severe thunderstorms as spring/summer events, but it is surprising how many strike in mid-winter, especially along the Gulf Coast and in Florida. When you are close to the warm, moist Gulf and intense Gulf lows form there and track NE, that can set off the conditions, even in January.

Posted by: MMCarhelp | April 26, 2010 6:47 PM | Report abuse

Tornadoes can also strike far to the north, in Canada. Edmonton, Alberta, at a latitude of 54 degrees north in a typical Continental climate, was hit by a big F4 tornado in 1987. Regina, Saskachewan, somewhat further to the southeast, has also been hit by significant tornadoes.

Posted by: MMCarhelp | April 26, 2010 6:51 PM | Report abuse

Add parts of India and Bangladesh to those areas frequented by tornadoes and severe weather. This area of the Indian subcontinent is the second-most tornado-prone region [after North America], and the mechanism is similar...wind interactions on the lee side of a prominent mountain range, in this case the Himalayas...similar to those interactions in the lee of the Rockies which give Tornado Alley its reputation. In the case of the Indian subcontinent the tornadoes tend to occur to the south of the mountains, but move southwest to northeast with frontal disturbances passing south of the mountains. Hence, Indian subcontinent tornadoes tend to be a feature of the dry monsoon.

BTW, it's also true that fatalities from large hail are rather common in India and Bangladesh; there are many people there and their housing is more poorly constructed than in the Midwest. People in India and Bangladesh tend to get caught outdoors in severe hailstorms far more frequently than in the American Midwest.

Posted by: Bombo47jea | April 26, 2010 7:13 PM | Report abuse

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