America gets more tornadoes than anywhere on Earth, and the national count has barely changed in forty years. What changed is where they land: out of the open Plains and toward the Mississippi, where storms more often come at night and houses are lighter.
1,200
Tornadoes a year · United States
Fig. 1 — The drift. Green: where the classic and Dixie alleys sit (schematic). ▲ rising, ▽ falling, 1979–2017. Dashed: the 1925 Tri-State track.
The average number of tornadoes reported in the U.S. each year, more than any other country (NOAA Storm Prediction Center). The national total has stayed roughly flat since 1979. Where they touch down has not.
8 states
where tornado reports and tornado-friendly weather rose from 1979 to 2017, every one of them east of the classic alley (Gensini & Brooks, 2018).
01 · Where they land
Texas leads the count. Florida leads per square mile.
Raw totals favour big states. Average tornadoes per year, 1991–2010, coloured by each state's 1979–2017 trend.
tornadoes per 10,000 sq mi a year in Florida — the highest density of any state.
11.7
per 10,000 sq mi in Kansas, the heart of the classic alley.
Florida's tornadoes are mostly weak and brief. The alley question is less about how many touch down than about where the violent, long-tracked ones form — and that is the pattern the map's markers follow.
02 · The drift
Fewer on the Plains, more in the Mid-South.
Over 39 years, national totals held steady while the activity slid east — in both tornado reports and the atmospheric ingredients that make them.
Trends apply to portions of each state. Report counts can rise just because more people are watching, so the authors checked them against the Significant Tornado Parameter, a weather-data index that doesn't depend on witnesses. The two agreed.
The shape of the shift
3 → 8
3 regions trending down, all in or beside the Plains alley; 8 states trending up, all east of it, clustered along the Mississippi and Ohio rivers.
39 years
of records, 1979–2017. The national total stayed roughly flat over the whole period.
03 · Why the east is deadlier
Night storms, light houses, and trees in the way.
On the Plains you can often see a tornado coming across open ground in daylight. In the Mid-South, a storm is more likely to arrive after dark, behind trees, and to hit a manufactured home.
Mobile homes' share of at-home tornado deaths · AP / NOAA
2.5×
Tornadoes between midnight and sunrise are about 2.5 times as likely to kill as daytime ones (Ashley et al., 2008). The Southeast has a high share of night tornadoes.
Mobile & manufactured homes
04 · The scale
How a tornado gets its number.
Since 2007, U.S. tornadoes are rated on the Enhanced Fujita scale from the damage they leave, then converted to estimated 3-second gusts.
The 2011 Super Outbreak produced three EF5s in four days. The 1925 Tri-State tornado came before any rating scale existed; it is widely regarded as F5-equivalent, but that is an estimate.
05 · Two storms that drew the map
One tornado in 1925. 362 in 2011.
The deadliest single tornado and one of the deadliest outbreaks in U.S. records both struck the eastern side of the map: the Ohio Valley and Alabama.
Deaths · NOAA NWS / NCEI
219 mi
Tri-State path across Missouri, Illinois and Indiana in about 3½ hours (dashed line on the map).
362
confirmed tornadoes, April 25–28, 2011. Alabama was hit hardest; April 27 was the deadliest tornado day in 85 years.
Why it matters · what to notice on the map
Tornado Alley doesn't have an official border, and the one people picture is out of date.
Find the drift arrow. It starts in Oklahoma, where trends fell, and ends among the eight ▲ states where they rose.
Follow the rivers. The rising states sit along the lower Mississippi and the Ohio, a region of trees, hills and towns, not open wheat fields.
Look at Tuscaloosa and the dashed Tri-State track. The deadliest events on record are already on the eastern side of the map.
Same storms, different outcomes. More night tornadoes and more manufactured housing (53% of at-home deaths) mean the same tornado is more dangerous in Dixie Alley than in Kansas.