60-Year Data: Tornadoes a Major Threat Beyond 'Tornado Alley'
The recently updated severe report database of the government's Storm Prediction Center, now including data from 1950 through 2010, indicates that tornado touchdowns are indeed most common in the Plains. However, tornado touchdowns have been widespread across the eastern two-thirds of the nation and have dotted the Rockies and the West Coast.
(Tornado touchdowns from 1950-2010, courtesy of NOAA)
Tornado touchdowns have historically been numerous in the Midwest and the Carolinas, as well as in the Deep South, which is sometimes referred to as "Dixie Alley" because of its elevated tornado threat. The southern heat and humidity are potential fuel for tornadoes, but the contrasting cooler, dry air is not as abundant as it is in the Plains.
The Atlantic Seaboard, including the major cities from New York to Washington, has been a relatively active region since 1950 compared with the Appalachian region just to the west.
This distribution of tornadoes often feeds into the myth that mountains protect communities from tornadoes. In fact, the more rugged terrain of the mountainous region does not disrupt tornado tracks or prevent formation of tornadoes; however, the slightly cooler and drier climate leads to conditions less conducive to formation.
In other words, the relative lack of tornado touchdowns in mountainous regions is because of the weather, not the terrain.
More numerous tornadoes where the air is relatively warm can also be seen in the Western U.S. as well. Local peaks in tornado touchdowns can be seen in the valley locations, such as southern Idaho, the Central Valley of California and the deeper Utah valleys.
The lack of moisture is the limiting factor for tornado production in the West, and it's also a limiting factor for the strength and longevity (how long a tornado remains on the ground) of western tornadoes.
(Tracks of tornadoes from 1950-2010, courtesy of NOAA)
Long-tracking tornadoes are rare in the West but are common in the Plains. Long-tracking tornadoes also occur with frequency in the South and Midwest. Although less common, they're also a danger in the mid-Atlantic region and Northeast.
This year so far has been particularly active in terms of tornado formation, with the Storm Prediction Center having accumulated more than 700 preliminary reports during that time. While the confirmed number might be slightly lower, the number is approaching half of the three-year average of 1,376 in fewer than four months.
And the most active months for tornadoes -- May and June -- are still to come.