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Tornadoes that hit Washington, D.C. suburbs in September 2001 illustrate a fact often stated by weather forecasters: "Tornadoes can and do happen any time of
Figure 1. Tornado Alley includes parts of Texas, Oklahoma, Nebraska, eastern Colorado and western Iowa, and is characterized by a high frequency of strong and violent tornadoes and a relatively consistent season from year to year.the year in just about any location." No one knows this better than National Severe Storms Laboratory Research Meteorologist Dr. Harold Brooks, who has studied the climatology of severe weather in the United States to better understand when and where tornadoes are most likely to occur. By understanding the threat posed by tornadoes in the United States, particularly the threat of strong and violent tornadoes, forecasters can more accurately predict them and communities can better prepare for them.

Through his study of tornado climatology, Brooks has revealed several important things. First, tornadoes occur most often in the central United States, commonly known as Tornado Alley. Second, the central plains have a repeatable annual tornado cycle, with the highest probability of tornado occurrence in the springtime. Finally, areas outside of tornado alley do not have a typical tornado season and experience fewer tornadoes. These findings have several implications.

In his study, Brooks used a data set of tornado reports from 1921 to 1995. He particularly focused on significant tornadoes, those rated F2 or greater on the Fujita Damage Scale. Even though only about 10 percent of tornadoes are significant, these tornadoes are responsible for the majority of deaths caused by tornadoes in the country, with violent tornadoes - those rated F4 or higher - claiming 67 percent of the total. In addition, these tornadoes typically cause millions of dollars in damage costs when they occur.

The primary area of the United States in which significant tornadoes occur most often is in an L-shaped region from Iowa to Colorado to Texas (see Figure 1), with the highest threat in Oklahoma. In addition, this area has a consistent season each year - from April through mid-June, with the most tornadoes normally occurring in May. These two facts - the conjunction of high frequency of strong and violent tornadoes and the relative consistency of the season from year to year from north Texas up into western Iowa - provide a natural, objective way to define Tornado Alley.

The concept of Tornado Alley may be very important for the emergency management community, Brooks said.

"It is relatively easy to keep awareness up in a region where events happen frequently and where the threat is confined to a relatively short period of time," he said. "In addition, it is typically easier to recruit volunteer storm spotters in such an area and to maintain their enthusiasm."

Tornado near Oklahoma City, OK, May 3, 1999.
Photo by Daphne Zaras, NOAA Public awareness was high, for instance, during the May 3, 1999 Oklahoma City tornado. Despite damaging almost 8,000 structures, fewer than 40 direct fatalities occurred. In addition to timely and accurate warnings from the National Weather Service and live coverage from local television and radio stations, people in the path of the tornadoes knew how to respond and did it.

In contrast, heightening awareness in an area where tornadoes rarely occur or occur over a broader season of the year is much more difficult, Brooks suggests. In fact, he believes a lack of public awareness in areas of the country where the threat of a tornado on any particular day is low is one reason for many of the high death toll events that have occurred in the past 20 years. During that period of time, only two of the 22 tornadoes in the United States have caused at least eight fatalities (representing the highest 10 percent of death tolls) in Tornado Alley. Those are the Andover, Kansas tornado on April 26, 1991, which had its fatalities in a trailer park, and the May 3, 1999, Oklahoma City tornado, which was the (inflation adjusted) biggest property damage tornado in U.S. history.

While almost one-fourth of all significant tornadoes occur in Tornado Alley, only nine percent of the major killers have, Brooks said. Thus, he found, the vast majority of high fatality tornadoes in recent years have occurred in areas such as the southeastern United States where tornadoes are an especially rare event on any given day.

"In these areas, people have not thought about what they are supposed to do to seek shelter from a tornado, and they typically don't make good decisions under pressure," Brooks said. "When their basic state of awareness is very low, they are less likely to respond to a warning."

Emergency managers who must prepare their communities for the rare significant tornado face a significant challenge.

Adapted from Clues from Climatology: When and Where Do Tornadoes Occur? by Keli Tarp, NOAA, 10/8/01

Additional information about the Severe Thunderstorm Climatology project can be found at http://www.nssl.noaa.gov/hazard

The National Severe Storms Laboratory, in Norman, Oklahoma conducts research to improve accurate and timely forecasts and warnings of hazardous weather events such as blizzards, ice storms, flash floods, tornadoes, and lightning.