Purdue News

April 1, 2005

Purdue scientists unravel Midwest tornado formation

WEST LAFAYETTE, Ind. – A Purdue University study of tornado formation indicates that twisters can develop in unexpected ways and at unexpected times and places, a discovery that presents a new twist to weather watchers across the country.

Tornado survey
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Although tornadoes are often conceived of as arising from springtime storms that develop in early evenings out of isolated weather cells, a new study spearheaded by Purdue's Robert "Jeff" Trapp indicates those conceptions often fail to hold, especially in the Midwest. Although far from the so-called "Tornado Alley," a region that falls generally in the plains of Texas, Oklahoma and Kansas, the Midwest still experiences a high number of the storms every year.

After examining data on more than 3,800 U.S. tornadoes, Trapp's team has found that many twisters develop within the line-shaped storm fronts that often sweep across the country. The twist is that these are tornadoes that are more likely to form late at night and in colder months.

"The upshot of our analysis is that tornadoes form under a broader set of circumstances than meteorologists once thought, and this is especially true if you live far from Tornado Alley," said Trapp, an associate professor of earth and atmospheric sciences in Purdue's College of Science. "If you're driving in the rain on an October midnight near Lake Michigan, remember that a tornado is not outside the realm of possibility."

Survey 2
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Trapp's study, which he performed with colleagues at the University of Oklahoma, Colorado State University and the National Severe Storms Laboratory, appears in the February issue of the journal Weather and Forecasting. As a first step toward improving our ability to predict tornado strikes, which often come with only a few minutes' warning, the group initially set out to find what types of storms generally produce the destructive funnels.

"In the heart of Tornado Alley, tornadoes most often develop from relatively small 'cell' storms that look like blotches on a Doppler radar weather map," Trapp said. "Over time, these cells frequently merge into line-shaped storms that can stretch hundreds of miles. The conventional wisdom has been that line storms don't often spawn tornadoes, but we found that a significant number did."

The group analyzed 3,828 tornadoes that were spotted in the United States from 1998 to 2000. Though 79 percent of these tornadoes came from cells, Trapp said, 18 percent came from line storms nationwide. But in the Midwest, those numbers varied widely. In Indiana, for example, about half of the tornadoes that developed came from line storms.

"This implies that we may be overlooking many tornado-breeding storms in the Midwest and elsewhere," Trapp said. "Complicating the issue is that what we know about tornado formation is based largely on the cell-type storms observed in Oklahoma and Texas."

Cell storms often form in springtime in the late afternoon, which is why most storm warnings in Tornado Alley are issued in the evening, Trapp said. But line storms often form later at night and also in the cool season, between October and March.

"These are the hours and months when people probably least expect severe weather," Trapp said. "In fact, they may not even see a tornado coming because of the lateness of the hour."

The silver lining to all these clouds is that tornadoes are less frequent overall in the Midwest – Indiana only experiences about 20 tornadoes a year, compared to about 52 in Oklahoma and 124 in Texas – and Midwest storms are often weaker than those of the Plains. But because many of these tornadoes may go undetected, Trapp said, people outside Tornado Alley should begin to shift their thinking.

"We're not trying to be alarmist with these findings," he said. "But we hope that people will stay alert to tornado risk even outside the traditional severe storm season."

Trapp said that the current study represents a step toward better weather prediction, but there's still much work to be done on the problem.

"This paper is meant to be the first in a series of studies of tornado development," Trapp said. "Now that we have a better picture of when and where they form, we can begin to develop tools that can serve a practical purpose. For now, we are merely trying to raise awareness among weather watchers about tornado-forming situations that, until now, have been, figuratively speaking, beneath our radar."

This research was funded in part by the National Science Foundation.

Trapp is affiliated with Purdue's climate and extreme weather group as well as with the Purdue Climate Change Research Center. The center promotes and organizes research and education on global climate change and studies its impact on agriculture, natural ecosystems and society.

Writer: Chad Boutin, (765) 494-2081, cboutin@purdue.edu

Source: Robert "Jeff" Trapp, (765) 496-6661, jtrapp@purdue.edu

Purdue News Service: (765) 494-2096; purduenews@purdue.edu


Related Web site:
Related release on Trapp's work


While people generally expect tornadoes to form from isolated storm cells in the evening in springtime, a survey of weather data led by Purdue researcher Robert "Jeff" Trapp has revealed that a significant number of tornadoes form under unexpected circumstances. Especially in the Midwest, many tornadoes form from "line-shaped" storms often associated with large weather fronts. Such tornadoes are more likely to form during winter months, late at night. These data were taken from a survey of 3,800 tornadoes that formed over the United States from 1998-2000. (Purdue University/Trapp laboratories)

Publication-quality graphics are available at https://ftp.purdue.edu/pub/uns/+2005/trapp-tornado.jpg and https://ftp.purdue.edu/pub/uns/+2005/trapp-graphic.jpg



Tornadoes from Squall Lines and Bow Echoes

Robert J. Trapp, Sarah A. Tessendorf, Elaine Savageau Godfrey and Harold E. Brooks

The primary objective of this study was to estimate the percentage of U.S. tornadoes that are spawned annually by squall lines and bow echoes, or quasi-linear convective systems (QLCSs). This was achieved by examining radar reflectivity images for every tornado event recorded during 1998-2000 in the contiguous U.S. Based on these images, the type of storm associated with each tornado was classified as cell, QLCS, or other. Of the 3,828 tornadoes in the database, 79% were produced by cells, 18% were produced by QLCSs, and the remaining 3% were produced by other storm types, primarily rainbands of landfallen tropical cyclones. Geographically, these percentages as well as those based on tornado days exhibited wide variations. For example, 50% of the tornado days in the state of Indiana were associated with QLCSs. In an examination of other tornado attributes, statistically more weak (F1) and fewer strong (F2-F3) tornadoes were associated with QLCSs than with cells. QLCSs tornadoes were more probable during the winter months than were cells. And finally, QLCSs tornadoes displayed a comparatively higher and statistically significant tendency to occur during the late night/early morning hours. Further analysis revealed a disproportional decrease in F0-F1 events during this time of day, which led the authors to propose that many (perhaps as many as 12% of the total) weak QLCSs tornadoes were not reported.


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