Patrick Springer, Published April 20 2014
Tornado danger often underestimated in Red River Valley
The arbitrary designation coincided with the Northern Pacific Railroad, extending from Duluth, Minn., to the Puget Sound, and supposedly marked the northern limit of tornadoes.
That spurious claim, touted by the railroad to lure settlers to areas north of the Tornado Belt in Dakota Territory, was obliterated in 1887 when a “whirlwind” blew through Grand Forks, killing five and destroying property.
Even today, more than a century later in an area so prone to formidable blizzards and floods, it can be easy to underestimate the dangers of tornadoes.
A fresh reminder of that reality came with the recent determination that some residents of the Red River Valley spend as much time under tornado warnings as certain areas in the heart of “Tornado Alley.”
Cass County, for example, spends on average almost 2½ hours per year under tornado warnings, a span similar to many counties in “Tornado Alley,” a turbulent swath of the southern and central Plains.
That finding – a visual indicator of the area’s significant tornado risk – was revealed in a map recently compiled by a weather researcher in Iowa who graphically depicts information from meteorological archives.
After generating the map earlier this month, researcher Daryl Herzmann of Iowa State University in Ames realized that counties with the most time under tornado warnings usually were very large.
That could explain why Clay County is under tornado warnings a bit more than 1½ hours per year, about an hour less than adjacent Cass County in North Dakota.
Still, even after adjusting for county size, the Red River Valley and surrounding areas are comparable to much of the Great Plains in time under tornado warnings.
‘Great Plains alley’
Weather analysts at the National Climatic Data Center also have come up with a way to compare tornado frequency and land area.
Over a recent 20-year period, North Dakota and South Dakota each averaged 4.7 tornadoes yearly per 10,000 square miles; Minnesota averaged 5.7 tornadoes.
Those are well above the national average of 3.5 tornadoes per year. By comparison, Oklahoma averaged 9, and Kansas 11.7 per 10,000 square miles.
No precise boundaries exist for “Tornado Alley,” and some weather experts argue that tornadoes are so widespread that the designation is meaningless.
But Greg Gust, a warning coordination meteorologist for the National Weather Service office in Grand Forks, said the zone extends even into southern Manitoba.
“We’re clearly in the classic Great Plains alley,” he said.
Colder weather in the northern Plains, however, shortens the tornado season. Although tornadoes in North Dakota have been observed as early as March and as late as November, most occur during the summer months of June, July and August.
“It’s a short season but a very robust season,” Gust said. “We get a lot of activity in a very short time.”
June is usually the area’s peak tornado month. The most notable example is North Dakota’s deadliest tornado, which struck Fargo on June 20, 1957, killing 10 people.
Twin tornadoes plowed through Wadena, Minn., on June 17, 2010, causing millions of dollars of property damage, requiring the school to be rebuilt.
The tornado that killed one person and demolished two trailer parks in Northwood, N.D., on Aug. 26, 2007, came after the June peak, but at the tail end of the normal tornado season.
Intensity and frequency
Because the northern Plains is sparsely populated, with few structures in rural areas to be damaged by tornadoes, their impacts tend to be less than in more densely populated areas, Gust said.
“The rating of the intensity is highly influenced by what it hits,” he said. Even so, high-intensity tornadoes “still are high-frequency occurrences around here,” Gust added, citing the Wadena and Northwood tornadoes as recent examples.
Over a 10-year period, about 60 percent of the tornadoes in Cass County are equivalent to those in a similarly sized area of eastern Oklahoma, an area known for the intensity and frequency of tornadoes, Gust said.
A more dense radar network and a bigger corps of trained weather spotters in the central and southern Plains have been shown to account for much of the higher frequency of reported tornadoes in those areas, he said.
“We have much less dense radar networks up here,” he said. The wider gaps in radar make it more difficult to detect tornadoes on the fringe of their reach.
Of course, the prairie was even more sparsely settled back in 1886, when some marketing mind for the Northern Pacific Railroad dreamed up the “Tornado Belt.”
The false reassurance likely was intended to help ease fears settlers might have in moving to the wide open prairie, said Nancy Godon, co-author of “Reshaping the Tornado Belt,” a book that explored the 1887 Grand Forks tornado, which debunked the myth that the northernmost reaches of Dakota Territory were outside the range of tornadoes.
She and her co-authors, including her meteorologist husband, Vince, determined that cyclonic winds tore through Grand Forks and continued through East Grand Forks, Minn., killing five people and destroying property along a 20-mile path.
“People didn’t really understand that tornadoes can happen anywhere,” Nancy Godon said, noting that severe tornadoes have occurred in Manitoba.
If the 1887 tornado were to happen today in Grand Forks, which now is much more densely populated and developed, the damage would be much worse.
“If that happened today, it would be devastating,” she said.
Readers can reach Forum reporter Patrick Springer at (701) 241-5522