Is Greater Grand Forks safe from tornadoes because the cities were built along the forks of two rivers?

That’s a myth often shared anecdotally in Greater Grand Forks, which has had tornadoes spin nearby but which has not been directly struck since modern records began being kept in 1950.

But is it true?

Absolutely not, says Aaron Kennedy, University of North Dakota atmospheric sciences professor. He said he has been able to watch tornadoes form in the distance from his office window at UND over the past several years.

“The river is a non-factor,” said Kennedy, who has heard the myth about the river’s forks.

The National Weather Service agrees.

“No place is safe from tornadoes,” the weather service notes on its website. “They can cross rivers, travel up mountains, roar through valleys and hit major metropolitan areas.”

National Weather Service meteorologist Brad Hopkins said that back in the 1800s, people believed tornadoes could not form north of Fargo. However, that notion has also proven to be false. Tornadic activity can occur even farther north, with twisters often reported in central Canada.

Another myth: Windows should be opened in the event of a tornado, to equalize pressure in a home. Not true, according to the National Weather Service.

The National Weather Service also does not encourage drivers to seek shelter from a tornado under an interstate overpass.

“Overpasses can concentrate the tornado winds, causing them to be significantly stronger. This places the people under them in an even more dangerous situation,” the NWS website states.

How tornadoes form

Exactly how tornadoes form is not completely understood, according to the National Oceanic and Atmospheric Administration.

What is known is that most result from supercell thunderstorms. These severe thunderstorms have persistent updrafts that can reach speeds of 100 mph. Within the storm, a strong vertical wind shear causes a horizontally rotating cylinder of air. The updraft lifts the rotating cylinder within the supercell. The rotating cylinder of air then narrows, becoming stretched, and spins faster and faster. It eventually forms a tornado.

The tornadoes produced on June 17, 2010 came from a group of supercell storms.

Although technology has gotten more advanced over time, it is nearly impossible to measure the actual wind speed inside tornadoes, as they can destroy just about any unprotected weather instruments placed in their path, according to the Weather Channel. So, the best way to measure a tornado's strength is by looking at the resulting damage. In order to truly classify a tornado, it must hit something, UND atmospheric sciences professor Aaron Kennedy said.

A majority of North Dakota’s tornadoes are classified as weak, which means they often rank as EF0 or EF1 tornadoes. The damage with EF0 storms, with winds 65 to 85 mph, is generally light. The tornado may peel shingles off of some roofs or damage some siding. Branches are broken off trees and shallow-rooted trees might be pushed over.

EF1 tornadoes, 86 to 110 mph, cause moderate damage, including severely stripping roofs, damaging mobile homes or causing windows to break.

EF2 twisters reach speeds between 111 to 135 mph and can cause considerable damage, including roofs torn off well-constructed homes and the destruction of mobile homes.

EF3 tornadoes have estimated wind speeds from 136 to 165 mph and cause severe damage. Entire stories of well-constructed houses can be destroyed and there may also be severe damage to large buildings such as shopping malls. Trees can also be debarked by the storms.

The wind speeds of EF4 tornadoes can reach 200 mph and cause devastating damage, including leveling well-constructed houses.

The most severe tornadoes are classified as EF5 storms. The Weather Channel describes the damage as “incredible.” Strong frame houses are typically leveled off foundations and swept away and automobile-sized missiles may fly through the air in excess of 100 meters. High-rise buildings will have significant structural deformation.

The F5 tornado that hit Fargo in 1957 is the northernmost F5 storm to hit in the United States.