A violent tornado that roared through southwestern Manitoba earlier this week has received a "high-end EF-2" rating from Environment Canada.

The preliminary rating was issued late Wednesday afternoon, following an assessment by meteorologists who surveyed the damage between Pierson and Virden, Man., following Monday night's twister.

A tornado rated EF-2 on the Enhanced Fujita Scale comes with wind speeds of 179 to 218 km/h and leads to roofs blown off homes, sheds destroyed and mobile homes flipped.

Environment Canada has called Monday's tornado a "rare" event, as radar imagery shows it stayed on the ground for 2½ to three hours. In Canada, tornadoes rarely stay on the ground longer than a few minutes.

Earlier, Environment Canada meteorologist Natalie Hasell said the team was looking at damage to buildings to help rate the twister. But any quick clean up by people in the area could present a challenge, she said.

"If it goes through a field, there's not a whole lot to rate there. But if it touched buildings that type of indicator can give us an idea how sturdy the structure was and then we can rate the damage," Hasell said.

"Once the damage is rated then we get the EF, that enhanced fujita scale rating."

Environment Canada says it's still surveying the area. It's asking people who may have taken photos of the tornado or the damage to call 1-800-239-0484 or email storm@ec.gc.ca.

How do we rate tornadoes?

On April 1, 2013, Environment Canada adopted the Enhanced Fujita Scale — or EF-Scale — to measure the strength of a tornado (it had been in use in the U.S. since February 2007). It's an improved version of the original Fujita Scale that was devised in 1971 by a pioneer in tornado research at the University of Chicago, Tetsuya (Ted) Fujita.

The new EF-Scale estimates three-second-gust wind speed inside a tornado based on the damage that's observed by examining a large number of indicators, ranging from residential housing to office towers to trees, as well as ground markings and meteorological data. The EF-Scale damage ratings are backwards-compatible with the original F-Scale, but the associated wind speeds have undergone major changes.

The EF-Scale ranges from 0, for a tornado that pushes over shallow-rooted trees and causes some damage to chimneys and signs, to 5, when houses are lifted off their foundations, vehicles are thrown 100 metres or more, and trees are uprooted and carried long distances.

Intensity Wind speed Type of damage

EF0

104-137 km/h Damage to trees, shingles, antennas and windows.
EF1

138-178 km/h Trees uprooted, cars overturned.
EF2

179-218 km/h Roofs blown off homes, sheds destroyed, mobile homes flipped.
EF3

219-266 km/h Walls, roofs destroyed, metal buildings collapsed, forests destroyed.
EF4

267-322 km/h Well-built homes mostly destroyed, heavy objects thrown long distances.
EF5

323 km/h or more Homes destroyed and/or blown great distances, roofs blown off larger structures, which are otherwise heavily damaged.