GREAT FALLS — Unlike those in "Tornado Alley" in the central part of the country, most Montana tornadoes are relatively small and usually touch down in sparsely-populated areas — but not always.
In 2016, an EF-3 tornado hit the town of Baker in southeast Montana. In 2015, a small tornado hit near Sidney in Richland County in eastern Montana, injuring one person and causing some damage.
In June 2010, a tornado hit Billings, causing significant damage to the MetraPark facility. Just several weeks later, two people were killed when a tornado struck a family ranch near Reserve in northeastern Montana.
Montana's tornado season generally spans from late May through early August. The two key ingredients for tornado formation are low-level moisture and wind shear. Wind shear is the change in wind direction with height up to the mid-levels of the atmosphere.
"When it comes to tornadoes in Montana, it's really about moisture. So to get tornado development, we really want a moist layer right at the surface of the Earth at the lowest levels. That is provided actually by the Gulf of Mexico," said Patrick Gilchrist, the warning coordination meteorologist for the National Weather Service in Glasgow.
The likelihood of tornadoes increases in eastern Montana, due to the proximity to the Gulf of Mexico, not the change in topography. Higher terrain in western Montana actually acts as ignition for severe weather outbreaks as it forces the air to move vertically creating lift in the atmosphere. The severe weather outbreaks can often last through the night, traveling as far as Minnesota.
While tornadoes are less common in central Montana, the threat still exists. There have been a few notable events in recent history. An F2 moved through Lewistown back in 1999 and a series of F3 tornadoes southeast of Big Sandy in Choteau County back in 1988.
The Enhanced Fujita Scale replaced the Fujita Scale in February 2007.
Damage assessments are utilized in determining the tornadoes scale, which can be difficult given the rural nature of eastern Montana.