May 6, 1965 — 51 years ago today — was one the deadliest days in Minnesota’s recorded tornado history. Thirteen people died and more than 680 were injured as “tornadoes roared like a gang of drunken devils for six hours through the Minneapolis area,” mostly in the north and west Twin Cities metro area, the Minneapolis Tribune recounted.
Such a severe outbreak of weather is uncommon in Minnesota. Never since has the atmosphere hit the state with a quartet of F4 tornadoes on a single day (two other tornadoes were determined to be an F3 and an F2), according to the Minnesota Department of Natural Resources.
But we have our share of cyclones. Since 1950, Minnesotans have experienced at least 1,724 tornadoes that killed 110 and injured 2,117, according to data kept by the National Oceanic and Atmospheric Administration.
Here’s a look at when, where and why tornadoes touch down in Minnesota.
In Minnesota, tornadoes have occurred with some frequency from April through September, but the state's twister season is historically at its height between May and August. Since 1950, 88 percent of reported Minnesota tornadoes occurred in those months.
As for time of day, the danger zone for tornadoes is in the afternoon and early evening. Eighty-two percent of tornadoes touched down between 2 p.m. and 8 p.m. That’s when we’re most likely to see the type of climatic cocktail that produces a tornado, said Harold Brooks, a senior research scientist at the National Severe Storms Laboratory in Norman, Oklahoma: Tornadoes materialize from mighty thunderstorms, when there’s warm, moist air near the ground, dry, cool air at upper levels, and another mechanism — like a front — that pushes the hot air up into the middle of the cool air.
“To make it the kind of storm we call a supercell — the rotating type of storm — you want to have the winds in the environment of the storm to have a certain special distribution,” Brooks said.
In the lower Great Plains region, those conditions are more common in spring, whereas Gulf Coast states reach peak tornado season in winter, Brooks said.
Big tornadoes uncommon
Some twisters stick out in our collective mind, like the aforementioned 1965 ones in the Twin Cities area ($50 million in damage), 1981’s Har Mar tornado, the 13-tornado rampage that tore through the St. Peter area in March of 1998 and the 2011 tornado that clobbered parts of North Minneapolis (followed by what some called inadequate response from the city). But such destructive events are relatively rare.
The intensity of a tornado is measured with the Fujita scale (which switched to the fairly comparable Enhanced Fujita scale in 2007), a 0 to 5 rating that indicates wind speed and likely damage.
The majority of tornadoes reported in Minnesota in the last six decades were F0s, also known as gale tornadoes — the type that wreak minor havoc on chimneys, trees and street signs, but rarely injure or kill anyone.
One percent were destructive F4 tornadoes that level well-constructed houses, lift even large vehicles into the air, and hurl massive objects around. Minnesota has seen only three known F5 tornadoes since 1950, the type that rip the bark from trees and damage steel-reinforced concrete structures.
Is Southern Minnesota more tornado prone?
A map of Minnesota’s recorded tornadoes shows that most of them appear to have touched down in the state’s southern two-thirds. Is there something about that rarefied climate of pines, taconite and lakes Up North that repels them?
Sort of. The air in Northern Minnesota tends to be colder, so it’s more difficult for tornado-causing warm air to gather near the ground, Brooks said.
But the lack of tornadoes in the north is also likely a function of reporting. National Weather Service staff at 122 forecast offices around the country (there are two headquartered in Minnesota: Chanhassen and Duluth) monitor weather in assigned areas. When they hear of a potential twister, an envoy is sent to inspect damage and determine whether or not it was caused by a tornado.
There are fewer people in northern Minnesota, and if a tornado touches down and nobody sees it or reports it, it’s not likely to make it onto official tornado registers.
“We have yet to find a good moose reporting system. They tend to not be very likeable animals if you’re trying to get them to tell you about weather events,” Brooks said.
To browse national tornado data by year, check out this Smithsonian Institute interactive.
Random Acts of Data is an occasional series by MinnPost data reporter Greta Kaul and news editor Tom Nehil. The goal: to answer questions about all things Minnesota using the vast amount of data at our disposal. If you have a question you’re wondering about, send an email to firstname.lastname@example.org with the subject line, “Random Acts of Data.”