The most violent tornado outbreak on record occurred on April 3-4, 1974, during an event dubbed the "Super Outbreak."

147 tornadoes touched down in a 24-hour period, making this the United States' most prolific tornado outbreak on record—and the benchmark for all subsequent tornado outbreaks—until a larger event unfolded on April 27, 2011.

The disaster broke the record for the most violent tornadoes in a single day, with a total of 7 scale-topping F5 twisters, 23 F4s, and 33 F3s reported during the outbreak. An additional F3 tornado touched down north of the border in Ontario.

The Super Outbreak occurred as a classic springtime low-pressure system swirled into the Great Lakes region on April 3.

Intense supercell thunderstorms developed and thrived in the warm, moist airmass on the southern side of the system. Many of these supercells each produced numerous long-track tornadoes, some of which stayed on the ground for dozens of miles.

Meteorologists were able to use radar imagery to issue advanced tornado warnings before some of the larger twisters—most notably the storm that hit Xenia, Ohio—but the outbreak still exacted a terrible toll. 335 people lost their lives during the Super Outbreak of 1974.

A generational tornado outbreak unfolded across the southern and eastern United States on April 27, 2011. The event holds the record for the most tornadoes ever recorded in 24 hours, with 247 tornadoes confirmed by survey crews through April 28.

Tornadoes ripped across portions of 18 states during the outbreak, with the brunt of the tornadoes focused on interior sections of the southeast. Alabama bore the brunt of the outbreak with 62 confirmed tornadoes striking the state April 27.

Six of the tornadoes were scale-topping EF-5s with winds of 201+ mph. These high-end tornadoes scrubbed well-built structures clean off their foundations, debarked trees, and left behind extreme scouring of dirt and asphalt from the ground.

This was one of the deadliest tornado outbreaks on record. 348 people died from the storms on April 27-28; most fatalities occurred in Alabama. One high-end, long-track EF-4 tornado that cut from Tuscaloosa to Birmingham was responsible for 65 deaths and more than 1,500 injuries.

The longevity of the tornadoes matched their ferocity. Survey crews found that the Tuscaloosa-Birmingham tornado was on the ground for more than 80 continuous miles. Some of the supercells cycled with multiple tornadoes over hundreds of miles.

Forecasters predicted this tornado outbreak well in advance, giving days of notice that April 27 would be a red-letter weather day that required significant attention and precautions.

Making matters worse is that the outbreak was immediately preceded by a powerful squall line with embedded tornadoes that tore across the affected areas earlier on the morning of April 27.

Many of the communities shattered by tornadoes later in the day were still reeling and without power from that morning's awful storms, a potential factor in at least some of the subsequent deaths and injuries.

One of the strongest and deadliest tornadoes ever recorded in the United States ripped through Joplin, Missouri, in the early evening hours on Sunday, May 22, 2011.

A powerful supercell entered southwestern Missouri to find an ideal environment to produce a significant tornado. The twister touched down just west of Joplin and grew more than one mile wide by the time it reached town.

The multiple-vortex tornado churned through south-central Joplin, reducing street after street of homes and businesses to unrecognizable rubble.

Survey crews assigned the tornado a scale-topping EF-5 rating based on indications of 201+ mph winds amid the rubble. Very few structures can withstand the 201+ mph winds needed to achieve EF-5 level damage. Experts used clues like an entire hospital shifting on its foundation as the basis for this tragic tornado's rating.

158 people died as a result of the immense size and scale of destruction left behind by the Joplin tornado. It was the seventh-deadliest single tornado ever recorded in the U.S., and the deadliest since the advent of modern severe weather forecasting and warning systems.

A 2.6-mile-wide tornado touched down outside of El Reno, Oklahoma, in the early evening hours on May 31, 2013, becoming the widest tornado ever observed.

The culmination of a series of tornado outbreaks in late May 2013, the storm ripped through central Oklahoma just one week after a deadly EF-5 tornado hit the nearby community of Moore.

The tornado touched down at 6:04 p.m. CDT and initially moved on a southeasterly track. The twister turned east over the next 15 minutes and rapidly expanded to its maximum width, killing several storm chasers who were caught off-guard by the tornado's growth.

After turning northeast, the tornado continued toward Interstate 40 where it killed several more motorists west of Oklahoma City. The tornado remained on the ground for a little more than 16 miles, killing 8 people and injuring two-dozen more before finally lifting at 6:43 p.m. CDT.

While mobile Doppler radar recorded winds in excess of 200 mph at the tornado's peak strength, suggesting that it may have reached EF-5 intensity, survey crews with the National Weather Service only rated the tornado an EF-3 based on the sparse physical damage left behind by the storm. The Enhanced Fujita Scale is a damage scale used to estimate wind speeds; actual wind speed measurements have no bearing on a tornado's EF-scale rating.

The historic El Reno tornado was one of 19 twisters reported across central Oklahoma on May 31, 2013. The parent supercell that produced the ultra-wide tornado contributed to a historic flash flood disaster across the Oklahoma City metro area, which killed an additional 14 people.

The longest confirmed tornado track in the United States cut a path measuring 219 miles through portions of southeastern Missouri, southern Illinois, and southwestern Indiana.

This tornado reportedly killed 695 people—a record that stands today as the deadliest single tornado in U.S. history.

Dubbed the Tri-State Tornado, the twister was part of a larger tornado outbreak spawned by a classic early-spring low-pressure system moving through the central United States. Warm, unstable air fuelled powerful supercell thunderstorms that formed in an environment ripe for tornado development.

The historic tornado remained on the ground for three-and-a-half hours, touching down near Ellington, Missouri, before lifting near the town of Petersburg, Indiana.

Numerous towns were completely or near-completely destroyed, including Annapolis, Missouri; Gorham, Illinois; Parrish, Illinois; and Griffin, Indiana.

Much of the intense damage and a large number of the injuries and fatalities occurred in and around Murphysboro, Illinois.

Although tornadoes wouldn't receive damage ratings for a few more decades, most tornado experts agree in hindsight that the Tri-State Tornado would've easily been rated an F5/EF-5 based on the level of devastation left in its wake.