Between April 3 and 4, 1974, the USA experienced the largest series of tornadoes since observations began. Within 24 hours, 148 tornadoes passed through 13 states.
A century event
The Super Outbreak of 1974 occurred at the tail end of a very strong La Niña. On April 1, a powerful low formed over the Plains and gradually shifted eastward. On its front, moist-warm and unstably stratified air was carried northward, and on its back, cold air was carried southward. Already on April 1 and 2, the first F2 and F3 tornadoes formed, which also claimed several lives. On April 3, a pronounced trough extended over large parts of the U.S., with the eastern half of the country lying in the area of a southwesterly high-altitude flow (see Figure 1).
Fig. 1: Weather situation on April 3, 1974; Source: NOAA
The 1974 super outbreak began at approximately 1 pm in Morris, Illinois. As the day continued, activity steadily increased, tornadoes became more intense, and tracks became longer. With 148 confirmed storms, it was the strongest outbreak to date. Only the 2011 super outbreak produced more storms between April 25 and 28, with 216 tornadoes. Still, the 1974 event remains unique, as 30 tornadoes in the two highest categories formed within 24 hours alone, including 23 F4s and 7 F5s. This number of F5 tornadoes on a single day is unprecedented to date; in 2011, there were "only" 4 (according to the EF scale, which had already been adjusted at that time). Often no storm of this category forms for many years, the last time the Moore tornado was rated as EF5 was in 2013!
The first F5 formed at about 15:20 local time, it was the Depauw/Daisy tornado. It claimed 6 lives. The next one hit the town of Xenia, Ohio at 16:30 (Figure 2). It caused enormous damage, 36 people lost their lives.
Fig. 2: F5 Tornado in Xenia, Ohio; Source: NOAA
A little later, another F5 passed through Brandenburg, Kentucky, killing 31 people. The next F5 was the Cincinnati/Sayler Park tornado in Ohio. The community of Tanner, Alabama, was hit particularly hard. It was hit by two F5 tornadoes in quick succession. Several aid workers and survivors of the first storm were killed in the second. The last F5 of the series moved over a distance of 128 km and completely destroyed countless buildings. In some cases, only the bare foundations were left (by the way, the subsequent categorization is also based on this damage pattern).
Fig. 3: Trajectories of the various tornadoes during the 1974 super outbreak.; Source: Evan Fisher (via Twitter)
During a period of 18 hours, tornadoes were continuously active, at the peak of the series even 15 at the same time! The technical tools of the meteorologists were much simpler at that time – both in terms of radar and satellites, and in terms of communication. In addition, there were large-scale power failures in the course of the destruction. In Indiana it was no longer possible for the meteorologists to differentiate the various counties. They therefore put the entire state under a blanket tornado warning. This was the first and only time so far in the United States. In addition to the United States, 30 people also died in an F3 tornado in Windsor, Ontario, Canada. The total death toll for these two days was 315 to 330, depending on the source. The estimated damage amounts to approximately $3.5 billion according to today's calculations.
Already last Friday, dozens of tornadoes moved across the U.S., with 132 reports received, according to NOAA (more information is also available in last week's blog). Investigations are still ongoing, however, and it is still too early for a final assessment in this regard. According to current reports, 24 people lost their lives so far, many are still in hospital. Today, Tuesday, and especially in the coming night, there is a threat of a repeat, the initial situation is almost a copy of last Friday. It is feared that many areas will be affected again within a few days.
Fig. 4: Probability for tornado formation today Tuesday, April 4, 2023; Source: NOAA