50th anniversary of the 1974 tornado super outbreak

An event of the century

The super outbreak of 1974 occurred at the end of a very strong La Niña phase. On April 1, a strong low formed over the Plains, which gradually moved eastwards. On its front, warm, humid and unstable stratified air moved northwards, while cold air moved southwards on its back. The first F2 and F3 tornadoes formed as early as April 1 and 2, which also claimed several lives. On April 3, a pronounced trough stretched across large parts of the USA, with the eastern half of the country in the area of a southwesterly upper-level flow (see Figure 1).

The 1974 super outbreak began at about 1 p.m. in Morris, Illinois. As the day progressed, the activity steadily increased, the tornadoes became more intense and the tracks longer. With 148 confirmed storms, it was the strongest outbreak to date. Only the 2011 super outbreak produced even more storms between April 25 and 28 with 216 tornadoes. Nevertheless, the 1974 event remains unique, as 30 tornadoes of 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; in 2011 there were "only" 4 (according to the EF scale, which had already been adjusted at the time). Often no storm of this category forms for many years, the last time the Moore tornado was rated as an EF5 was in 2013!

The first F5 formed at around 15:20 local time, it was the Depauw/Daisy tornado. It claimed 6 lives. The next one hit the town of Xenia in Ohio at 16:30 (Figure 2). It caused enormous damage and 36 people lost their lives.

A little later, another F5 passed through Brandenburg in Kentucky, killing 31 people. The next F5 was the Cincinnati/Sayler Park tornado in Ohio. The community of Tanner in Alabama was hit particularly hard. It was hit by two F5 tornadoes in quick succession. Several helpers and survivors of the first storm were killed in the second. The last F5 in the series covered a distance of 128 km and completely destroyed countless buildings. In some cases, only the bare foundations remained (incidentally, this damage pattern is also used for subsequent categorization).

During a period of 18 hours, tornadoes were continuously active, at the peak of the series even 15 at the same time! The technical aids available to meteorologists at the time were much simpler – in terms of radar, satellites and communication. In addition, there were large-scale power outages in the wake of the destruction. In Indiana, it was no longer possible for meteorologists to differentiate between the various counties. They therefore placed the entire state under a blanket tornado warning. This was the first and only time this had ever happened in the USA. In addition to the United States, 30 people died in an F3 tornado in Windsor, Ontario/Canada. Depending on the source, a total of 315 to 330 people died on these two days. The estimated damage amounts to around 3.5 billion dollars according to today's calculations.

Typical tornado season and current situation

Tornadoes can generally occur in any month. Due to the lower temperature level and lower irradiation, but also due to the lower moisture content of the atmosphere and its often more stable stratification, tornadoes are rather rare in the winter months. This value increases significantly in spring and reaches its peak in early summer. Further information on the typical tornado season can be found in this blog, for example.

Over the past two days, a strong area of low pressure has once again caused several tornadoes to form. The following map provides an overview of the reports received (T = tornadoes, W = strong winds, H = hail). The front will move further east today, Wednesday, with an increased risk of tornadoes, especially in North Carolina and Virginia.