Especially during spring and early summer, severe thunderstorms with tornadoes form in North America with more or less regularity. While most of these events produce "merely" a handful of tornadoes, extreme events can occur if the constellation of pressure distribution, wind shear, and moisture is ideal for the storms. The most devastating and very deadly tornado outbreak to date occurred exactly 12 years ago. Within 4 days 362 tornadoes formed!
The super outbreak was caused by a powerful high pressure trough that moved into the Southern Plains (Texas, Kansas, Oklahoma, and parts of Nebraska, Colorado, and New Mexico) on April 25. An extratropical low also developed in advance of this high trough and subsequently moved northeastward. Thus, the "main ingredients" for a major storm event were in place. On the one hand, moist-labile air was carried northward from the Gulf of Mexico, and on the other hand, the trough provided the necessary lifting effects and thus served as an initial trigger for the formation of supercells. Behind the trough, a very strong jet stream in middle layers of the troposphere (wind speeds between 150 and 180 km/h) brought additional strong wind shear. The thunderstorm cells began to rotate.
Chronology of a catastrophe
In the days leading up to the storm outbreak, the Storm Prediction Center (SPC) predicted a multi-day threat of severe storms with strong tornadoes and large hail. During April 25, the first supercells formed, which also produced tornadoes. An existing tornado warning was soon replaced by a tornado warning of "particularly dangerous situation". The states affected were Arkansas, Missouri, Oklahoma, Texas and Louisiana. The most devastating tornado hit the city of Vilonia that day, resulting in four deaths here. A total of 42 tornadoes and five deaths were counted for the first day of the super outbreak.
A severe weather warning of the highest level (high risk, level 5 out of 5) was issued for the same region on April 26. A negatively-directed upper-level trough (trough axis from northwest to southeast) led to the formation of two surface lows. One low moved northeast along the Mississippi River into Wisconsin, spawning several tornadoes in its path. Most of them were relatively weak and destroyed agricultural buildings in particular, but fortunately did not hit any large towns. However, hail up to 5 centimeters in size also caused some damage. The second ground low formed further south over Texas and "benefited" from the more favorable atmospheric conditions. It was able to deepen further and produce various tornadoes. The strongest was rated as EF3, with wind peaks of up to 260 km/h. Fortunately, no fatalities occurred on this day despite 55 tornadoes.
Fig. 1: Tornado tracks on a piece of open land; Source: NOAA National Weather Service
The situation was quite different on the following day. Compared to the first two days, the greatest potential for severe weather shifted further east. After a first series of tornadoes in the morning hours, more and stronger thunderstorm cells formed during the afternoon. These heralded the peak of the super outbreak. In some areas, the SPC issued not only the highest warning level for severe weather, but also an extremely rare and high tornado probability of 45% within a 25-mile radius – rightly so. From mid-afternoon on, the thunderstorm cells really exploded. By late evening, 207 tornadoes were counted, 70 of which were rated as strong tornadoes (EF2+). On this day alone, 316 people died and about 3000 were injured, some of them seriously. Worth mentioning are the four EF5 tornadoes. These had wind peaks of at least 320 km/h, although certain were well above that. However, an exact determination is not very easy, because on the one hand the wind field is very small and the probability of an existing wind measuring instrument in this area is therefore extremely low. On the other hand, most measuring instruments would be destroyed by such winds anyway. In many cases, the maximum wind speed is inferred from the damage caused (and consequently the tornado is categorized). Nowadays, the Doppler technology of radar systems can also be used to estimate the approximate wind peaks.
Fig. 2: Remains of a foundation after an EF5 tornado in Hackleburg.; Source: NWS Birmingham
On April 28, the active zone moved further toward the East Coast. More tornadoes formed from Georgia south to near the border with Canada. Most of the 40 tornadoes were weaker, but 3 people still died that day.
Balance and classification
EF5 tornadoes are very rare and statistically occur only once every few years. The last one occurred in Moore, Oklahoma in 2013. Four in one day is extremely unusual, it is the second highest value for a single day after the most severe super outbreak to date between April 3 and 4, 1974, when as many as seven EF5 tornadoes were recorded within 24 hours.
A total of 324 people were killed and more than 3000 injured over the entire period. Of the 362 tornadoes, 31 were fatal and caused $12 billion in damage. With the exception of the 1936 Tupelo–Gainesville tornado outbreak, the death toll in a 24-hour span has never been higher than on April 27, 2011, but at that time the technical capabilities for early warning were not available either. During the 2011 super outbreak, the average lead time was about 24 minutes, saving many lives. Incidentally, today marks the 34th anniversary of the world's deadliest tornado, which claimed 1,300 lives in Bangladesh.
Fig. 3: Overview of all tornadoes from April 25-28, 2011; Source: U.S. Tornados
April 2011 was not only historic because of that outbreak. A total of 751 tornadoes were recorded for the entire month, far exceeding the previous peak of 542 tornadoes within a month (May 2004). Typically, the tornado season in the U.S. lasts from March through June.