What caused the tornadoes?
In the South and Southeast, all the ingredients came together to create a once-in-a-generation outbreak of severe weather and tornadoes. Unstable air had been in place for several days and a cold front was approaching.
Normally, this type of set-up would create severe weather, but not to this effect. This time, a strong jet stream drove deep into the southeastern states with westerly winds of more than 100 mph. At the same time, a deepening surface low formed low-level winds from the south/southeast, creating a severe turning of the winds with height or wind shear. With this extreme wind shear and instability, storms had little trouble starting to rotate and producing tornadoes.
The mid-Atlantic area had a similar but much less extreme setup. It certainly had the warm air, but the mid/upper levels of the atmosphere were warmer, making in not as unstable as in the South.
How does this outbreak stack up against historic storms in the past?
This did not quite eclipse the Super Outbreak of 1974, during which 148 tornadoes struck 13 states. Yet both caused tremendous damage and loss of life. About 330 people were killed in the outbreak of 1974 and almost 5,500 were injured.
On Wednesday, 139 tornadoes were reported in at least 10 states. This week, 200 tornadoes were reported in at least 15 states, though many reports appear to be multiple reports of the same storm.
What is making this tornado season so violent?
A large part of the answer lies in the airflow in the upper portion of the atmosphere, at the jet stream level. There has been a parade of energetic storm systems, accompanied by strong jet stream winds, diving down into the southern states this spring. These dips, or "troughs," in the jet have helped set up major clashes between cold air to the north and warm, humid air from the Gulf of Mexico to the south.
The Washington Post