Since 1950, there have only been 19 F/EF4 tornadoes in the U.S. during the last month of the year and only 2 F/EF5 tornadoes.

The last EF4 tornado to strike the U.S. during the month of December was during the Christmas Outbreak of December 2015.

The last EF5 tornado to strike the U.S. during the month of December was in 1957.

An EF5 tornado is the strongest designation a tornado can receive. Exceptionally rare, these tornadoes can produce wind speeds higher than 200 mph. The last EF5 tornado to strike the U.S. was Moore, Oklahoma, in May of 2013. That was 3,125 days ago and the longest streak on record.

This event was caused by a volatile atmospheric set up that was primed to produce violent and long-track tornadoes. Friday featured unseasonably warm and record-setting temperatures that felt more like Spring than mid-December. This warmth, combined with high humidity, provided ample fuel for the storms.

As the day progressed, the wind fields strengthened helping to create the dynamics, or “spin,” in the atmosphere needed to produce tornadoes.

A cold front charging through the region provided the trigger for the storms, which with all the ingredients in place created ripe conditions for a tornado outbreak.

Compounding the favorable atmospheric ingredients is the fact that the United States is currently in a La Niña pattern, which historically increases tornado frequency across the Mississippi Valley.

And climate change may also have played a role.

Research reveals that climate change may be causing tornado alley to shift East, out of traditional tornado alley of the Great Plains and into parts of the Mississippi Valley.

While meteorologists and climate scientists cannot yet say that tornado frequency is increasing globally due to climate change, it can be said with relative certainty that tornado frequency and associated vulnerability is increasing for the Mississippi Valley and Midwestern regions of the United States.

During a Saturday evening press conference President Joe Biden addressed what role climate change may have played in the disaster. “Well, all I know is the intensity of the weather across the board has some impact as a consequence of the warming of the planet. The specific impact on these specific storms I can’t say at this point. I’m going to be asking the EPA and others to take a look at that,” Biden said. “But the fact is we all know everything is more intense when the climate is warming. Everything.”

Kentucky Gov. Andy Beshear described the tornado as the most devastating one to hit the state, estimating that it might have killed at least 70 people or more than 100.

That will make this event not just one of the deadliest tornado events in Kentucky history, but also in U.S. history, and potentially the deadliest December outbreak on record.

Nighttime tornadoes are more than twice as likely to cause fatalities than their daytime counterparts. That’s because tornadoes are harder to see at night, and people who are sleeping often don’t have a way to be woken when warnings are issued.