Last weekend's tornadoes in the south and Midwest are believed to be the deadliest on record for December. So far, about 90 people are confirmed to have been killed in western Kentucky, Arkansas, Illinois, Missouri and Tennessee.
Kentucky Gov. Andy Beshear said Thursday that 16 people in his state are still missing following the storms.
Forecasters say there was an indication of a possible significant weather event days ahead of the storm — but they had no idea the storms would be so violent.
"As we got closer to the event, it became the upper-level pattern is matching up well, the temperatures have stayed extremely warm at the ground, and nothing's happened to cool that off," said Harold Brooks, a scientist with NOAA's National Weather Service Storm Lab.
Brooks says large outbreaks of violent tornadoes are not uncommon in December, nor are large upper-level systems.
But one rare factor that led to last week's extreme weather event was the large area of warm, moist air on the ground. The region had been experiencing record warm temperatures, and there have been no recent cold fronts to cool down the Gulf of Mexico.
"The Gulf temperatures look like they should look a couple of months ago, and so they're very high," Brooks said. "That means when air comes out of the south, it's bringing up a lot of warm, moist air, which is the fuel for the thunderstorms. So, that's the key thing — that warm, moist air fuels what's under first, and then the strong, upper-level system brings in the winds, which are necessary to make those storms rotate and be potential tornadoes."
Warmer temperatures are one ingredient for tornadoes that can be attributed to climate change. But Brooks says it's hard to make a direct link to climate change because tornadoes mostly depend on the wind.
"The fact that that may actually decrease in the future makes the climate connection a lot harder in tornadoes than it does in a lot of other things," Brooks said. "It's not as straightforward is as temperature records or heavy rainfall, which both are really pretty directly related to the planet warming."
As scientists continue to study last week's tornado outbreak, Brooks made one message clear — people need to pay attention to potential weather events before they happen, so families can make potentially life-saving plans.