With a large swath of the U.S. under the threat of severe weather this week, it's a good time to review your severe weather safety plan. All modes of severe weather are possible, including damaging winds, hail, and tornadoes. While tornadoes are dangerous at any time of day, they are even more deadly when they occur after dark.

Tornadoes can happen at any time of day, but research has shown that nighttime tornadoes are twice as deadly as ones that occur during the daytime hours. One study, published in 2008 and led by Ashley Walker at Northern Illinois University, found that while only 27.3 percent of tornadoes happen at night, they account for 39.3 percent of all tornado deaths. The study examined 48,165 tornadoes documented in the U.S. between 1950 and 2005.

So why are nighttime tornadoes more deadly than their daytime counterparts? There are a few reasons. First, people are more likely to be asleep at night, making it more difficult to hear tornado warnings and to seek shelter. Second, it's harder to see a tornado at night, even with a lot of lightning. Trained spotters are less likely to be awake to see a nighttime tornado, which makes it more challenging to confirm that there's a tornado on the ground.

There are also a few other factors at play. While storms that occur during the day tend to have plenty of instability to work with, the low level jet tends to fuel nighttime severe weather. A strong low level jet can keep the severe weather threat, and thus the tornado threat, going long after the sun sets.

Research has also shown that the Southeast is more prone to nighttime tornadoes. Tennessee leads the pack, with 45.8 percent of its tornadoes happening in the dark. In Arkansas, 42.5 percent of tornadoes occur between sunset and sunrise, while in Kentucky it's 41.5 percent.

The Southeast also has a larger number of people living in mobile homes, which are far more susceptible to storm damage than traditional homes. Those living in mobile homes are 15 to 20 times more likely to be killed in a tornado than someone living in a permanent structure.

Taking steps to stay safe when tornadoes are in the forecast is crucial, especially if nighttime tornadoes are possible. First, make sure all of your warning devices, such as cell phones and NOAA weather radios, are fully charged. Second, turn up the volume on your alerts so they are loud enough to wake you up.

Identifying a safe location prior to severe weather is also vital. If you live in a mobile home, you may want to consider heading to a storm shelter or a friend or family member's sturdy home. If you live in a traditional home, go to an interior room in the lowest level of your home during a tornado warning. You want to have as many walls between you and the outside as possible.

Having a plan in place can save your life. Also, make sure you have your Weather & Radar app alerts turned on when severe weather is in the forecast.