A deadly, fierce December for tornadoes

December is typically the quietest month for tornadoes, but at least 20 deaths so far this month make it the second-deadliest December on record, according to the Storm Prediction Center.

An average December has only three tornado-related deaths, based on data since the early 1950s, said Greg Carbin, warning coordination meteorologist for the prediction center. Until last week's ferocious storms just before Christmas, only 10 people had died in tornadoes this year, putting 2015 on track to be the least deadly year from tornadoes.

Instead, this is the first time that December will be the deadliest month of the year, Carbin added.

December usually has only 24 tornadoes, but at least 60 twisters have been reported so far this month, according to data from the National Centers for Environmental Information.

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Carbin said the deadliest December was in 1953 with 43 fatalities.

Unseasonable warm temperatures are the main cause for the severe storms this month, said Frank Strait, senior meteorologist with AccuWeather. It's also been very humid, he said, and that warm, muggy air provides the fuel for thunderstorms to form.

El Niño has brought warm, spring-like temperatures to much of the southern and eastern U.S. this fall and winter. Typical results from El Niño are wet and stormy conditions across the southern U.S.

Strait said that after a couple more days of storms, cooler temperatures should shut off the severe weather threat for much of the nation.