(CNN) -- A rare mix of factors combined to cause widespread severe weather and dozens of reported tornadoes across the southern United States Wednesday, experts said.
The National Weather Service had already received more than 100 reports of tornadoes by Wednesday night, and that number appeared to be climbing rapidly.
"This could be one of the most devastating tornado outbreaks in the nation's history by the time it's over," CNN Meteorologist Sean Morris said.
Several meteorological conditions combined Wednesday to create a particularly dangerous mix, he said. A storm system that brought severe weather to parts of the South Plains earlier this week was heading east, a cold front was moving across the Deep South and upper levels of the atmosphere were conducive for severe storms.
"It is tornado season, but an intensive event like this only will occur maybe once or twice a year," he said. "It's very rare to have all these ingredients come together."
The worst tornado outbreak in U.S. history occurred in April 1974, when 148 twisters touched down in 13 states over a 16-hour period, according to the National Weather Service. The agency said 330 people died and 5,484 were injured in a path of damage that covered more than 2,500 miles.
That month saw a total of 267 tornadoes -- the largest number recorded since 1950, according to the weather agency.
Authorities are still assessing damage from Wednesday's storms, and it could be days before officials establish how many tornadoes hit.
By Wednesday night, authorities said dozens of people had been killed as storms swept through Alabama, Mississippi and Arkansas.
And dramatic images of massive funnel clouds and flattened buildings left little doubt about the storms' strength.
"The storms are just amazingly explosive and they're covering a very large area," said Greg Carbin with the National Weather Service Storm Prediction Center in Norman, Oklahoma.
With the latest outbreak, April may turn out to be a historic month for tornadoes, he said.
The Weather Service received unofficial reports of 39 tornadoes on Tuesday alone. The long-term average for confirmed tornadoes in April is 116.
"We may finish out April with more than 300 tornadoes," Carbin said. "It looks like it will be a record-breaker as far as sheer numbers go. The numbers for April are definitely on a record pace."