(CNN) -- A widespread outbreak of severe weather across the Deep South may be one for the record books, as the National Weather Service received reports of more than 150 tornadoes.
"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.
The unofficial number of twisters reported Wednesday by the Storm Prediction Center was 151. The same system continued to spin off tornadoes early Thursday.
The long-term average for confirmed tornadoes in April is 116.
"It was certainly a rare day that there were so many tornadoes that caused so much damage in populated areas," CNN Meteorologist Jacqui Jeras said.
An official count is not immediately available, but Jeras said she expects that the numbers will reveal a historic April with a record number of twisters.
Authorities were still assessing damage from the storms early Thursday, and it could be days before officials establish how many tornadoes hit.
Several meteorological conditions combined Wednesday to create a particularly dangerous mix, Morris said. A storm system that brought severe weather to parts of the South Plains earlier this week headed 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," Morris said. "It's very rare to have all these ingredients come together."
The deadly phenomena can be spawned from rotating thunderstorms called supercells, which can stretch across areas as large as entire counties.
A tornado can form when a mesocyclone -- a rotating column of air -- is created within a super cell storm and is stretched to create a funnel-like structure that can reach speeds of up to several hundred miles per hour.
"Almost like when you pull the drain in your tub," said CNN Meteorologist Chad Myers. "The entire storm spins and is focused and angled down."
"When you have a storm all by itself like we saw yesterday, that's when you get this structure," he added.
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.
By early Thursday, authorities said hundreds of people had been killed as storms swept through Georgia, Alabama, Mississippi, Tennessee 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.
"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."
CNN's Ed Payne contributed to this report.