( 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 .
NEW : tornadoes can be spawned from rotating thunderstorms called supercells . Authorities are investigating damage from the storms . Chief meteorologist : ` ` It is very rare to have all these ingredients mixed together '' The worst tornado outbreak in U.S. history occurred in April 1974 .