(CNN) -- With the official state death toll now at 337, the Wednesday through Thursday tornado outbreak is the second-deadliest single day for tornadoes since recordkeeping began, the National Oceanic and Atmospheric Administration said.
The outbreak is also on record as creating the most number of tornadoes in a single day.
Records go back to 1680. Since then, there has been only one other date in U.S. history on which more people died during a severe weather outbreak, according to the National Weather Service.
Weather officials say the reason why so many perished was due to the size and path of the tornadoes. Meteorologists rely on what is called an "Enhanced Fujita Scale" to rate the severity of tornadoes.
The lowest ranking, EF0, applies to twisters with recorded 3-second wind gusts of 65 to 85 mph, according to the National Weather Service. The highest, an EF5, is assigned to tornadoes with speeds of more than 200 mph.
The weather service has so far recorded 11 tornadoes with EF3 ratings or higher that struck central and north Alabama on Wednesday. Some of the twisters were three-quarters of a mile wide and traveled dozens of miles, experts said.
"That's an astounding amount for a single-day tornado event." said Krissy Scotten, a weather service meteorologist in Birmingham, Alabama. "It's one of those instances where you had very large tornadoes on very long tracks hitting heavily populated areas."
"When you put that together, you're going to see large loss of life and massive devastation," Scotten said.
According to the National Weather Service:
-- The deadliest single day for tornadoes was on March 18, 1925, with 747 fatalities across seven states.
-- As of Saturday morning, the Wednesday-Thursday outbreak has seen 337 fatalities across six states.
-- The third-deadliest tornado outbreak was on March 21, 1932, when 332 people died.
-- An EF4 tornado that struck Pleasant Grove, Alabama, was produced by an astounding supercell thunderstorm that began in Newton County, Mississippi, and dissipated in Macon County, North Carolina. That supercell lasted 7 hours and 24 minutes and covered 380 miles, producing several strong to violent tornadoes along the way.
-- The agency estimates that on Wednesday-Thursday, there were 211 tornadoes.
-- The largest previous number of tornadoes on record in one event took place from April 3-4, 1974, with 148 tornadoes.
More information at: http://www.noaanews.noaa.gov/april_2011_tornado_information.html