Violent storms ranked among nation’s deadliest

The violent storm system that killed hundreds in several southern states this week ranks among the deadliest tornado outbreaks in U.S. history.

At least one of the 22 killer tornadoes that struck the country over the past four days has been named an EF-5 strength twister by the National Weather Service, and more may gain that ranking as damage continues to be assessed. Only about 1 percent of all tornadoes reach the most powerful ranking. Six of the twisters have been designated EF-4 strength, which means their winds were above 160 mph.

Wednesday's E-5 Smithville, Mississsippi tornado killed 14 people, destroyed nearly 200 buildings, and even swept away "well-built modern 2-story homes that were bolted to their foundations," according to Wunderground blogger Jeff Masters, who has a great rundown of the strength of each twister. The tornado was a half-mile wide and its winds were gusting at more than 200 mph. It's the first E-5 twister (an older measurement system that's roughly comparable to the new EF one) to strike Mississippi since 1966, the AP reports, and managed to completely disappear a 1965 Chevy pick-up truck that was parked in front of one of the houses in its path. Scientists measure tornado strength by surveying damage after the fact, and then estimating what the wind speed of the twister was.

The 318 people declared dead so far over the four-day storm makes it the fourth deadliest tornado outbreak in U.S. history. The last comparable storm was in 1932, when 332 people died.

This month alone, a record-breaking 835 tornadoes have been reported. The all-time record number of twisters in an entire year was 1817. Susan Buchanan, spokeswoman for the National Weather Service, said that May is usually the month with the most tornadoes, "which tells us this may be just the beginning." The single deadliest tornado occurred in 1925, killing 695 peopled in Missouri, Illinois, and Indiana.

The National Weather Service has posted a video that shows you what the outbreak looked like from space, below.

Scientists know that tornadoes are caused by supercells: "rotating thunderstorms with a well-defined radar circulation called a mesocyclone," according to NOAA's web site. But still no one quite understands how they form during thunderstorms.

But meteorologists say they have a better understanding of how the storm system that formed the tornadoes came into being. Harold Brooks of the National Severe Storms Laboratory in Norman, Okla tells the AP the storm system was caused by a "trough" over the western United States that interacted with a strong jet stream coming from the south. The warm, wet winds to the east of the trough interacted with the dry winds from the west, creating ideal conditions for a thunderstorm.

UPDATE: This post has been updated to reflect the latest death toll.

(Destruction in Tanner, Alabama after Wednesday's tornado: AP)