Friday's tornado emergency sent a chill through Oklahoma City and its environs, in large part because it came less than two weeks after a powerful twister drove through the same area. The attention given to the weather pummeling America's midsection plays off the devastation left behind on May 20 in Moore, Okla. — and that contributes to the perception that this year's storm season has been far worse than usual.

It hasn't been, says Cliff Mass, a weather researcher at the University of Washington.

He noted that the season had an unusually quiet start, weather-wise. "We're catching up a little bit," he told NBC News on Friday. "There's no big-picture association with global warming, or anything else. ... It's just that people are very sensitized to this after Moore."

The month of May is prime time for storm activity in Texas and Oklahoma, he said. "Then the tornado frequency moves northward as you get into the latter part of the season," Mass said.

If there's any consolation for Oklahomans suffering through yet another threat to lives and property, it's the fact that meteorologists have increased their capability to track and anticipate storms over the past decade. "It's just amazing to watch the technology being thrown at it," Mass said.

The answers to some of the scientific questions surrounding twisters are clearer today than they've ever been — but other questions still pose mysteries: 

Friday's storm reportedly spawned a "multiple-vortex tornado." What's that?

Multiple-vortex tornadoes rank among the most violent and damaging storms. In such a case, the center of the tornado's wind funnel spawns two to seven smaller twisters, or subvortices. These mini-twisters circulate around the edge of the storm cloud at speeds that can range up to 100 mph faster than the winds in the main funnel. The subvortices typically last less than a minute.

The storm that flattened Joplin, Mo., in 2011 had multiple vortices, as did a powerful storm that swept over Indiana last year. (This picture of the Indiana storm actually shows the subvortices.) The National Weather Service's Storm Prediction Center says multi-vortex tornadoes are probably behind most reports of multiple tornadoes hitting at once.

Purdue University tornado researcher Ernest Agee says one characteristic of a multi-vortex storm is a pattern of asymmetric damage. One side of a structure might look relatively untouched, while the other side could be completely destroyed. "Those individual vortexes are very destructive," he said. 

Why is Tornado Alley more prone to deadly twisters?

The classic explanation has focused on Tornado Alley's geography: The Rocky Mountains tend to impede the eastward flow of moist air, while the Great Plains allow frigid air to stream southward from Canada and meet up with warm, humid air from the Gulf of Mexico. However, the National Weather Service's Storm Prediction Center says this is a "gross oversimplification" when it comes to explaining the origin of tornadoes.

Mass cites an array of factors that include strong vertical instability and a large amount of wind shear during the spring. "Everything comes together to make this the spot for tornadoes," he said.

National Weather Service

The National Weather Service provided this preliminary damage path for the tornado that swept through the region around El Reno, Okla., on Friday.

"Tornado Alley" generally refers to the region centered in Texas, Oklahoma, Kansas and points north, where tornadoes are most frequent — but multiple studies indicate that the deadliest twisters occur to the east, in a region that's come to be known as "Dixie Alley." The reasons for that have to do with geography and demographics as well as meteorology in the southeastern United States: Storms tend to move faster, and they're more likely to strike at night. There are more trees and other obstructions to raise havoc. Population densities are generally higher, and the region has many manufactured homes that lack basements in which to take shelter.

Are tornadoes a uniquely American phenomenon?

The United States has the highest incidence of tornadoes, with an average of more than 1,000 every year, according to the National Climatic Data Center. But other regions of the world have twisters as well. Canada is No. 2 with about 100 per year, followed by northern Europe, western Asia, Bangladesh, Japan, Australia, New Zealand, China, South Africa and Argentina. Britain has more tornadoes than any other country, relative to its land area. "Fortunately, most UK tornadoes are relatively weak," the data center says.

Will climate change make tornadoes worse? More frequent?

"The short answer is, we have no idea," said Michael Wehner, a climate researcher at the Lawrence Berkeley National Laboratory. For years, Wehner has been studying the climate models for extreme weather, and he's a lead author for the next report from the Intergovernmental Panel on Climate Change as well as the federal government's latest national assessment on climate change.

One problem is that the observational record for tornadoes has not been uniform over time. "It has a bias to it, because more people are living where tornadoes occur, and more people are out looking for them," Wehner said. That contributes to the perception that tornadoes are happening more frequently than they used to.

The other big problem is that current climate models don't have the resolution that's needed to simulate the localized, violent activity of a tornado. Currently, global models are built up from atmospheric interactions on a scale of 100 kilometers (62 miles). Improvements in computer power could soon bring that down to a scale of 25 kilometers (16 miles). That should make it possible for scientists to simulate the weather phenomena that give rise to tornadoes, but not the tornadoes themselves, Wehner said.

What about other storms?

When it comes to rainstorms, researchers say there's a clearer link to climate change. "The metric that I like to look at is the daily amount of rain for a storm that happens once every 20 years," he said. "That storm, in a much warmer world, would happen more frequently." For example, if the world follows a "business-as-usual" scenario, he projects that the average temperature would rise 11 degrees Fahrenheit (6 degrees Celsius) by the end of the century, and that a once-in-20-years rainstorm would come around every five to 10 years on average.

That doesn't necessarily mean tornadoes would be more frequent, however. In fact, the current projection calls for wetter spring weather in the northern U.S., and drier weather in the Southwest — with Tornado Alley right in the middle. "There's some evidence that there might not be a change" in the character of a tornado season, Wehner observed.

Slideshow: Tornadoes ravage Plains

Tom Pennington / Getty Images

Images document the devastation left behind in Oklahoma after the May 20 tornado.

Launch slideshow

More about tornado science:

• Why tornadoes seem as if they're on the rise
• Flash interactive: What causes tornadoes?
• Full coverage of the Oklahoma tornadoes

This report incorporates information from a May 20 posting on the science of tornadoes and a Cosmic Log item about multiple-vortex tornadoes from March 2, 2012.

Alan Boyle is NBCNews.com's science editor. Connect with him by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding him to your Google+ circles.

Tony Gutierrez/AP

An aerial view of a neighborhood Moore, Okla., destroyed by a May 21 tornado.

As America’s “Tornado Alley” braces for more storms this week, one of the nation’s top meteorologists says that changing habitation patterns have made living in the region like playing an increasingly deadly game of Battleship – where hits on densely populated targets like Moore, Okla., will only grow more common.

In the Great Plains states of Oklahoma, Texas, Kansas and Nebraska, which combined are hit by an average of more than 250 tornadoes per year, cities and suburbs have grown as much as 20 percent in the past decade, while rural counties have lost population.

Bill Hooke, director of the American Meteorological Society, compares tornado strikes to Battleship, the board game in which players take turns guessing where on a numbered grid their opponent’s ships are hidden.  Battleships are the biggest targets. And in Tornado Alley, the battleships – the cities – are getting even bigger.

 “Think of the Midwest as a blank sheet of graph paper,” said Hooke, “with the towns and cities being the ships. ... When you keep adding people, and more urban sprawl, and farms turn into housing developments, tornadoes are much more likely to hit something.” That increases the chance that when a tornado hits a populated area, it will be a densely populated area.

At the same time, he said, the smaller targets on the grid are disappearing from the map, as rural towns and counties lose people to metropolitan areas like Oklahoma City, Dallas, Kansas City and Omaha. “With that added into the picture, you’re exchanging many smaller encounters for fewer bigger ones,” said Hooke. “You have fewer events, fewer collisions, but when they do happen (the area) will be more populated and the damage will be greater.”

All six of Oklahoma’s fastest growing counties are in the Oklahoma City and Tulsa areas. While the state as a whole grew 8.7 percent between 2000 and 2010, suburban Cleveland County, site of Moore, grew 23 percent. Meanwhile, 23 of Oklahoma’s 77 counties, nearly all of them rural, lost population.

In Kansas and Nebraska, the rural/urban divide was even more pronounced. In both states, about three quarters of counties lost people between 2000 and 2010, while the population as a whole grew more than 6 percent and suburban counties grew as much as 30 percent.

On Monday and Tuesday, tornadoes battered Kansas, but were concentrated in rural counties with populations of less than 11,000 that have been losing people. No injuries were reported.

The increased risk to metro areas may be especially apparent in Oklahoma City, said Kevin Simmons, an economics professor at Austin College in Texas who is an expert on the economic impact of natural hazards. Oklahoma City is “the buckle of the Tornado Belt,” said Simmons.

“The National Weather Service has a color-coded map of the places where tornadoes are most likely to strike,” said Simmons. “You can see it centering on Central Oklahoma.”

During a major tornado outbreak in May 2011, said Simmons, an EF 5 tornado swept through outlying western and northern areas of metropolitan Oklahoma City. Nine people died as the twister cut a 75-mile swathe through four counties. The tornado was similar in strength to this month’s Moore storm, which was also classed EF 5, meaning winds exceeded 210 mph. But it hit areas that were twice as far from the urban core and far less densely populated. Piedmont, Guthrie and El Reno, Okla., the three largest towns in that storm’s path, have densities that range from 83 to 203 people per square mile. Moore’s density is 1,892.

“You take a similar tornado and put it through Moore, 24 people perish,” said Simmons.

Slideshow: Tornadoes ravage Plains

Tom Pennington / Getty Images

A monster tornado hit Moore, Okla., Monday afternoon, leaving at least 24 dead.

Launch slideshow

Simmons also noted that the kinds of Oklahoma towns that are now losing population are more amenable to community shelters than cities like Moore. “The rural areas have an opportunity for community shelters that the suburban and urban areas don’t have,” he said. “In a small town five streets wide, they have the potential to get everybody into the shelter in a limited amount of time.”

He cited the example of Tushka, Okla., a tiny town two hours southeast of Oklahoma City that was hit by an EF 3 tornado – a tornado with wind speeds of up to 165 mph – in April 2011. Two elderly sisters died in their home, but most of Tushka’s population crowded into two community shelters, one underground and one above ground, and were safe.

“Now imagine doing that in the suburb of a major metro area,” said Simmons. “It’s a different phenomenon. “

Tushka’s shelter may be less crowded the next time a tornado comes near, however: The town's population fell from 345 in 2000 to 312 in 2010.

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Shalyn Phillips / TVNWeather.com

An eastward advancing cold front is to blame for the series of tornadoes in recent days. Shown is a tornado near Wichita, Kan over the weekend.

By Douglas Main, LiveScience

Tornado season has been relatively quiet this year. There were only 72 tornadoes nationwide in April, 70 percent below the 10 year average, according to the Weather Channel. But within in the last week, tornado outbreaks have been erupting from North Texas to Minnesota.

Why do these tornadoes seem to be hitting all of a sudden?

An eastward advancing cold front is to blame. This pocket of cold air has run into warm air from the Gulf of Mexico. Like a wedge, the cold front has caused the warm air to rise, since it's less dense, said Jeff Weber, a scientist with the University Corporation for Atmospheric Research in Boulder, Colo.

"It's kind of the perfect setup," Weber told LiveScience.

This rising warm air has created thunderstorms that have in turn spawned tornadoes, which draw their rotation from the system's abundant wind shear, which is a change in wind speed and direction with altitude. In this case, winds have blown north off the Gulf of Mexico, and interacted with currents moving east along with the cold front, Weber said. This helps create swirling gyres that can be flipped vertically and create tornadoes, he said. [Infographic: Tornado! How, When & Where Twisters Form]

This follows a relatively calm spring that saw few tornadoes, which is largely due to unusually cold temperatures throughout much of the country. The cold can in turn be blamed on the fact that the jet stream, the ribbon of wind that stretches across the Northern Hemisphere, dipped farther south than usual. This brought with it frigid Arctic air and prevented warm, moist air from the Gulf of Mexico from advancing northward.

Last week, with the jet stream shifting north and the atmosphere more conducive to tornado formation, an outbreak of an estimated 16 twisters erupted in North Texas on May 15. The tornadoes ranged in strength from an EF-1 in the town of Millsap that caused damage to roofs, to an EF-4 in Granbury that completely destroyed houses.

The Texas tornadoes were worsened by the fact that the jet stream dipped south during this outbreak, adding extra rotational power to the twisters, Weber said.

In the next few days, this system should soon weaken, Weber said. That's because the center of the cold front's circulation is now over Minnesota and Wisconsin, and warm air has filled the center of the country. This means there will be less interaction between masses of cold and warm air, which fuel tornadoes' destructive power, Weber said.

The Weather Channel stat comes from a tweet by meteorologist Mike Bettes. (The Weather Channel is a venture of NBCUniversal.)

Email Douglas Main or follow him on Twitter or Google+. Follow us @livescience,  Facebook or  Google+. Article originally on LiveScience.com.

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