The US National Weather Service has issued tornado warnings for much of Northern Texas as at least two tornadoes reportedly touched down in the Dallas-Fort Worth area.

 A large...extremely dangerous...and potentially deadly tornado has been confirmed, the NWS said. To protect your life... take cover now!

Most people might not associate tornadoes with urban areas or states outside of Kansas, but nearly every state in the US is afflicted by them, and cities are routinely struck.

The reality is, any city can be hit by a tornado, Kenneth Blumenfeld, visiting assistant professor in the Department of Geography at the University of Minnesota told ABC News last June.

If you have sufficient moisture in the atmosphere, instability, some lifting mechanism and the proper wind patterns as they go up into the atmosphere, you can get a tornado. Those conditions tend to be more common in some areas than others, but the fact is, they can arise just about anywhere, Blumenfeld told ABC.

In 2009, Weather Channel blogger and severe weather expert Greg Forbes said he had analyzed statistics on tornadoes, and found that metro areas were hit about three times as often as the rest of the non-metro areas in their states.

Not one metro area I studied had a lower tornado density than its whole parent state average. Remarkable! Forbes wrote.

Northern Texas falls roughly within Tornado Alley, the unofficial name for a swath of land in the middle of the US that sees higher numbers of tornadoes than more coastal regions.

Between 1991 and 2010, Texas saw an average of 155 tornadoes per year, according to the National Oceanic and Atmospheric Administration's Climatic Data Center.

Tornadoes generally stay on the ground for an average of five minutes, though they can touch down for anywhere from an instant to several hours, according to NOAA.

NWS meteorologists confirmed that they sighted a tornado near Kennedale, Tex. at 2:25pm Eastern time. At 3:03pm Eastern, meteorologists also detected a severe thunderstorm capable of producing a tornado six miles southwest of Justin, Tex.

At 3:40pm Eastern, CBS affiliate KTVT Dallas said there were reports of a tornado moving through the western sections of Plano, Tex., which lies north of Dallas.

All flights at Dallas Fort Worth airport have been grounded because of the tornado threat, according to CNN.

Representatives for the Dallas Police Department told the Associated Press that possible tornadoes have touched down in the southern part of their city.

There are no official estimates of casualties yet, though several injuries have already been reported in Kennedale and Lancaster, Tex., according to KTVT.

Pictures posted on the KTVT website show funnel clouds, damaged semi-trailer trucks tossed off the road, downed trees and other damage along the I-20 highway. Some Texans reported seeing baseball-sized hail.

The deadliest tornado incident in the US occurred in March 1925, when one or more tornadoes cut a 219-mile path through Missouri, Indiana and Illinois and killed 695 people.