Why so many tornadoes are striking US?

Deadly tornadoes are spawning across the United States. Only today 48 tornadoes were reported with a few fatalities. This massive storm system also spawned deadly tornadoes on Leap Day, which raked Kansas, Nebraska, Illinois, Indiana, Missouri, Kentucky and Tennessee. The severe storms killed at least 12 people and included a strong EF-4 twister in Harrisburg, Ill., a rarity for February.

As of this morning, the severe storm risk area covered an estimated 162 million people, or 56 percent of the United States, according to weather experts at the National Oceanic and Atmospheric Administration (NOAA).

While the main tornado season runs from spring to early summer, this year's early outbreaks show that tornadoes can form under a variety of conditions and strike during fall and winter, too. This year's mild winter and warm start to meteorological spring has upped the risk of dangerous storms.