Interactive: Intensity of U.S. tornadoes

In an average tornado season, the United States sees hundreds of EF-1 rated tornadoes — ranking lowest on the Enhanced Fujita scale.

In 2011, the U.S. saw a record 638 tornadoes rated EF-1. That year was also busy with EF-5 tornadoes, with nine on record, and 73 deaths. Damage is extremely severe in an EF-5 — which is the rating assigned to the twister that struck Moore, Okla. in 2013.

For a closer look at 1950-2013 data click here.  For a closer look at 2003-2013 data click here.

A tornado is assigned an official rating after someone from the National Weather Service, a federal agency, assesses the damage it caused. According to the NWS, several factors are considered, such as whether something received minor damage or if a structure or piece of structure blew away.

NWS officials today are assessing the strength of the tornadoes that swept through the U.S. Sunday evening and Monday morning. As of this afternoon, there were more than 30 confirmed tornadoes.

More are expected in the next day as storms move East through other states, including Mississippi, Alabama and Tennessee.

For a closer look at 1950-2013 data click here. For a closer look at 2003-2013 data click here.

Immediate severe weather alerts are available to anyone in the U.S. via a weather radio app called Storm Shield, which is available for iPhone and Android devices. For Storm Shield information on social media, find it on Twitter @stormshieldapp, and on Facebook.

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