Here's some background information about tornadoes, which are funnel-shaped clouds that form under thunderclouds and contain rapidly rotating air.
Facts: Most tornadoes form from severe thunderstorms. Hurricanes can also produce tornadoes.
Tornado winds may exceed 300 miles (483 kilometers) per hour.
Tornadoes can lift cars, mobile homes and animals into the air.
Tornadoes are sometimes called "twisters."
The damage path of a tornado is usually less than 1,600 feet wide.
On average, tornadoes travel at around 10-20 miles per hour.
The average tornado is on the ground about five minutes.
The most destructive and deadly tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.
A tornado over a body of water is called a "waterspout."
The United States has the highest number of tornado occurrences in the world with an average of more than 1,000 tornadoes reported each year.
According to the National Weather Service, in 2017 there were 35 tornado-related deaths in the United States.
Most of the tornadoes in the United States strike in Tornado Alley, which spans the Midwest and the South.
Tornadoes usually occur during the spring and early summer, most often in the late afternoon and early evening.
A tornado watch is issued by the National Weather Service when atmospheric conditions promote the forming of tornadoes.
A tornado warning is issued when Doppler radar detects a mesocyclone in a thunderstorm, or when a funnel cloud has been spotted.
A tornado emergency is enhanced wording in a tornado warning indicating a large tornado is moving into a heavily populated area. Significant widespread damage and numerous fatalities are likely. The term was coined by forecasters in May 1999 and is used sparingly.
Enhanced Fujita Scale: The Enhanced Fujita scale became operational on February 1, 2007, and is used to assign a tornado rating based on estimated wind speed and damage the tornado causes.
EF0 is the weakest point on the Enhanced Fujita Scale and EF5 is the strongest.
An EF5 tornado can tear a house off its foundation.
Rating EF0: Wind speed between 65 and 85 miles per hour.
Rating EF1: Wind speed between 86 and 110 miles per hour. Moderate damage. Peels the surfaces off roofs; mobile homes pushed off foundations or overturned; moving cars blown off roads.
Rating EF2: Wind speed between 111 and 135 miles per hour. Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off the ground.
Rating EF3: Wind speed between 136 and 165 miles per hour: Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.
Rating EF4: Wind speed between 166 and 200 miles per hours. Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.
Rating EF5: Wind speed 200 plus miles per hour. Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yards); trees debarked; incredible phenomena will occur.
Timeline: March 18, 1925 - One of the worst tornado disasters in the United States. 695 people in the tri-state area of Missouri-Illinois-Indiana are killed. It is the longest-lived and has the longest path of any recorded US tornado.
1950 - The United States begins keeping official records about tornadoes.
February 2, 2007 - At least 20 people are killed in Lake and Volusia counties in Florida after at least three tornadoes touch down in the middle of the night.
March 1, 2007 - At least 20 people are killed, one in Missouri, 10 in Alabama, and nine in Georgia from a string of tornadoes. In Alabama, eight of the 10 killed are teenagers from Enterprise High School in Enterprise, Alabama.
February 5, 2008 - At least 56 people are killed, 32 in Tennessee, 13 in Arkansas, seven in Kentucky, and four in Alabama from a string of tornadoes.
March 14, 2008 - A tornado reaching EF2 strength at times hits downtown Atlanta, damaging the World Congress Center, CNN Center, the Georgia Dome, and many other buildings.
May 9-11, 2008 - A series of tornadoes kills 22 in three states including six in Ottawa County, Oklahoma; 13 in Newton County, Missouri; one in Jasper County, Missouri; one in an area of Purdy in Barry County, Missouri, and one in Laurens County, Georgia.
April 14-16, 2011 - At least 114 tornadoes touch down in Oklahoma, Kansas, Louisiana, Mississippi, Alabama, Missouri, Illinois, Georgia, South Carolina, North Carolina, Virginia, Maryland, Arkansas and Pennsylvania. Of the 46 fatalities reported, 23 occur in North Carolina.
April 25-28, 2011 - An outbreak of 201 confirmed tornadoes occurs from 8:00am ET April 25 to 8:00am ET April 28, 2011. There are approximately 321 fatalities in six states during the entire outbreak. The majority of fatalities occur in Alabama, where as many as 243 people are killed. Other states reporting fatalities are Mississippi, Tennessee, Georgia, Virginia and Arkansas. In terms of multi-day outbreaks, this outbreak holds the record for the largest number of tornadoes.
May 22, 2011 - An EF5 tornado strikes Joplin, Missouri, killing at least 158 people. It is the deadliest single US tornado since federal record-keeping began in 1950. The tri-state tornado of 1925 is still the deadliest tornado in US history.
May 24, 2011 - Tornadoes strike Arkansas, Oklahoma and Kansas, killing at least 18 people.
August 3, 2011 - The Storm Prediction Center's final report for April 2011 shows 753 tornadoes touched down across the United States, breaking the previous monthly record of 543 tornadoes in May 2003.
May 20, 2013 - An EF5 tornado hits the Moore, Oklahoma area. The path of the tornado is 14 miles long. Twenty-four people are killed.
January 20-23, 2017 - Twenty people are killed - more than in all of 2016 - after an outbreak of twisters in Georgia and Mississippi. More than 80 tornadoes are reported over three days across Texas, Arkansas, Florida Louisiana, Mississippi, Alabama, Georgia and South Carolina - with more than 60 reported on January 21 alone. according to the National Weather Service.
Top 10 Deadliest Single US Tornadoes: (Source: NOAA/NWS Storm Prediction Center)
March 18, 1925 - Tri-state area of Missouri, Illinois and Indiana - 695 fatalities.
May 6, 1840 - Natchez, Mississippi - 317 fatalities.
May 27, 1896 - St. Louis, Missouri - 255 fatalities.
April 5, 1936 - Tupelo, Mississippi - 216 fatalities.
April 6, 1936 - Gainesville, Georgia - 203 fatalities.
April 9, 1947 - Woodward, Oklahoma - 181 fatalities.
May 22, 2011 - Joplin, Missouri - 158 fatalities.
April 24, 1908 - Amite, Louisiana and Purvis, Mississippi - 143 fatalities.
June 12, 1899 - New Richmond, Wisconsin - 117 fatalities.
June 8, 1953 - Flint, Michigan - 116 fatalities.
Top 10 Costliest Tornadoes since 1950 (in 2015 dollars): (Source: NOAA/NWS Storm Prediction Center)
May 22, 2011 - Joplin, Missouri - $2.8 billion (actual cost) - $2.92 billion (adjusted for inflation)
April 27, 2011 - Tuscaloosa, Alabama - $2.45 billion (actual cost) - $2.56 billion (adjusted for inflation)
May 20, 2013 - Moore, Oklahoma - $2 billion (actual cost) - $2.09 billion (adjusted for inflation)
June 8, 1966 - Topeka, Kansas - $250 million (actual cost) - about $1.81 billion (adjusted for inflation)
May 11, 1970 - Lubbock, Texas - $250 million (actual cost) - $1.5 billion (adjusted for inflation)
May 3, 1999 - Moore/Oklahoma City, Oklahoma - $1 billion (actual cost) - $1.4 billion (adjusted for inflation)
April 27, 2011 - Hackleburg, Alabama - $1.3 billion (actual cost) - about $1.35 billion (adjusted for inflation)
April 3, 1974 - Xenia, Ohio - $250 million (actual cost) - $1.19 billion (adjusted for inflation)
May 6, 1975 - Omaha, Nebraska - $250 million (actual cost) - $1.09 billion (adjusted for inflation)
April 10, 1979 - Wichita Falls, Texas - $277 million (actual cost) - about $898 million (adjusted for inflation)