As far as tornadoes are concerned, April this year was the second most active April on record, with 351 recorded in a preliminary count by the National Weather Service Storm Prediction Center; April’s average is 258.
Tornadoes are spawned by severe thunderstorms, and this April there were official reports of severe weather on all but three days. While thunderstorms can occur virtually anywhere in the U.S., severe activity is particularly common in the Great Plains, Midwest, and Southeast.
While tornadoes have touched down in every state, they are common in an area termed “Tornado Alley,” which encompasses the lowland areas of the Missouri, Mississippi, and Ohio river valleys and includes regions of Texas, Nebraska, Kansas, and Oklahoma.
A secondary peak sometime occurs throughout much of Mississippi, Alabama, Georgia, and Tennessee, sometimes referred to as “Dixie Alley”; however, this region exhibits a wide degree of variability.
One of the Most Destructive Forces in Nature
Tornadoes are rapidly rotating columns of air, connected to the base of the thunderstorm and in contact with the ground. Due to its geography, the United States has the most tornado development of any country, with approximately 1,200 tornadoes occurring each year. These storms are some of the most destructive forces in nature, causing an average of 70 deaths and 1,500 injuries per year.
The development of a tornado under a supercell is referred to as tornado genesis and is an active area of academic research. It is generally accepted that tornado genesis occurs when rotational momentum from the parent supercell becomes stretched and concentrated. Just as an ice skater increases their rotation through the contraction of their arms, a tornado gains rotational momentum as the updraft and downdraft of the storm stretch and contract the rotating air parcel into a thin column.
The strong upward motions within the tornado typically result in the formation of a dark condensation funnel, attached to the cloud base. As the funnel cloud makes contact with the ground, debris and dust is often lifted into the air around the tornado, forming a debris cloud. Often this debris cloud is visible on radar in the form of a “radar debris ball”.
Most tornadoes rotate cyclonically, (counter-clockwise in the Northern Hemisphere), with forward wind speeds ranging from stationary to about 70 mph. Tornadoes generally have rotational wind speeds ranging from 40 to 110 mph, although they can have winds up to 300+ mph. Weaker tornadoes generally last about 10 minutes and travel only short distances, while stronger tornadoes can last for several hours and can travel hundreds of miles.
Tornadoes tend to vary in intensity regardless of shape, size, and location, although stronger tornadoes tend to be larger than weak ones. Longer track tornadoes also tend to be stronger than those that have a short track; however, these correlations are quite weak given the considerable complexity of severe weather systems.
The annual aggregate losses from severe thunderstorms have accounted for approximately one half of all U.S. catastrophic insured losses since 1990. And April is not the month in which the highest number of tornadoes is typically recorded—May is.