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What's the science behind tornado outbreaks?

By Alex Sosnowski, AccuWeather senior meteorologist

Spring to early summer is prime time for tornadoes across the United States, but what ingredients must come together to produce an outbreak of tornadoes?

Isolated tornadoes can occur year-round across the U.S., but the prime months for tornadoes are from March to July, with the biggest months being April to June. There is a secondary tornado season in the south-central U.S. from September to November.

The number of tornadoes closely corresponds to the months of the year when there are the most thunderstorms.

Static AP Tulsa Tornado

A tornado hit the Tulsa, Oklahoma, area on March 30, 2016. Several people were injured and multiple homes and businesses were damaged or destroyed. (AP Photo / Larry Papke)

On average, when a particular weather pattern produces 10 or more tornadoes over a roughly 24-hour period, a tornado outbreak has occurred.

"The term tornado outbreak is relative to the geographical area," according to AccuWeather Lead Storm Warning Meteorologist Eddie Walker.

"A typical tornado outbreak for the Plains and Mississippi Valley typically constitutes a couple dozen strong tornadoes but in New England, an outbreak may be as little as half of a dozen tornadoes," Walker said.

In order for a tornado to form over land, a severe thunderstorm must first develop.

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Most thunderstorms need four ingredients to become severe, according to Walker.

"These ingredients are: moisture, instability, vertical forcing and wind shear," Walker said.

The geographical area of the Plains and Mississippi Valley often allow these ingredients to come together in just the right way to not only produce a small number of tornadoes, but a large number of tornadoes that constitute an outbreak.

Static severe thunderstorm / tornado outbreak setup

Meteorologists measure moisture by sampling to what point the air must be cooled to in order to become saturated. This measurement is the dew point temperature.

When the air is moist it feels humid. A typical warm and humid day in May may have temperatures in the 80s F with a dew point temperature in the 60s to the lower 70s F, thanks to a flow of air off the Gulf of Mexico, which happens often.

Instability refers to the ability for a batch of air to continue to rise through different layers of the atmosphere.

When the atmosphere is stable, air cannot rise or may only rise slightly. Shallow clouds may struggle or the sky may remain clear when the atmosphere is stable.

When the atmosphere is unstable, air may be able to rise tens of thousands of feet above the ground. This can occur with ease when the air aloft is much cooler than the air near the ground.

A warm and humid day is often enough to allow bubbles of air to rise.

When air rises it cools and moisture condenses. When the air is warm and moist to begin with, towering clouds may form first, then thunderstorms may soon follow.

However, moisture and instability alone may not be enough to produce a severe thunderstorm.

The two remaining components, vertical forcing (lift) and wind shear, are key players in severe thunderstorms.

Without these two features, there can be a garden variety thunderstorm that mushrooms up on a lazy summer day. The storm may produce torrential rain and gusty winds for a few minutes over a small area then fall apart.

Lift is usually caused by an approaching wedge of cool air (cool front) or dry air (dry line). In the case near the Central states, this air usually comes from Canada or the Rockies.

When lift is present, an unstable atmosphere can allow a rapid formation of a severe thunderstorm that towers 25,000 to 40,000 feet or more above the ground.

Static May 22, 2011 Satellite Photo

This image shows an outbreak of severe thunderstorms over the central and southern Plains on May 22, 2011, moments before the Joplin, Missouri, tornado, occurred. (NOAA satellite)

Next, wind shear, which is the change in wind direction and speed through different layers of the atmosphere, comes into play.

Wind shear is greatest when winds aloft are strong and often when the jet stream is in the same general area.

Static Wind Shear Severe Storm Ingredient

It is wind shear that helps severe thunderstorms rotate. It is during rotating severe thunderstorms that tornadoes are most likely to develop.

Wind shear occurs when the air well above the ground is moving along a faster pace than air close to the ground. Friction causes this and usually causes the air near the ground to blow at a difference direction than aloft.

This difference in speed creates a long tube of slowly rotating air or a horizontal vortex tube.

Tornado formation

In the vicinity of a thunderstorm, instability and lift then bend that tube of air from the horizontal to the vertical. This rotation then causes the entire thunderstorm to spin.

With so much air rising in a severe thunderstorm, there must be corresponding downdrafts. It is believed that when these rapidly descending downdrafts are enhanced by the overall spin of the thunderstorm, a tornado forms.

Most tornadoes are most likely to form during a mature, severe thunderstorm, rather than a storm that developed over a period of several minutes and then fell apart. This is because it usually takes some time for the updrafts and downdrafts to develop and increase in strength and for the thunderstorm to develop rotation.

In order for a tornado outbreak to occur, the four atmospheric conditions must be able to support many long-lived severe thunderstorms.

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