You may have noticed severe storms that spawned over 130 tornadoes Easter Sunday and early Monday morning, but what ingredients must come together to produce an outbreak of tornadoes? A tornado outbreak occurs when a particular weather pattern produces 10 or more tornadoes over a roughly 24-hour period.
Spring to early summer typically is the time of year when these outbreaks occur across the United States with the prime months for tornado outbreaks extending from March to July, with the biggest months being April to June. The number of tornadoes closely corresponds to the months of the year when there are the most thunderstorms. In order for a tornado to form over land, a severe thunderstorm must first develop and for outbreaks to occur certain conditions must be in place for long lasting severe thunderstorms.
The ingredients needed to produce such an outbreak are instability, moisture, lift and wind shear. The geographical area of the Plains and Mississippi Valley often allow these ingredients to come together in just the right way to produce these outbreaks.
Typically warm, humid air from the Gulf of Mexico clashes with cold, drier air from the north. Warm air at the surface rises into a colder atmosphere above creating instability. Moisture and instability can produce thunderstorms, but will not produce a tornado outbreak alone as there are two other factors.
Lift is usually caused by an approaching wedge of cool air from a cold front. 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.
The final ingredient is wind shear. Wind shear is the change in wind direction and speed through different layers of the atmosphere. The greater the winds are in the upper atmosphere will determine the greatest amounts of wind shear. This occurs when the jet stream is in the same general area. Wind shear is what helps thunderstorms rotate. The air well above the ground moves at a faster pace than air close to the ground. Friction causes the air near the ground to blow at a difference direction than aloft which causes a long tube of rotating air to form. Instability and lift bend the tube of air causing the entire thunderstorm to spin.
With so much air rising in a severe thunderstorm, corresponding downdrafts occur. It is believed that when these rapidly descending downdrafts are enhanced by the overall spin of the thunderstorm, a tornado forms. Most tornadoes are likely to form during a severe thunderstorm that has been ongoing for awhile since it takes time for the updrafts and downdrafts to develop and increase in strength.
With all the ingredients needed to produce tornadoes, it just so happens that we find them coming together more in the United States and Canada which means North America has more tornadoes than anywhere else in the world by far.