Photo Credit: Severe Warning Systems, Joey Marino, Radarscope
According to the National Weather Service, supercells are the least common type of thunderstorm. However, they are the most dangerous type of thunderstorm because they tend to produce all types of severe weather such as damaging winds, very large hail, and of course, tornadoes. A thunderstorm needs to have an organized and strong updraft to produce damaging winds and large hail, but a mesocyclone must be present for tornadogenesis to occur. A mesocyclone is a cyclonically rotating vortex of air in a thunderstorm. The mesocyclone is often found near the updraft on the backside of a storm.
For a mesocyclone to form, there needs to be a few things happening in the atmosphere. There needs to be both a change in wind direction and an increase in wind speed as you increase with height. This will create a horizontal spin in the lower atmosphere (stage 1). Even in an environment where there is no thunderstorm activity, a mesocyclone has now formed. Now that the mesocyclone has formed, the air within the updraft tilts the rotating column of air from horizontal to vertical (Stage 2). Once the rotation is tilted vertically, a locally lowered cloud base known as the wall cloud, typically forms at the base of the updraft (Stage 3). It is in this region of the supercell where a tornado will form. The first graphic sbove provides a great representation of the formation and maturing stages of a mesocyclone.
Tornadoes associated with supercells can range from weak EF-0’s to violent EF-5 tornadoes. A great example of a supercellular tornado is the EF-4 Solomon-Chapman, Kansas tornado that occurred on May 25th, 2016. An isolated supercell thunderstorm had formed over Ottawa county in north central Kansas. This supercell had produced 4 tornadoes during its lifespan. One of the tornadoes was a long-lived wedge tornado that was on the ground for 90 minutes and had a path length of 26 miles. That is astonishing since the average tornado is only on the ground for approximately 10 minutes. In the picture above, you can see the stunning structure of the massive rotating wall cloud as well as the wedge tornado as it heads in the east direction.
However, supercells are not the only type of thunderstorm that can form a tornado. Quasi-linear convective systems (QLCS), squall lines and bow echos are known to have the capability of producing tornadoes as well as other types of severe weather. These types of thunderstorms generally are a line of thunderstorms that can travel for hundreds of miles and can sustain themselves for hours.
Unlike supercells, tornadoes with these types of storms are produced by the formation of mesovorticies. Mesovorticies are compact couplets of spinning air with very high vertical spin that form on the leading edge of a line of thunderstorms. These do tend to produce strong damaging winds but can also produce weak and brief tornadoes. They also are low level storm features and typically happen at least 1 km above the surface. Meanwhile, the mesocyclone of a supercell is more common in the mid-levels. In order for a tornado to have a chance at forming, the mesocyclone has to work its way down into the low-levels and form low-level rotation.
A prime example of this occurred just recently on March 29th, 2018. A line of severe storms was moving through parts of southern Texas late Tuesday night into Wednesday morning. As these storms were moving through Corpus Christi and Woodsboro, TX, a few mesovorticies had formed along the leading edge of the line of storms. Multiple tornado warnings were issued as radar had detected the rotation associated with this mesovorticies.
Below is a screenshot of the velocity scan from the NWS office in Corpus Christi as the line of storms was moving through. A velocity scan is a type of radar image used by meteorologists to observe the strength of winds and rotation associated with a thunderstorm. You can see that the mesovorticies are circled in blue and the rotation is shown by using light red and green colors.
During that morning, many damage reports were being sent to the NWS office in Corpus Christi. And so, a survey team was sent out to survey the damage. Windows were blown out and roofs were damaged in Woodsboro, TX and several mobile homes were flipped over in Seadrift, TX. The survey had presented enough evidence to conclude that a few weak EF-0 tornadoes and EF-1 tornadoes had formed briefly as the line of storms was moving through the area.
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