SIOUX CITY, Iowa (KCAU) – Some parts of the country, including right here in Iowa have already seen significant severe weather through the first few weeks of spring, including several strong tornadoes.
Many of those tornadoes were produced by the rarest but often the most violent type of thunderstorms, supercells. Although supercells are the most rare mode of thunderstorms, supercell tornadoes are the most common type of tornado. This is because tornadoes need instability (warm air at the surface/colder air aloft) and plenty of wind shear (increase in wind speeds/change in wind direction with height) to form, similar to the ingredients needed for supercell development.
Also, one of the main characteristics of a supercell thunderstorm is a persistent rotating updraft. These updrafts, given adequate shear, often associated with supercell development, tilt that rotating updraft from a horizontal position to a vertical and fed by warm, moist air from the surface, also common in the warm sector (wedge between fronts ahead of low-pressure system) and can result in tornado formation.
While many supercells do become tornadic, not all supercells end up producing tornadoes. However, supercell thunderstorms have been behind some of the most violent tornadoes and tornado outbreaks in US history. Some infamous supercell tornadoes include both the Super Outbreak of 1974 and 2011, the two largest single-day outbreaks in history. The only 59 F5/EF5 tornadoes to ever hit the US, including Moore, Oklahoma, tornadoes of 1999 and 2013 and 2011 Joplin, Missouri, were also spawned by supercell thunderstorms, as were the Pilger, Nebraska, twin tornadoes in 2014 which were rated EF4.
While most tornadoes from the most tornadoes, especially the strong to violent and most destructive tornadoes are produced by supercells, some tornadoes are non-supercell related and are instead embedded in squall lines or quasi-linear convective systems. These, however, tend to be weaker and much shorter-lived than supercell tornadoes, which can oftentimes be long-tracked and long-lived. Non-supercell tornadoes can also be much harder to detect since they’re often embedded within a fast-moving line of storms, many times occurring late at night or early in the morning.
Tornadoes can also take on various appearances, from thin and rope-like, how most tornadoes begin and end, to monster wedges, commonly seen with only the most violent and destructive storms.
Aside from rope and wedge, there is the classic cone tornado, which appears wider at the top and narrower at the bottom (think an upside-down traffic cone). These have wider paths than a rope tornado, although they usually start as a rope-type tornado, but a much narrower path than a wedge tornado. These tornadoes can be on the weaker side, however, tend to be on the stronger side at times, like the Pilger, Neb., tornadoes in June 2014, which began as cones before evolving into large stovepipe tornadoes.
Similar to a cone tornado, a stovepipe tornado, which again looks just like it sounds, is about the same width, top to bottom. Depending on the width of these, stovepipe tornadoes can leave fairly wide paths of destruction and can be strong and even violent, as was unfortunately proven in the Pilger tornadoes. However, stovepipe and cone tornadoes usually leave narrower paths of destruction than those of a wedge tornado.
Some of the most destructive and devastating tornadoes in history were wedge tornadoes. These types of tornadoes look wider than they are tall and oftentimes are a half mile wide or larger. In fact, the 2013 El Reno, OK tornado, which is the widest tornado on record, was a massive 2.6 mile wide wedge tornado. The 1999 and 2013 Moore, OK EF5 tornadoes, the 2011 Joplin EF5 and Tuscaloosa EF4 tornadoes were all wedges.
In the strongest, most violent tornadoes, multiple vortices may develop, which appear as small circulations around the main tornado. And on rare occasions, the vortices may become independent of the parent tornado and form a secondary tornado, like what happened in the Pilger, NE twin tornadoes.
In the wake of any tornado, from a thin, weak rope to a monster wedge, and everything in between, the local National Weather Service Office (you can find yours by going to http://weather.gov and searching your location) sends out surveyors to assess damage and assign an EF rating using various damage indicators and degrees of damage. Using these assessments and estimated winds at the approximate time the storm hit allow them to assign a rating between EF-0 and EF-5.
However, many may have previously heard of the Fujita scale (F0-F5), which is what the EF or Enhanced Fujita scale is derived from. The difference between the two scales is that the original Fujita scale, created by Dr. Ted Fujita was based only on damage sustained, didn’t take into account differences in construction of structures. The F scale also based it’s rating on the worst damage regardless of how isolated the damage was (ex. a single shed was totally destroyed so the tornado was an F4 despite F2 damage elsewhere).
The EF scale was put into operation on February 1, 2007, effectively replacing the previously used Fujita (F) scale. While some aspects of the original tornado rating scale were preserved, the EF scale includes 28 damage indicators, each with various degrees of damage and estimates of lower-bound and upper-bound wind speeds that would be most likely to cause the degree of damage seen. Each damage indicator includes descriptions of typical construction and each degree of damage provides thorough descriptions of damage that may be seen. The EF scale takes the basic ideologies of the F scale and as it says in the name, enhances the scale to provide more precise and accurate ratings and assessments of tornadoes.
For more information on the Fujita and Enhanced Fujita scales, visit : https://www.spc.noaa.gov/efscale/.
It’s also extremely important to note that even a seemingly small rope tornado can be a destructive and devastating tornado and a large, menacing wedge isn’t necessarily, automatically a catastrophic tornado, however, they do tend to be on the stronger side. Likewise, tornadoes can take on various appearances that may not be one of the “shapes” discussed above. For more information on tornadoes, numerous resources can be found at by clicking here.
And as severe weather season ramps up in Siouxland, visit KCAU 9’s Weather page by clicking here to get all the latest updates.
You can also download the KCAU9 Weather app in your device’s app store, by searching KCAU-9 Weather.