Severe Weather Awareness Week: Tornadoes
When Is The Peak Time of Year for These Twisters?
Severe weather season is upon us and this week is Severe Weather Awareness Week across South Dakota. Each day this week we will tackle a different severe weather topic. Earlier this week I broke down the difference between a Watch and a Warning, while yesterday Blaise discussed severe thunderstorms. Click the links to learn more about those stories.
Today’s topic is probably the one that fascinates the most people: tornadoes. These twisters come in all shapes and sizes, and no two are the same. Some may touch down for just a few seconds as others may be on the ground for hours, tracking over tens if not hundreds of miles. There are numerous types of tornadoes, but the three most commonly referred to are categorized as rope, cone/elephant trunk, and wedge tornadoes. Check out the photo, courtesy of Tempest Tours, to see what each one looks like. In order to be considered a wedge, the tornado has to be at least as wide as it is tall.
Many people think tornadoes are rated based on their wind speed, but the rating is actually created after the storm has passed using the type and severity of damage produced. This rating system is named the Enhanced Fujita (EF) Scale. Before the Enhanced Fujita Scale, there was the “Fujita Scale,” named after Dr. Ted Fujita, the scientist who came up with the scale back in 1971. The EF Scale was implemented on February 1, 2007 for the United States (April 1, 2013 for you Canadians), after the Fujita Scale Enhancement Project of the Wind Science and Engineering Research Center at Texas Tech University. The newest scale was developed after meteorologists and civil engineers studied previous damage patterns and created a new scale to align wind speeds more closely with associated storm damage. The resulting scale can be seen in the photo below.
EF-0 = Minor or no Damage; Example – Branches broken off trees
EF-1 = Moderate Damage; Example – roof severely stripped
EF-2 = Considerable Damage; Example – roofs torn off well-constructed houses
EF-3 = Severe Damage; Example – Entire stories of well-constructed houses destroyed
EF-4 = Extreme Damage; Example – Well-constructed and whole frame houses completely leveled
EF-5 = Total Destruction; Example – Strong-framed, well-built houses leveled off foundations are swept away
Around 95% of tornadoes rate as less than an EF-3, with roughly 77% of tornadoes falling in the EF-0 to EF-1 categories. Very few, 0.1% of all tornadoes, are categorized as an EF-5. Generally the United States sees around 1,250 tornadoes per year, averaging roughly 1 to 2 EF-5 tornadoes per year.
Tornadoes could happen at any point of the year but they are most common in the late spring with April, May, and June, being the peak months in the United States. April is the month that generally features the most violent tornadoes (EF-3 to EF-5), while on average, the month of May is the most active, seeing more tornadoes than any other month at roughly 275 tornadoes.
In South Dakota, the 30 year average (from 1991-2010) is 36 tornadoes per year, but since 2000 the average is actually down slightly, near 29 tornadoes per year. The peak month for tornadic activity in the Rushmore State is the month of June as just about half (16 of 36) of our annual tornadoes occur during the month of June.
In 2016 there were only 16 reported tornadoes in South Dakota, which was the fourth lowest total this century.
Nowhere in the United States is safe from these tornadoes as all 50 States have recorded a tornado, but the most common places to find these twisters are to the east of the Rocky Mountains. These areas are called “Tornado Alley” and “Dixie Alley.” Two other areas of enhanced activity are called “Hoosier Alley” and the “Carolina Alley,” also to the east of the Rocky Mountain range. These areas have no agreed upon boundaries, but you can find the rough representations of these locations in the image below.
One thing we want to remind you, tornadoes are unpredictable and are very deadly. These warnings should be taken very seriously. Make sure to get to your lowest floor, into an interior room away from windows, and cover your head. If you are in your car, the best thing to do is to get out and find a low lying area, like a ditch, and cover your head. NEVER seek shelter under an overpass, as the overpass acts as a wind tunnel and can suck you out into the storm.