Tornado Risk

from NOAA Tornado Frequecy

Though tornadoes occur all over the Earth, not limited to any specific geographic location, some parts of the world are much more prone to tornadoes than others. Globally, the middle latitudes (that would be us), provide the most favorable environment for the creation of tornadoes because this is where cold, polar air meets warmer, subtropical air, generating precipitation along the air mass collisions. In addition, air in these mid-latitudes often flows at different speeds and directions at different heights conducive to creation of rotation within the storm.

Tornadoes have been documented in every state of the United States, and in terms of absolute count, the United States leads the world, with an average over 1,000 tornadoes (EF-0 to EF-5) recorded each year. A distant second is Canada, with around 100 per year. Interestingly, the places that receive the most frequent tornadoes are also fertile farmland area. This is due in part to the high number of convective rain storms in these areas. One of the main difficulties with tornado records is that a tornado, or evidence of a tornado must have been observed. If a tornado occurs in a place with few or no people, it is not likely to be documented. Much of what we know as tornado alley of the central plains was very sparsely populated until the 20th century, and so the historical record before 1950 may not be accurate.

Tornado intensity is measured by the Fujita Scale, F, (also known as the Fujita-Pearson Scale) and the Enhanced Fujita Scale, EF, that imperfectly links damage to wind speed, but is relatively easy to apply in practice without much additional expenditure of time or money. The scale is used to rate the intensity of a tornado by examining the damage caused by the tornado after it has passed over a man-made structures. Generally speaking the intensity of tornadoes ranges from F0 (or EF-0) to F5 (or EF-5).

In the United States, there are two regions with a disproportionately high frequency of tornadoes. Florida is one and "Tornado Alley" in the south-central U.S. is the other. Florida has numerous tornadoes simply due to the high frequency of almost daily thunderstorms and the southeast and Gulf Coast are not far behind. However, despite the violent nature of a tropical storm or hurricane, the tornadoes they create tend to be weaker than those produced by non-tropical thunderstorms.

Frequency of stronger tornadoes in the U.S.

So should I worry, should I consider installing a tornado shelter in my home (beyond the partially above ground basement that my house already has)? John Nelson of IDV Solutions put 62 years worth of tornado data from NOAA on a map. John plotted each tornado's path and used brightness for its level of intensity. IDV Solutions is a cool company that sell what it calls Visual Fusion software. Visual Fusion is data visualization software for building interpretive images from virtually any data source. These images are used to connect all data in a single view, enabling those to whom columns of numbers do not speak to interpret data in a visual, interactive way to improve understanding and insights. John’s visualization shows that tornadoes in general arrive from the southwest and travel to the northeast, but more importantly for me the brightness of the lines indicates the intensity of the storm and the storms that have occurred in the Piedmont of Virginia have not been intense tornadoes. However, his data visualization lacks a time parameter. Looking at the frequency graph of tornadoes from NOAA it appears that the frequency of intense storms has not increased since the peak in the 1970’s, but the overall frequency of all tornadoes surpassed the 1973 peak in 2011, but fell again in 2012. Of course 2013 could be a very big year for tornadoes. Nonetheless, it looks like for my home, building or installing a FEMA 320 certified storm shelter is not a top priority. I may get one someday if I ever have a large sum of extra money, but I may just reinforce a below ground section of my basement with a FEMA 320 safe room kit.

from John Nelson IDV solutions showing tornado intensity and direction

In storm shelters there are two options: a site-built shelter or a commercially manufactured shelter. A site-built shelter is one that is built into your house during construction- building a closet or bathroom to meet the necessary design standards developed by Texas Tech and adopted by FEMA. The FEMA 320 standard is not easily retrofitted into a standing house. There are also commercially manufactured shelters that can be purchased, they range from the old stand-alone Auntie Em style storm cellars, to ones that are designed to be installed in the floor of a garage or underneath and as part of the steps of a pre-manufactured home.

If you choose to purchase and install a tornado shelter, make sure the shelter you select has been approved, tested and certified by the National Storm Shelter Association to meet FEMA 320 standards. On the market there are above-ground, below-ground and partially below-ground models that have been tested and certified. For any shelter which is partially or completely above-ground, the walls must be resistant to debris impact. Make sure the shelter you select has been tested for debris impact resistance by Texas Tech or if it is a below ground shelter the door has been tested and certified as resistant to debris impact. 

detail from John's map showing Tornado intensity in parts of NC, VA and  MD

Finally, if a tornado watch has been called (and scrolls across your TV or announced on the radio) it means that tornadoes are possible in the area and you should think about how you will protect yourself and your family this is your chance to get ready. Hopefully, you already have and emergency plan, and storm supplies (like a radio, flashlight and water) and are ready to act quickly if a tornado forms in your area. A tornado warning means a tornado has been sighted by weather radar and you should act immediately to seek shelter. If you are in a house, go to the lowest level such as a basement or storm cellar. If there is no basement, go to an interior room such as a closet, hallway or bathroom. Try to protect your head from flying debris or broken glass-blankets, bicycle helmets. If you are in a mobile home, you should leave immediately and seek shelter elsewhere. If you are outside and cannot get to shelter, crouch beside a strong structure or lie flat in a ditch or low-lying area and try to cover your head and neck. Get as far away from trees and cars as you can. A car is not safe in a tornado and parking a car under an overpass is not effective protection.