Is Tornado Risk Increasing ?

Last week the Commonwealth of Virgina had a statewide Tornado Drill. According to Tim Keen, acting Chief of Prince William Fire and Rescue, “Tornadoes can happen anytime, anywhere in Virginia, they don’t have a season. Knowing what to do during a tornado warning can save your life.”

My HOA did not participate in the drill, but you can make preparations on your own. Being prepared for an emergency is a good thing. So:

• Build an Emergency Kit. This kit is useful to have on hand for all emergencies.
• Create or review your Emergency Plan, and know where your shelter will be. Make sure all family members know the plan and where they will meet.
• Sign up to receive alerts

You can get emergency alerts sent to your email from your county or just by enabling weather alerts on your phone. If a tornado watch has been called (and an alert appears on your phone, scrolls across your TV, you receive an email or it is announced on the radio) it means that tornadoes are possible in the area and this is your chance to get ready. Hopefully, you already have and emergency plan, and storm supplies in your Emergency Kit (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. Do not take refuge in a mobile home. 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.

Though tornadoes occur all over the Earth, 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 recorded each year. A distant second is Canada, with around 100 per year. Severe thunderstorms accompanied by tornadoes, hail, and winds cause an average of 5.4 billion dollars of damage each year across the United States.

In the United States, there are two regions that have historically had a disproportionately high frequency of tornadoes. Florida and the southeast is one and "Tornado Alley" in the Great Plains of the U.S. is the other. Florida and the southeast have numerous tornadoes simply due to the high frequency of almost daily thunderstorms. However, despite the violent nature of a tropical storm or hurricane, the tornadoes they create tended to be weaker than those produced by non-tropical thunderstorms.

Climate scientists have wondered if the frequency and location of tornados has been changing with increased temperatures. A recent paper; “Spatial trends in United States tornado frequency” by Vittorio A. Gensini of Northern Illinois University and Harold E. Brooks ?has looked into the possibility that tornado frequencies are changing across the United States. Their study built on the work of others, notably Farney& Dixon “Variability of tornado climatology across the continental United States” . Int. J. Climatol. 35, 2993–3006 (2015) and Agee, Larson, Childs & Marmo, A. Spatial redistribution of US Tornado activity between 1954 and 2013. J. Appl. Meteorol. Climatol. 55, 1681–1697 (2016).

The scientists found that the national annual frequencies of tornado reports have remained relatively constant, but they found that location of tornado occurrence may be changing. The scientists findings indicate a decrease in the traditional "Tornado Alley" of the Great Plains and an increase in the Southeast's . The scientist have also found a shift in the timing of tornado season, but were unsure if this was due to rising global temperatures or natural variability. 

STP frequency

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.

Despite improvements in records detecting spatial shifts in tornado frequency has proved challenging given the small areas and duration of tornado-producing thunderstorms and deficiencies in tornado reporting database. So the scientists used a proxy for tornadoes, the Significant Tornado Parameter (STP) to substitute for tornado frequency. STP is designed to identify and track atmospheric ingredients large storms capable of producing EF2-EF5 tornadoes. However, even if the right winds, moisture, and instability are present, thunderstorms and tornadoes won't form without a strong enough trigger to spark them. The STP does not account for whether or not a trigger is present, and no research has been done to examine the co-variant relationship of tornadoes occurrence location with STP. In addition, the time frame of the study was just a few years, so we are not sure how robust their findings are.