Several recent tornadoes have been given official ratings via radar scans, the practice causing a bit of a stir in the meteorological community. The massive El Reno, Oklahoma tornado was classified as an EF-5 based on a Doppler radar indicated wind speed of 296mph. The traditional way of estimating wind speed and intensity is via a damage survey along the tornado path (damage surveys originally rated the tornado at EF-3). Some feel that using radar will negatively impact the consistency of the current methods. Others think that radar should take the lead in future storms. My personal belief is that while damage surveys should still be performed on any tornado, using radar-based wind speeds when available to assign a rating is a good idea. Here is my main argument why.
The Fujita Scale has always been a past-tense measurement. When Dr. Ted Fujita first devised the scale in 1971, there were no Doppler radars to determine wind speed. The only way to estimate wind speed was through evaluating the damage caused by a given tornado. While reliable, the Fujita Scale also had a major drawback in that it assumed buildings of a given type were all constructed to the same standard. In several cases, tornadoes were downgraded when it became apparent that builders took shortcuts like not anchoring roofs or loosely attaching homes to concrete slabs. The more recent Enhanced Fujita Scale uses more thorough analysis of damage and accounts for differing levels of workmanship in construction. Since it is based on the original Fujita Scale, it too is only applicable after a tornado has destroyed something.
Imagine if hurricanes had no ratings and no wind speeds given until after they destroyed a coastline. It would undoubtedly be much harder to prepare for them. This is the problem facing residents and government officials who deal with tornadoes on a regular basis. The Fujita Scale is determined using the opposite method of the hurricane Saffir-Simpson scale. The wind speed dictates what category the storm falls into, giving forecasters, community planners and emergency response teams the information they need to make wise decisions. Of course a primary difference is that a hurricane’s large size and slow progression means wind speeds can be monitored by multiple radars, satellites and airplanes.
The biggest benefit in my view to assigning ratings based on wind speed relates to building construction. The National Weather Service states that only 1% of all tornadoes are at or above the EF-4 level. This is true only when counting tornadoes that were surveyed for damage. The ones that were not surveyed or went unseen by either radar or spotters do not count. With no damage survey, we do not know how powerful they were. A tornado in an open field can easily have EF-4 level winds but only do EF-2 levels of damage to trees and crops. This technical loophole gets repeated year after year and leads to an underestimate of average tornado intensity and has dire effects on building strength and codes.
The devastation in Moore, OK shows how violent tornadoes can be. While not many storms reach that level of potency, the question is how do designers build to provide adequate shelter when faced with winds of 150mph, let alone 250mph. Currently in the most tornado prone parts of the country, the minimum national standard for residential home wind resistance is a 3 second 90mph gust. Individual states and cities may have higher standards but this is still somewhat alarming. This design standard is not even enough to protect against a slow moving EF-1 storm. The 3 second gust ratings also do not include loss of structural integrity due to flying debris impacts. Repeated impacts can open holes in walls, break windows and weaken the structure to the point of failure, even with the sustained winds far below the gust value.
The design standards along the south Atlantic and Gulf coasts on the other hand are much higher, up to 150mph in some areas. This is in part due to the way that hurricanes behave and are monitored. With a hurricane strike, a large number of houses are guaranteed to encounter powerful sustained winds. The historical data backs up this assumption with detailed information on wind field size, intensity and duration. Adhering to a higher standard is therefore not just the moral thing to do, but it makes economic sense. People can survive inside a properly rated structure and repairing a slightly damaged structure is much less expensive than rebuilding completely. With tornadoes, the area affected is far smaller but the intensity is concentrated. By designing to the minimum standard, builders take the gamble that a significant tornado will not strike. Most of the time, this is true. When it is false, it is devastating to these buildings and those seeking shelter within their confines.
The main drawback is the lack of Doppler radar coverage at ground level in many locations. Unless a storm passes fairly close to a stationary radar, it is very hard to get accurate data. There are several mobile Doppler trucks used by research organizations but they are relatively rare (the El Reno storm was scanned by a Doppler On Wheels). These mobile units are the most accurate when it comes to scanning the extreme low levels of the atmosphere. With high risk weather forecasts usually giving at least 2 days notice, positioning a mobile radar into a region where severe weather is likely is possible. However, getting the radar into a clear position to scan a tornado within a given storm in that region is still a challenge.
Getting a better understanding of the life cycle intensity of tornadoes is very important. Without this data, structures will continue to be built to a much lower standard putting people’s lives and property at risk. Accurate wind speeds have helped coastal regions enhance the building standards for their most threatened areas. There is no reason that the residents in Tornado Alley should not have the same protection.