Why the May 31st El Reno Tornado was Downgraded to EF-3
Posted on Saturday, August 31, 2013 at 9:00 AM MST
The deadly tornado that ripped through Canadian County, OK on May 31st has been downgraded back to EF-3 on the Enhanced Fujita Scale. Back in early June, the National Weather Service initially rated the tornado EF-3 after surveying the damage, but after receiving data from several doppler radars in the greater Oklahoma City area that measured tornadic winds between 220 and 300 mph, they upgraded the tornado to an EF-5. If you Google the Enhanced Fujita Scale, any hit you get will say that an EF-5 tornado has winds above 200 mph. So why did the tornado get downgraded back to EF-3?
The answer lies in a small technicality of how the Enhanced Fujita Scale, or EF-scale, is defined. The National Weather Service's website states that "the EF-scale is still a set of wind estimates (not measurements) based on damage. It uses three-second gusts estimated at the point of damage based on a judgement of 8 levels of damage to the 28 indicators listed below." The indicators reference different types of structures, trees, etc. In other words, the EF-scale is a damage scale, not a wind scale, and the EF-rating is given based on the magnitude of the damage the storm does. The wind speed is estimated based on the magnitude of the damage and how well a stucture was built (a tornado that completely destroys a poorly built structure will receive a lower EF-rating than if it destroys a well-built structure).
Left: May 31st El Reno Supercell from near Weatherford, OK
Right: May 20th Moore Supercell from Norman, OK
One of the best examples to show how the EF-scale works is to compare the El Reno tornado to the EF-5 tornado that struck Moore, OK on May 20th. For this example, let's pretend the doppler radar data does not exist for either tornado. The images from Moore were amazing - entire neighborhoods wiped clean down to the concrete slabs. These were well built homes, too. It was very clear that there was widespread EF-4 to EF-5 damage. It turned out that most of the damage was rated EF-4. Amongst all the destruction in densely-populated Moore, there was only a very small pocket of EF-5 damage found near one of the elementary schools. You will soon find out that surveying damage can be much trickier in rural areas.
The El Reno Tornado did not directly impact any heavily-populated areas. It passed over fields and farmland south of Interstate 40 near El Reno. It did strike some homes that were of comparable construction to the homes in Moore. All of these homes impacted by the El Reno tornado that I saw still had inner walls intact (many had some outer walls standing, too), and quite a few of them still had at least part of their second floor intact. When you compare them to the homes in Moore that were wiped clean down to the slab, the difference in the magnitude of damage becomes quite apparent. That's why the El Reno tornado's EF-rating was downgraded. Could it have produced EF-5 damage somewhere? Absolutely, but distinguishing EF-3 damage from EF-5 damage in an open treeless prairie with nothing but dirt and grass to look at is next to impossible.
To wrap things up, let's talk briefly about how the doppler radar data comes into the equation. As far as the EF-scale is concerned, the data means absolutely nothing, but the data will be very useful to help scientists in their quest to better understand tornadoes. Because of the way doppler radar works, the 220 to 300 mph winds the radars measured were between 500 and 2500 feet above the ground. This does not necessarily translate to 220 to 300 mph winds at the ground level. Unless instruments are deployed in the tornado's path, the only way to know the wind speed at the surface is to estimate it by surveying the damage. It may not be the most accurate method in the world, but it's the best one we have today. The El Reno tornado certainly could have produced winds capable of causing EF-4 and EF-5 damage, but we will never know whether it actually did, so it will go into the official record books as an EF-3.
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