Photo of the El Reno Tornado at 6:28 p.m. CDT, when it was near peak strength. By Nick Nolte, taken on May 31st, 2013; accessed through Wikimedia Commons.
The EF3 tornado that struck Oklahoma farmland just outside of El Reno on May 31st, 2013 was the largest tornado ever recorded in U.S. history. As David Neal described in his earlier blog post, this tornado was devastating in many ways – killing 8 storm chasers, frightening an area that had experienced the killer Moore tornado 11 days prior, and narrowly missing suburbs within the densely populated Oklahoma City metro area. But just how strong was this record-breaking tornado?
700 mb and 850 mb isobaric surface maps of wind flags and isopleths of heights of each surface over the ground over CONUS from 12 UTC (about 7 a.m. CST), May 31st 2013. Map 1 from https://www.spc.noaa.gov/obswx/maps/700_130531_12.gif, Map 2 from https://www.spc.noaa.gov/obswx/maps/850_130531_12.gif.
David already discussed the meteorological ingredients that were abundant enough to produce a storm of this magnitude; ample heat and moisture, a stationary frontal boundary, and a dry line near the Oklahoma City metro created an area primed for explosive instability on the afternoon of the 31st. The maps above show how the upper levels of the atmosphere supported unstable conditions that day. The data plotted on these maps were collected by weather balloons and show 2 dangerous ingredients for thunderstorm development: wind shear and cold temperatures aloft. First, there was both directional shear (change of wind direction with height) and speed shear (change of wind speed with height) over central Oklahoma. Second, the decreases in temperature (22 ℃ to 12 ℃ from 850 mb to 700 mb) indicate a cooling upper atmosphere, which helps warm surface air rise further and produce more powerful storms. Ultimately, these factors (along with other helpful factors at even higher levels of the atmosphere) combined to support the storms that developed in Oklahoma that afternoon.
6.3 micron Visible satellite loop (from 2:15 to 8:15 p.m. CDT) from the GOES-14 visible imager over OKC the afternoon of the 31st and a 12 micron infrared still image of the same area from the POES AVHRR imager taken at 22 UTC (5 p.m. CDT). Both images downloaded from Scott Bachmeier’s satellite blog at https://cimss.ssec.wisc.edu/satellite-blog/archives/13130.
Strong storms certainly developed over central Oklahoma that day. You can see, in the visible satellite loop, small cumulus clouds combining and eventually becoming huge cumulonimbus clouds – the boiling clouds covering most of central Oklahoma as the loop progresses. David described the dynamics that led to these storms eventually developing out of the raw ingredients, but these satellite images help show just how powerful the OKC thunderstorms were. Their overshooting tops (the tallest parts of a severe thunderstorm, located directly over its updraft) are identifiable in both the visible and infrared imagery, designated by wrinkly and churning spots on the cloud tops in the visible loop and dark red/black colors in the high-resolution POES image. Since infrared imagery directly measures cloud temperature, that image is proof just how high surface air was carried by the cumulonimbus’ updrafts through the atmosphere (thus cooling the air proportional to its altitude) and therefore how strong these storms were.
Radar loop of the El Reno tornado’s base reflectivity (left) and base velocity (right). Captured between 6:13 -6:15 p.m. CDT on May 31st, 2013. GIF from the SPC’s Publications Page archive, courtesy of Jeff Snyder/ARRC, at https://www.spc.noaa.gov/publications/edwards/st-anim.htm.
Given these favorable conditions, it was likely that supercell thunderstorms would form and there would be some tornadoes that afternoon. The El Reno tornado took full advantage of the favorable conditions and became a monster, clocking in at a maximum 2.6 miles wide per National Weather Service damage surveys. The above radar loops were taken by the University of Oklahoma’s Rapid-scan X-band Polarimetric Radar (RaXPol) mobile radar, providing a high-resolution look inside the beast. These scans were also taken at an angle of 2° above the horizon, meaning they represent conditions significantly above ground level. Each dotted-line circle on the maps represents 2km in distance from the radar. On the left is a reflectivity loop, showing how dense the clouds, precipitation, or other material in the atmosphere is at certain locations; on the right is a velocity loop, which shows the motion of air in the atmosphere towards (green) or away from (red) the radar, with a small (or in this case, 2 mile wide) “couplet” of the two colors indicating the tight and incredibly fast rotation of a tornado. The scans show how large, strong, and well-structured the tornado was at these higher altitudes, even including an eye in the gigantic tornado’s center and a smaller satellite tornado rotating inwards towards the main tornado (the small velocity couplet appearing NW of the main rotation in the relative velocity loop). In fact, the tornado was so immense that the mobile radar had to flee to a safer distance as the storm moved towards them (seen as the tornado’s apparent jump in the last 3-4 frames).
Visible satellite images by MODIS of the OKC metro before and after May 31st, 2013. GIF taken from Scott Bachmeier’s satellite blog at https://cimss.ssec.wisc.edu/satellite-blog/archives/13130.
Ultimately, the record size of the tornado is what killed 3 storm chasers – Paul Samaras, Tim Samaras, and Carl Young – as they thought they were far enough away from the small, visible tornado funnel while driving directly into the much larger, invisible, and still tornadically strong wind field. They were tragically struck and killed by a sub-vortex in that larger wind field while trying to avoid the visible funnel. The above true-color satellite image betrays the true size of the tornado and the extent of its strong winds on the scale of the OKC metro area. In all, this tornado was a textbook case of ingredients coming together for a dangerously perfect storm, and the above satellite images show that while the storm chasers’ fatalities were tragic, this record-breaking tornado could have had a much more violent impact had it tracked a few more miles to the east instead of dissipated while still over farmland. Oklahoma City and El Reno dodged the largest bullet the meteorological community has ever recorded, and we can keep analyzing the record-breaking tornado to be prepared if a similarly perfect setup ever happens again.