A tornado touches down near El Reno, Okla., Friday, May 31, 2013. Pictured is the main funnel, not the anticyclonic tornado that flanked it. (AP Photo/Omaha World-Herald, Chris Machian)

Not only was Friday’s El Reno tornado the widest on record, but it was also flanked by an incredibly rare tornado that rotated in the reverse direction.

The National Weather Service confirmed Tuesday that a rare “anticyclonic” tornado touched down about three miles southeast of the monstrous 2.6-mile wide EF5 tornado that killed 9 people, including four storm chasers.

An anticyclonic tornado is one that spins clockwise rather than the conventional counterclockwise (cyclonic) direction. Precious few tornadoes are anticyclonic, and, of those, most are typically in the form of waterspouts or non-supercell thunderstorms (i.e. thunderstorms without a rotating updraft).

“Anticyclonically rotating supercells with tornadoes are extremely rare,” says a National Weather Service website.

Leading tornado researcher Joshua Wurman (of the Center for Severe Weather Research) and his team were in the field monitoring the deadly EF5 twister when they spied another funnel, but spinning backwards, on their two “Doppler on Wheels” mobile radar units.

“At that point we bailed east towards Oklahoma City,” Wurman said. “I’m very happy my team had a radar out there. We only knew about [the anticyclonic tornado] because of the radar; otherwise we may have driven into it.”

Amazingly, Wurman’s encounter was not El Reno’s first with cyclonic and anticyclonic tornado pairings. On April 24, 2006, such a duo touched down in the area. Storm chaser Ken McCallister caught both twisters on video. See below (footage of the anticyclonic tornado begins 12 minutes and 50 seconds in):

The Weather Channel’s severe weather expert Greg Forbes penned a blog post explaining how this couple formed, using the analogy of two opposing swirls on the periphery of a raging stream:

Imagine watching a stream flowing northeastward, with water flowing fastest in the middle of the stream. Near the edges, because of friction, the water flows slower. This sometimes results in eddies or whirlpools. The ones on the southeast side turn clockwise, while the ones on the northeast side turn counterclockwise. These eddies form because of the cyclonic and anticyclonic shears — changes of wind speed — on the two sides of the strong stream (jet).

It works that same way with the jet of air flowing into the thunderstorm. The updraft of the thunderstorm then concentrates those eddies into tornadoes. The counterclockwise eddy is the one that most often becomes the tornado. But once in about a thousand times the jet will be so strong that an anticyclonic tornado can form.

Paul Robinson, a tornado data analyst on Wurman’s team, said Friday’s anticyclonic tornado formed in similar fashion. He added that when they’re grouped together, the anticyclonic tornado is typically weaker than its cyclonic sibling, but sometimes can be “very pronounced.”

“I think [anti-cyclonic tornadoes] they’re probably more common than we think, but many are unobserved because they’re weak,” Robinson said. “In well-developed tornadoes, we’ve seen in them in roughly 20 percent of cases.”