Storm Chasers Capture Twin Tornadoes on Video

by Brett Israel

Date: 11 April 2011 Time: 04:51 PM ET

A whopping 27 tornadoes were reported across Iowa over the weekend, including twin tornadoes that were caught on film by a freaked-out storm chaser.

"Multi-vortex! Multi-vortex!" shouted the stunned cameraman when lightning illuminated the funnel clouds he and his fellow storm chasers were pursuing. The footage was captured April 9 in Pocahontas County, Iowa, and may show the beginning of several destructive twisters, one expert said.

Storm survey teams have confirmed 10 tornadoes across five counties so far. The hardest hit town was Mapleton, home to 1,200 people in Monona County, where tornadoes leveled about 60 percent of the town, reported the Des Moines Register. No one was killed, but about a dozen injuries have been reported so far.

Behind the big twisters

Iowa's largest tornado was an EF-3 tornado on the Enhanced-Fujita tornado damage scale. The twister measured three-quarters of a mile wide and had winds of up to 165 mph (266 kph). Iowa is in the heart of Tornado Alley, where textbook tornadoes form when warm, moist Gulf of Mexico air collides with cool, northern air, creating massive storms. The warm air rises and hits wind shear, a layer of the atmosphere where the winds change direction over a short height and create the rotation — think of a pinwheel with air pushing in opposite directions on the top and bottom.

Below the pinwheel, a cap of warm air bottles up surface heat below about 10,000 feet (3,000 meters). Heat builds until it punches through the cap, triggering a thunderstorm. With enough air shooting up and down, the pinwheel is knocked on its side, creating a huge rotating mass of clouds called a mesocyclone — the hallmark of a tornado-spawning supercell storm.

Science of twin tornadoes

A multiple vortex tornado is a different beast, packing several vortices inside the main vortex.

"It's almost like you have that circulation, and then if conditions are right you have circulation embedded within circulation," said Karl Jungbluth, a meteorologist with the National Weather Service in Des Moines, Iowa. Jungbluth is on the storm survey team that is piecing together exactly what happened during the weekend's storms.

Multiple vortices are usually visible only early in a storm before they break off into tornadoes of their own, which is probably what happened in Pocahontas.

"This circulation was so big, and in nature circulations can only live up to a certain size and intensity, then they break off into two, three or four tornadoes," said meteorologist Mike Smith, chief executive officer of Weather Data Services, a part of AccuWeather.

The storm survey team is studying the paths of destruction, looking at the "bread crumbs" — debris from destroyed houses — and comparing these clues with radar images of the storm to figure out exactly how many tornadoes formed. But Pocahontas is located in a hard-to-see spot for radars, Jungbluth said, so they may not ever know what happened below the supercell storms.

"It's just a mess," Jungbluth told OurAmazingPlanet.

Email OurAmazingPlanet staff writer Brett Israel at bisrael@techmedianetwork.com. Follow him on Twitter @btisrael.