A deadly three-day storm spawned a reported 267 tornadoes over the weekend, according to the Storm Prediction Center in Norman, Okla. But not all of these were single funnel clouds some were what you might call tornado twins and triplets.
Storm chasers following the course of the raging storm captured spectacular footage of a phenomenon called a multivortex tornado as the storm barreled through Oklahoma. In these twisters, multiple funnel clouds rotate around each other in a single storm.
The multivortex twister one of five confirmed tornadoes that hit Oklahoma on April 14 was the largest, an EF-3 tornado on the Enhanced-Fujita tornado damage scale. It had winds of up to 165 mph (266 kph) and killed two people near the towns of Tushka and Atoka.
Oklahoma 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 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.
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 was on a storm survey team that investigated a similar multivortex tornado in Iowa earlier this month.
These extra funnels usually come in groups of two to five and last only a few minutes. Multiple funnel clouds are usually visible only early in a storm before they break off into tornadoes of their own. A storm's circulations can only live up to a certain size and intensity, then it splits into two, three or four tornadoes, meteorologist Mike Smith, chief executive officer of Weather Data Services, a part of AccuWeather, told OurAmazingPlanet.
A multivortex tornado is hard to confirm without video. A signature of a multivortex tornado is intense damage where these funnel clouds touch down, and lesser damage where only the winds of the parent tornado swept through, according to the SPC. This often creates a hit-or-miss pattern of destruction in the tornado's path.
Storm survey teams are studying Oklahoma's tornadoes' paths of destruction, looking at the debris from destroyed houses and comparing these clues with radar images of the storm to figure out exactly how many tornadoes formed.
- Infographic: Tornado! An Inside Look at Tornado Season
- Twisted Science: Why Tornado Forecasting Is Tough
- 12 Twisted Tornado Facts
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