Tornado Alley, LLC.

Sean Casey's TIV (Tornado Intercept Vehicle) 2

is a 7-ton monster that looks as if it were made to survive the apocalypse. Maybe so, but the real challenge for the TIV 2 in proving its worth is just getting into position. For tornado chasers like Casey, who seek to document these malevolent forces of nature, finding and catching them requires as much science and smarts as it does bravado.

Tornado Alley, which opened late last month just as tornado season began ramping up, is Casey's IMAX directorial debut and the culmination of all this effort. In his documentary of following twisters around the notoriously tornado-stricken Midwest, Casey darts between the science-research group Vortex 2, which is seeking to understand more about how tornadoes form, and his TIV 2 team, which tracks the elusive but deadly storms and tries to catch them on camera.

Chasing a tornado begins days before one ever touches down. Casey consults with his team's meteorologist, Brandon Ivey, a fellow storm chaser and researcher since he was 16, who checks storm patterns and forecasts in multiple states. "Say for the next day Kansas looks like the place to be," Casey said. "We get into position the night before so that we're within range of all of Kansas." Once the chasers select a state, they check the forecasts again in the morning, hoping to spot the ingredients for a twister. "It's surface observations, how far north has the moisture come from the Gulf, what are the winds aloft like, what are the temperatures aloft?" he says. Massive amounts of moisture coming from the Gulf of Mexico combine with cold northern fronts create prime conditions for tornadoes. Those factors help Casey and his crew refine their search to a 10,000-square-mile area—or about one-eighth the size of Kansas. "You get to that area around 1 pm, and then you wait for the storms to form," he says.

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It's not enough just to find a brooding storm—the team needs to sniff out a storm that will not only produce a tornado, but also give Casey a good shot on camera. "You pick the storm that's the most isolated," Casey said. "You pick the storm that has the most fuel from that moisture-rich air mass." When Casey picks the storm, the chase begins.

Once on the road, the team looks for a mesocyclone, when the entire core of a storm begins to rotate. Another good sign (good for chasers, anyway) is if altitude measurements show a lower cloud base: That means a higher chance for tornado touchdown. And if a tornado appears, getting in the right position is key. "You're chasing that storm from the front," he said, "because whenever you get behind a storm, a storm that's moving at 30 miles per hour is very hard to catch back up with." In fact, Casey usually isn't "chasing" a tornado—he's trying to get into the path of one. "As the storm starts to get nearer to tornado-genesis, you're slowly letting that storm catch up to you," he said.

Figuring out where the tornado will touch down—and how to get in its path—requires even more meteorological knowledge. This time the key is an element of the storm called the rear-flanking downdraft. These downward-flowing winds are formed by warm, moist air being blown upward by the storm and interacting with the cold, dry air flowing downward—causing a major downdraft at the rear of an already-spiraling storm. That helps to create the vortex that tornadoes are so famous for.

When Casey and his team see a downdraft happening on their local Doppler radar, they move in for the shot. "As the tornado forms, you've got to get as close as you can," Casey said. "You have to be in position, or you're going to miss your chance." Those chances are few: Tornado season lasts just 10 weeks, and during that time Casey sees only about 20 tornadoes. Of those 20, the TIV team might get two chances to get inside of one. "But then to get into a tornado that looks impressive—the funnel all the way to the ground, and it's picking up stuff—you get one chance out of three years."

Despite the danger and chaos all around, Casey used a virtually unmodified IMAX camera to film Tornado Alley; he added just an air-jet system to keep stray raindrops off the lens. Otherwise, the rugged TIV 2 vehicle does a good job of keeping camera and crew safe—at least, while they're inside. When filming, Casey must shoot through an open window in the vehicle's turret and then jump in and close it before the howling winds hurl any debris into the vehicle.

The experience inside a twister is indescribable, Casey said. At least, you can't describe it with family-friendly terms. "It's not like you're calm and can take it all in," he said. "You're surrounded by these winds that are blowing at 150 miles per hour," he said, "and there are colorful comments—and expletives—being whispered by all."

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