Tornadoes aren’t like tropical storms or hurricanes, which forecasters can track for days as they approach the coast. “What we’re dealing with is on a much, much smaller scale,” says Greg Carbin, the warning coordination meteorologist at the Storm Prediction Center run by NOAA's National Weather Service. “It forms faster, it dies faster, and it covers a much, much smaller area.”
Of course, as we saw in Moore, Oklahoma, a tornado can still do catastrophic damage. The Moore tornado hit 16 minutes after a warning was issued (that’s using “the time the tornado began to produce damage to residences” as a measurement, per Carbin). It doesn’t sound like much to an outsider, but that’s actually three minutes more warning than the average twister, and those minutes can save lives.
The problem is that tornadoes are essentially impossible to predict until they're on the verge of forming: an ordinary thunderstorm can transform into a tornado in under 20 minutes. So at the Storm Prediction Center, Carbin explains, they focus on the thunderstorms.
“We look at in terms of ingredients,” he says. Thunderstorms form all the time, but storms with the potential to become tornadoes require moisture—from humidity or dew—and they require the interaction of both cold and warm air. The last ingredient is wind shear, to redistribute the storm’s precipitation “so that it doesn’t fall into the updraft and choke the storm off.” The updraft of warm air then “begins to spin in a cyclonic motion,” and if all the elements are in place, a tornado appears.
If all the ingredients line up just right, the Storm Prediction Center issues a “tornado watch" across broad swathes of territory. “We’re looking at a couple days out," says Carbin. "The difficulty gets to be...knowing where the tornado will form. That is an incredibly difficult thing to do with any kind of lead time.” Only when a twister is actually detected do the sirens howl.