The tornado is one of the most powerful, yet elusive, forces on Earth," says John Hart, a senior forecaster at the Storm Prediction Center  in Norman, Oklahoma. "Despite decades of research, we are still not able to predict the actual path of a tornado."
At best, meteorologists can be alert to the potential for tornadoes. Tornado-conducive conditions include warm air, humidity near ground level, and fast-moving winds (say, about 70 knots) about 2 miles above ground level, traveling in different directions from mile-high winds moving at about half that speed.
A tornado forms when warm air rises, cooling as it moves upward and thereby causing moisture to condense, creating a cloud. The spinning air keeps rising, forming a violently rotating column of air with extremely low pressure under the cloud base. A special type of thunderstorm called a "supercell," which spins, twists, and turns at its core (sometimes visibly so) creates the strongest tornadoes.
Doppler radar (the same technology that will land you a speeding ticket on the highway) uses light and sound waves to determine wind direction, looking for intensifying rotation—a clue that a tornado is about to form. Radar is mounted on about one hundred fifty 90-feet towers set up around the country to detect tornadoes. "50 miles from the radar site is not a bad range for detecting a larger, classic Midwestern tornado situation," says Corfidi. But tornadoes developing outside that range might not be detected.
To address this limitation, a new, and more sensitive, radar network, WSR88D (Weather Surveillance Radar 1988 Dopplers), was put into use in 1998, and it now covers virtually all the United States. It excels in detecting severe weather events since it allows time for early notification of damaging winds, and it significantly increases tornado warning time, because tornadoes can be predicted before actually reaching the ground.
However, because the radar is tilted up slightly, it is not able to detect small tornadoes that occur close to the ground. In fact, if a diminutive tornado occurs in a sparsely populated area, the only evidence of its existence may be the trail of destruction it leaves behind.
To aid in detecting tornadoes as they are forming, five satellites orbiting 22,000 miles over the Earth take snapshots of cloud formations every few minutes. "Just as a doctor reads an X-ray, trained meteorologists analyze the images for cloud structures known to be associated with tornadoes," says Hart. "Sometimes, a few puffs of clouds in the right place can make all the difference."
Once suspicious thunderstorms have been identified, radar keeps a close watch for rotation deep inside them, and local weather spotters are alerted. "If either the radar or the spotters indicate a strong threat of a tornado, a warning is issued," says Hart. Unfortunately, those warnings rarely give those in the tornado's path more than 10 or 20 minutes' notice of the impending event.
Despite knowing how tornadoes form, it remains virtually impossible to predict when one will develop. "To predict a tornado at a specific spot and time, we would need sensors not only on the ground but also extending to the upper atmosphere a mile or two above the Earth, placed every 10 miles. That would just cost too much money," says the Storm Prediction Center's forecaster Stephen Corfidi.
Then there are days when all of the ingredients for a tornado are present, but nothing happens. "If we issued a watch for every one of those situations, we'd be crying wolf so much that everyone would stop believing," says Corfidi. The question is, which one is worth blowing the siren over?"
For The Record
Forecast: Warm, chance of thunderstorms
Fact: The most destructive twister ever occurred on March 18, 1925, when the "Tri-State Tornado" hit Missouri without warning and "lived" for 5 hours, carving a 300-mile path of destruction. Back then, folks hadn't yet thought of using the newfangled radio as a way to warn of storms, so no one knew it was coming. The tornado killed 689, injured 3,000, and left 15,000 homeless.
Tornado Lore and More
Did you know that Elvis Presley survived a tornado? Have you ever wondered why tornadoes seem to always strike mobile home parks? Click here  for the answers to these and other wild weather questions.