MEMPHIS, TN—Officials from the National Weather Service issued a severe weather alert for all basements in Tennessee Tuesday after a deadly new weather phenomenon ravaged scores of residential downstairs areas, leaving every other part of the houses completely untouched. The recently discovered targeted cyclones, known as basement tornadoes, tore through cellars all over the state, killing dozens and injuring hundreds.

Calling the recent devastation the worst indoor weather event in U.S. history, weather service director Dr. John L. Hayes said that millions of residents who have for years been taught to seek shelter from twisters by taking refuge in their homes will have to drastically adjust long-held assumptions about tornado safety and preparedness.

A homeowner searches in vain for a moving box full of old halloween costumes.

"If there is a violently rotating column of air in your basement, do not go downstairs," Hayes said. "Wait in an attic or at the top of a stairwell. If possible, find shelter in a structure lacking any kind of subterranean open space at all, such as a mobile home."

"Unless, of course, regular tornadoes are sighted in the area, in which case you should immediately get to your basement, provided you have one," Hayes added.

Meteorologists have measured the spiraling basement winds at speeds of up to 200 mph—powerful enough in some cases to drive a box of dryer sheets six inches into solid concrete. In all, the cyclones have caused hundreds of dollars in property damage by toppling artificial Christmas trees, smashing jars of fruit preserves, and overturning ping-pong tables.

Doppler radar shows how basement tornadoes form just over the foosball table in this Knoxville-area cellar.

"Some people lost their entire basements," said Knoxville-area relief worker Dan Weiss, who personally observed a dozen rec rooms that were completely destroyed, and a half-dozen more that might have been destroyed, though it was difficult to say for sure. "Everything they had ever stored was suddenly lost. So much extra stuff gone to waste."

Using Doppler radar, interior satellites, and computer models, researchers have determined that basement tornadoes form when warm dank air collides with a cool dry draft, creating a significant drop in pressure, and causing wind velocity to increase and whirl in a vortex of dust, debris, and cobwebs. Predicting where and when a basement tornado will strike is challenging, however, because they seem to jump from cellar to cellar by traveling through sump pumps.

"All we can say for certain is that unfinished basements are twice as likely to develop tornadoes," said Allan Boyer, a controlled meteorogist at the National Storm Prediction Center in Norman, OK. "Also, when residents leave their washing machines and dryers running it creates the ideal environment for tornadoes, because of the extra spinningness it causes in the air."

The NSPC has developed a scale for ranking the intensity of basement tornadoes with the lowest rating, B0, only causing mild shag-carpet disturbances, and the highest, B5, capable of destroying workbenches and water heaters. According to NSPC statistics, independent-minded siblings between the ages of 16 and 19 who always want to be alone are most likely to be injured by basement tornadoes. The second highest at-risk group is families huddling in basements waiting for normal tornadoes to pass.

The outbreak of basement tornadoes has also raised a slew of privacy issues, pitting so-called storm hunters against homeowners, since those who study the deadly new phenomenon say they must gain access to private residences to be able to set up their equipment and observe the storms. Angry residents have already forced dozens of researchers camping in their cellars out of their homes, and at least two have been shot at or violently assaulted for peering through basement windows.

While the Federal Emergency Management Agency and the NWS have been working together to develop new nationwide evacuation guidelines, officials say it is a complicated process.

"Even if a storm warning goes into effect hours before a tornado strikes, we caution citizens against attempting to flee the area in automobiles, since this will dramatically increase their chances of being severely injured or killed by car tornadoes," FEMA administrator R. David Paulison said. "This could shape up to be the worst development in natural disasters since the 2003 spate of earthquakes that struck directly under doorways and large desks."