Tornadoes are so unusual in Utah that no one had ever been reported killed by one there until yesterday, when a strange year for twisters took another unexpected turn.

The state ranks 40th in the country for twister occurrence, averaging only about two a year since 1950, when records were first kept. There was no tornado watch or warning in effect yesterday, although a severe thunderstorm warning had been issued seven minutes before the funnel cloud did its damage in downtown Salt Lake City.

The tornado "was a bit of a surprise for us," said Donald Wernly, a warning coordinator with the National Weather Service. He said the last tornado that occurred in Salt Lake County was in 1989.

"Trying to create a tornado in the intermountain west -- in Utah -- is a very difficult proposition," he said. Based on the structure of the upper atmosphere, "the feeling for today was that there was a chance for showers and some thunderstorms. . . . But the wind shear, or turning of the winds in the atmosphere, that you need" for the kind of heavyweight storm that typically generates tornadoes "appeared not to be present."

Based on preliminary damage inspection, officials classified the twister as an F-2 on the Fujita wind damage scale used to rank tornadoes, in which an F-5 is the most devastating. The F-2 classification signifies winds between 113 and 157 mph. Jim Campbell, deputy director of the National Weather Service's western region, headquartered in Salt Lake City, said initial estimates put the winds at about 100 mph.

Meteorologists speculated that there might have been some low-level circulation close to the ground, possibly related to the Great Salt Lake nearby, that could have focused the energy, turning a more benign storm into a tornado-maker, Wernly said at a briefing late yesterday at a National Oceanic and Atmospheric Administration office in Silver Spring.

Though they are one of nature's most lethal outbursts, tornadoes are perhaps the least understood. The difficulties researchers have in chasing them down have been dramatized (and somewhat fictionalized) in Hollywood movies. But they are in fact very difficult to study and almost impossible to predict more than about a half-hour in advance. The Weather Service manages to give at least some warning in about 65 percent of cases, officials said, and hopes to improve warnings to cover perhaps 80 percent by 2005.

The United States is the most prolific producer of tornadoes in the world, followed by Australia. Most of those in the United States are concentrated in the central states of the "tornado belt." Texas leads with about 100 each year. But they can occur in any state. Although they are most common from April to June, and tend to occur late in the day, they can strike in any month.

Tornadoes tend to form at the bottoms of big thunderstorms known as "super cells." Air close to the ground gets sandwiched between layers of wind moving at different velocities, scientists say, and the trapped air mass starts to tumble like clothes in a dryer. Then, as rising hot air pushes on one end of the can, it tips up until it is rotating around a vertical axis. This great spinning mass of rising air -- a "mesocyclone" -- spins up into the base of the thunderhead.

This configuration does not necessarily result in a tornado, researchers have learned, but it can under certain conditions. At ground level, the airflow is violently turbulent as dirt and debris are sucked up into the funnel, where air pressure is 10 percent to 20 percent below normal.

The storm that spawned the lethal twister in Salt Lake City was not a super cell, however, but was a weaker type known as a pulse storm, according to Joseph Schaefer, director of the National Severe Storms Forecast Center in Boulder, Colo. In such a case, the dynamics are even less well understood.

The year has been unusual in the annals of tornado events, Schaefer said. "It started with a vengeance. We had an extremely active January," when tornadoes ravaged the mid-South, causing death and destruction across Tennessee, Arkansas, Mississippi and Missouri. One of Little Rock's oldest neighborhoods was hit.

Then the tornado activity fell below normal until April, picked up in May and June, and dipped again in July and August. Yesterday, Schaefer said, the preliminary and unofficial tally puts the total reported nationwide for the year at 1,087, compared with 1,204 by the end of August last year.

Twisters have seemed to take a bead on cities of late, but as far as scientists know, it's "really random bad luck," Schaefer said. A theory some decades back that the extra heat of metropolitan areas kept tornadoes from forming in those areas has "died a graceful death," he said. Among the population centers struck in recent years, in addition to Salt Lake City and Little Rock, are Nashville and Miami. Going as far back as the 1950s, he said, the list would include St. Louis, Waco, Tex., and Flint, Mich.

"One thing we're finding out," Schaefer said, "is that even weak, ephemeral tornadoes can be dangerous if they happen in the wrong place. . . . The thing that really keeps me awake at night is the thought of a tornado hitting a major sporting event."

Staff writers Ceci Connolly and Manuel Perez-Rivas contributed to this report.

Where the Tornado Struck

The tornado struck about 1 p.m., pushing over trees and vehicles and ripping through tents set up around the Salt Palace Convention Center. The funnel cloud also tore into the roof of the Delta Center and blew out windows in the Wyndham Hotel.

1. Thunderstorms develop in warm, moist air in advance of cold fronts. These thunderstorms often produce large hail, strong winds and tornadoes.

2. Before thunderstorms develop, a change in wind direction and an increase in wind speed create an invisible, horizontal spinning effect in the lower atmosphere.

3. Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.

4. An area of strong rotation, two to six miles wide, extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

The Fujita Scale

F0

40-72 mph

Light damage: Tree branches broken, chimneys damaged.

F1

73-112 mph

Moderate damage: Trees snapped, roofs damaged, windows broken.

F2

113-157 mph

Considerable damage: Trees uprooted, roofs torn off, mobile homes demolished.

F3

158-206 mph

Severe damage: Walls torn off, cars overturned.

F4

207-260 mph

Devastating damage: Houses leveled, cars thrown.

F5

261-318 mph

Incredible damage: Houses lifted off ground, cars thrown hundreds of feet.

SOURCES: National Weather Service, World Book Encyclopedia