Utah's top weather forecaster says the Aug. 11 killer tornado struck Salt Lake City so swiftly there was no time to warn of the danger.

"The skies were essentially just a few puffy clouds in the morning," said William J. Alder, meteorologist in charge of the National Weather Service regional office, Thursday. "By 11, 11:30, the clouds started to build, and we started getting concerned that we might have more of a weather day than we had previously anticipated."Alder spoke during a meeting of the joint federal-state Hazard Mitigation Survey Team, held in the State Office Building. Following the session, team members toured now-familiar scenes of destruction. One member compared the damage to that of a relatively mild earthquake of 5 on the Richter scale.

On the morning of the tornado, warm southerly winds were blowing in ahead of a trough of colder air that entered Utah from Nevada. Meanwhile, a breeze was blowing off the Great Salt Lake. The two systems converged in the center of Salt Lake Valley, creating wind shear.

Thunderstorms formed on the Oquirrh Mountains around noon. Hail fell in Herriman.

"The atmosphere destabilized extremely rapidly," Alder said. "Things blew up right over the valley."

Suddenly, the mix of winds and storms combined to generate a strong, rare F2 tornado. Allen A. Crandy, 38, a contractor attending the yearly Outdoor Retailer Summer Market north of the Salt Palace, was killed by debris. Another 80 people were injured. The twister also caused $170 million in structural damage and destroyed 1,000 trees.

In the Midwest, where thunderstorms form and build over long stretches of plains, forecasters often have enough time to issue tornado warnings. But this one did not develop in such a leisurely way. It didn't float in with the weather systems that usually approach from the west.

"This basically did not come from Nevada. It just developed right over the valley here . . . and then it just progressed right up the Avenues," Alder told the Deseret News.

It began as the least damaging type tornado, called an F0, striking around 400 to 500 S. Navajo St. (1340 West) about 12:41 p.m. As it churned for 4.25 miles it gathered force, turning into an F2. The ferocious winds of the F2 storm continued for another 3.75 miles, the funnel extending 100 to 200 yards wide, before it blew from the upper end of the Salt Lake Avenues district at 12:55 p.m.

An F0 tornado has winds of 40 to 72 miles per hour, causing light damage; F1, moderate damage, is from 73 to 112 mph; F2, "considerable damage," has winds of 113 to 157 mph.

Later, analyzing radar data from the time of the tornado, meteorologists could see circulation patterns developing. "But we see that a number of times in the summer with severe weather, like the thunderstorms," Alder said. With thunderstorms, Utah can sometimes expect hail, strong winds and rain.

"You just don't think tornado in Utah. You just don't think that. And possibly if the radar had maybe a lower scan angle we might have been able to detect it sooner, but I really don't think so." If forecasters had glimpsed the formation a bit earlier, they still would not have had enough lead time to get out any significant warnings, Alder believes.

Once the tornado started its rampage up the Salt Lake Avenues, radar images showed circulation patterns. "But by then it was too late, of course, to get anything out. I mean, it had already done the damage."

Asked whether the impact would have been more severe had the twister struck busier parts of downtown Salt Lake City, Alder replied, "You would have had equal amounts of damage no matter where it would have tracked through the metro area.

"Just be thankful it didn't go over a trailer park."

According to Alder, this apparently was not Utah's first killer tornado. In 1884, a seven-year-old girl was killed while camping with her family near the Weber River 23 miles east of Wanship, Summit County. News reports of the time said a tornado downed trees on the family's tent, killing the girl and injuring two other people.

Ninety tornados have been documented in Utah since 1950, he said. The only F3 storm -- one whose winds of 158 to 206 mph are capable of inflicting severe damage -- plowed into the Uinta Mountains near Chapeta Lake. "You had over 1,000 acres of trees just totally uprooted," Alder said.

That one struck on Aug. 11, 1993 -- six years to the day before the Salt Lake tornado.