Those killed and left homeless by tornadoes that ravaged northern Illinois Tuesday received little warning from the National Weather Service, whose forecasters in Chicago were frustrated by a twister that did not appear on radar and was not reported by observers until most of the violence ended.

"By the time we issued the warning, two-thirds of the ball game was over," said Paul Dailey, meteorologist in charge of the service's Chicago Forecast Office, which was responsible for tracking the storm.

Forecasters said they first learned of the twisters about 3:45 p.m. CDT and issued a warning six minutes later. According to reports from the scene, the first of several tornadoes touched down at 3:30, and forecasters said those were on the ground for 10 to 15 minutes.

Dailey and his colleagues said they were watching closely radar images of the severe thunderstorm that spawned the tornadoes. But the radar screen never showed telltale signs of a tornado, they said, nor did they hear from the small army of volunteer "weather watchers" who are trained by the service and supply forecasters with crucial storm information.

"We're puzzled," said Richard Koeneman, a meteorologist in the Chicago Forecast Office at O'Hare International Airport. "That tornado was on the ground for 10 or 15 minutes, and no reports came into us. We don't know why."

Unlike hurricanes, which gather energy slowly and are so large that they can be tracked easily by satellite, tornadoes are mercurial creations that form and strike suddenly.

Scientists have learned that tornadoes most often are associated with severe "supercell" thunderstorms. The Chicago office had been tracking such a storm all day Tuesday across Illinois from northwest to southeast. Forecasters reported an unconfirmed sighting at 1:30 p.m. CDT of a tornado associated with the storm in Pecatonica about 80 miles northwest of Joliet.

No warning was issued because the report was unconfirmed, they said.

Another indication of trouble came at 3:20 p.m., when large hail was reported in Aurora. Within minutes, forecasters issued a severe thunderstorm warning for that area but soon heard reports that the brunt of storm was on its southern edge. They issued another warning at 3:37 p.m.

Dailey said yesterday that the tornado probably first touched down before or immediately after the second thunderstorm warning. Yet forecasters then had no idea that they were dealing with anything worse than a severe thunderstorm.

Radar failed to show evidence of a storm structure called a "mesocyclone," a rapidly rotating wall of clouds in the center of the thunderstorm that appears on the radar screen as a hook-shaped echo. The mesocyclone produces the smaller, but far more intense, funnel that marks a tornado.

Difficulty in detecting, let alone predicting tornadoes, results because radar cannot detect something as small as a tornado funnel. Unless ample rain is in the mesocyclone, radio waves issued by the radar have nothing to bounce off.

The forecasters said that, more than ever, they are awaiting the long-delayed deployment of a new Doppler radar system called NEXRAD. Such a system could have helped them to detect the signature of a tornado in the works, perhaps giving as much as 30 minutes' warning.

"We would certainly like to have had Doppler yesterday in this office," Koeneman said. "We would have liked to have the state of the art. But we didn't."

Doppler weather radar can detect not only the presence of storms but also provide a profile of wind movement within a storm. Information about rapidly rotating winds would be invaluable for predicting and detecting tornadoes. The Doppler NEXRAD system is scheduled for installation in Illinois next year, but technological problems and funding shortfalls have delayed the program repeatedly.

Forecasters said yesterday that they would like to have issued earlier warnings but had no reason to do so until the first report of a funnel cloud came from state police at 3:45 p.m. A few minutes' warning, however, might not have helped the victims, they said.

"Powerful tornadoes are often not survivable," Koeneman said. "If you hear the warning and get into the closet and the house is blown away and all that's left is the foundation, you're probably not going to survive."