With 270,000 fans expected to swarm Grant Park this weekend for Lollapalooza, the city and the concert promoter have been working for months on a severe-weather plan that takes into account wind, tornadoes, lightning and hail.

The 2012 Lollapalooza Severe Weather Plan makes emergency decisions a joint process, declaring that "the internal decision to suspend festival activities and move patrons to a safe area rests with the (promoter) C3 Presents partners, in conjunction with City of Chicago Public Safety Officials."

But the written plan is unclear on what would happen if the safety officials and the promoter are in conflict — a situation that can stifle quick decisions. That type of conflict likely contributed to tragedy in the weather-related collapse of a stage structure last year in Indianapolis.

An independent expert who analyzed what happened at the Indiana State Fair, where seven concert-goers were killed, said plans should leave no ambiguity about who makes the final call in an emergency.

"It is not in the best interest of public safety to have ambiguity about who is responsible for the decision about whether the show should be delayed," said Charlie Fisher of Witt Associates, which conducted an eight-month independent assessment of emergency preparedness by the Indiana State Fair.

"It should be very clear: 'This is who is going to make the call,' and 'This is how the call is going to be made.'"

Fisher said he was speaking generally about concert safety, not about what was happening in Chicago, because has not reviewed the plan here.

The Indiana State Fair Commission has updated its emergency management plan to designate the fair's newly hired chief operating officer as the ultimate decision-maker on evacuation, said Fisher, who has reviewed the revised plan in Indiana.

C3's written weather plan for a music festival it runs in Austin, Texas, acknowledges that city officials would have final authority in any emergency, something Lollapalooza's weather plan does not do.

C3 spokeswoman Shelby Meade said that even if it's not in writing, C3 staff know that city public safety officials are the ultimate emergency decision-makers at Lollapalooza.

But the advice of a state police captain to evacuate the grandstand at the Indiana State Fair initially went unheeded — despite the fact that the fairgrounds were under state police jurisdiction — because the State Fair director and the band's tour manager disagreed with the police recommendation. The conflict likely delayed evacuation of the Indiana State Fair grandstand Aug. 13, 2011, as high winds approached and may have contributed to the seven deaths and more than 40 injuries that occurred when the stage structure toppled over into the audience.

As the storm drew close, the Witt report found, authorities on the ground gave conflicting advice. The state police captain made his recommendation to evacuate. The tour manager for the band, Sugarland, wanted the concert to go forward.

Without clear guidance on who should make the decision, Indiana State Fair director Cindy Hoye agreed to go forward with the concert, explaining later to investigators that "nobody is going to tell (the band) what to do." The Witt report found that by the time Hoye decided to defer to the police captain, it was too late. The structure collapsed as the fair director and police captain were on their way up to the stage to order an evacuation.

Bands would not have a say in a weather evacuation of Lollapalooza in Chicago, but the promoter clearly does, according to the official plan. Delores Robinson, spokeswoman for Chicago's Office of Emergency Management and Communications, said stopping or evacuating the concert would be "a joint decision, a collaborated decision between C3 Partners and city of Chicago public safety officials."

Melissa Stratton, director of news affairs for Chicago police, said that "should the need arise, the city will take whatever steps are necessary to ensure the safety of event attendees and residents, while working closely with the event organizers at every step."

Both C3 Presents and Chicago's emergency management office say the kind of conflict that undermined safety at the Indiana State Fair could never happen at Lollapalooza.

"Because of the strong relationship and shared role C3 and the city have taken in developing this festival and ensuring fan safety, there is no conceivable scenario where there would be a conflict about how to proceed," Meade said.

"I don't see there arising a conflict," agreed Robinson, the emergency management office spokeswoman, "because everyone there is operating for the safety of the public."

Meade said "there is no ambiguity" about how emergency decisions are made: "Only the city and C3 are involved in the decision-making process during an emergency and we are in full agreement on a unified approach."