PHILADELPHIA — Philadelphia’s mayor jokes that Pope Francis has personally guaranteed perfect weather for his visit next week, but emergency planners still have to be prepared in case Mother Nature has other ideas.

A sudden thunderstorm moving in on one of Francis’ events on the spacious, tree-lined Benjamin Franklin Parkway could put a projected crowd of a million people or more in the path of potentially deadly lightning and dangerous winds.

It’s a risk that’s inherent to the final leg of Francis’ six-day, three-city U.S. tour. His events in Washington and New York are indoors, aside from occasional rides in the Popemobile, but in Philadelphia his big public events are outside.

Organizers expect about 500,000 people for a festival and concert featuring Francis on Sept. 26 and more than a million people at his Mass on Sept. 27. About 40,000 people are expected for the pope’s Sept. 26 speech on immigration and religious freedom at Independence Mall.

The National Weather Service will provide real-time weather monitoring to the city’s emergency operations center. If they detect trouble, officials will send an alert via text message to people signed up for them and broadcast instructions on the video boards lining the parkway, the city’s emergency management director Samantha Phillips said.

Next comes the challenge of getting people to safety.

Officials will direct people to nearby parking garages because of their ease of access. They also will encourage people to seek cover on their own at hotels, restaurants and other closed spaces.

“The amount of time needed to get that many people to move to a safe place in some sort of orderly fashion means you’d have to get the word out well in advance,” Penn State meteorology professor Jon Nese said. “You have to have a plan in place, but implementing it could be a logistical nightmare.”

Philadelphia’s plan is based in part on Washington, D.C.’s evacuation plan, “Safe Haven,” and the city’s experience in evacuating the Benjamin Franklin Parkway during the Made in America music festival on Labor Day weekend in 2014, when there was torrential rain, thunder and lightning.