A perfect storm is brewing at U.S. airports ahead of the busy summer travel season: Longer lines to get through security screening.

Airlines and airports are warning fliers to get to the airport two hours early for domestic flights and three hours for international trips. Traffic is up, TSA staffing is down and the security workers who remain are charged with carrying out more stringent measures in the wake of recent terror attacks.

Passengers on American Airlines in Philadelphia, and four other cities, got a taste of the longer waits over spring break, March 14 to March 20, when lines through screening checkpoints were an hour or longer.

A total of 6,800 passengers missed their flights in Philadelphia, Los Angeles, Miami, Atlanta, and Dallas Fort Worth, said American spokesman Ross Feinstein. He declined to specify the numbers for each city.

"For that one week, Philadelphia was in the top five for American with regard to passengers delayed at TSA checkpoints who missed flights due to the lines," said Feinstein. "We continue to speak with the TSA and Department of Homeland Security because we are trying to find common sense solutions to fix this issue. It's unacceptable for passengers to be waiting in TSA lines for an hour."

The reasons for longer wait times include more people traveling, fewer airport screeners because of Congressional budget cuts, and tightened airport security measures after the Paris terrorist attacks and the crash of a jet flying between Egypt and Russia, believed to have been brought down by a bomb.

TSA adopted more extensive security tactics after an inspector general report last summer found that 67 out of 70 mock weapons and explosives got past TSA agents in regular security tests.

Expedited security lines, known as TSA PreCheck, were thought to be the answer to longer lines when launched in 2012.

The program which gives vetted passengers, who pay an $85 fee, faster screening -- shoes and belts and light jackets stay on, laptops and liquids stay in bags -- has not attracted the once-a-year flier. Only 25 percent to 30 percent of passengers use it on any given day, TSA said.

Even the PreCheck lanes can be long, but passengers move through them twice as fast as in standard lines, the agency said.

TSA screeners nationwide have been cut by Congress from a peak of 47,147 in 2013 to 42,525 this year at nearly 440 airports.

"The main thing to understand, there's been a huge increase in passenger volume across the country," said TSA spokesman Michael McCarthy. The number of passengers going through security checkpoints has risen from 643 million three years ago to 740 million predicted in 2016.

Philadelphia International Airport will see 7.4 percent more passengers this year, and some U.S. airports are looking at double-digit passenger growth, McCarthy said. "Nationwide, we're seeing about 7 percent to 8 percent more passengers on average."

The longer waits have distressed passengers.

Philadelphia airport has seen an increase in tweets about the TSA. "We get tweets about the lines, and we get questions about how to sign up for PreCheck," said airport spokeswoman Mary Flannery. "Overall, both questions and concerns about TSA have increased."

Looking to the summer when more families and vacationers will be traveling, the airport is working with the airlines, and TSA, about their needs and to coordinate staffing "to get passengers to their destinations as quickly as possible," Flannery said.

Seattle-Tacoma International Airport, which saw 12.9 percent passenger growth last year, is hiring 90 private contractors for the summer to speed up the lines.

The workers will take over some TSA duties, including telling passengers to take out their laptop, empty their pockets, throw out water bottles, and remove their shoes. They will shift passengers within the queue to balance the lines, and take bins that have been through x-ray machines to the front of the line, said airport spokesman Perry Cooper.

The TSA said it plans to hire more staff, use more canines, and pay overtime to keep officers on the floor to open additional lanes this summer, McCarthy said.

Philadelphia airport once had nearly 800 TSA officers in the mid-2000s and today has about 650, said Joe Shuker, president of the American Federation of Government Employees (AFGE) Philadelphia local.

The officers staff seven Philadelphia terminals, in two shifts, and other TSA officers screen passengers' checked bags before they go on planes, Shuker said.

The emphasis used to be getting passengers through security quickly, he said. Then last year, the TSA directed officers to perform more thorough screening checks. "Now the emphasis is on security -- don't worry about the lines -- make sure you are mitigating the threat," Shuker said.

"It's almost like you can't have both, unless we get more people," Shuker said. "If we had more staff, we could open up more lines, and we'd be able to bring more people in. But with the staff we have, you can only open so many lanes."

lloyd@phillynews.com

215-854-2831

@LoydLinda

BY THE NUMBERS

740M
Passengers expected to travel through U.S. airports this year.

694M
Fliers passed through airport security checkpoints in 2015.

42,525
TSA officers will staff U.S. airports in 2016.

47,147
TSA agents were working in airports in 2013.

Source: Transportation Security Adminisitration