Slidell —Willa Cristina of Harahan knows how to celebrate a milestone — she marked her 85th birthday last year by jumping out of a plane, so she was determined to make her granddaughter’s 18th birthday just as special by planning another skydiving excursion for family and friends.

Nicolette Spencer of River Ridge, who turned 18 on Thursday, and her friend Brigette Bruno, who turned 18 last month, followed Cristina’s example by tandem skydiving with instructors from Skydive N’Awlins at Slidell Municipal Airport on Sunday morning.

But a sudden thunderstorm grounded the rest of the party, including the intrepid grandmother who had planned to take the leap with her oldest granddaughter, Kristin Klein, who had traveled from her home in Richmond, Va., to celebrate her 33rd birthday.

“What a bummer,’’ Cristina said as she watched sheets of rain blow across the tarmac. “At least one birthday girl got to go.’’

Indeed, Spencer ended up with as much of an adventure on the ground as in the air. As family scanned the sky and spotted a red, white and blue chute floating down, they were sure it was Spencer, but it turned out to be the second jumper, Bruno. The weather change had forced the Cessna 182 to make a course change, and Spencer and her instructor landed in a pasture northeast of the airport. They got a lift back to the airport from the surprised property owner.

Cristina took the disappointment of her canceled dive in stride. She had her big moment last year, inspired by the example of former President George Herbert Walker Bush who celebrated his 85th birthday by skydiving. Family and many friends turned out to watch her big moment, and her son Paul Spencer said she showed everyone else up by jogging back to the hangar after landing.

The only family member to miss Cristina’s birthday jump last year was her husband, Robert Cristina, who died last month. A veteran of WWII, he had to parachute out of his plane when it was shot down over Italy.

“He said, ‘I’m not going to watch any idiot jump out of a perfectly good airplane,’’’ she said with a laugh.

Her granddaughter had been one of the admiring throng, and that’s when Cristina promised they would do it again when Spencer turned 18, the minimum age set by the company.

“I tell everyone about my grandma,’’ said Spencer, who is senior at Mt. Carmel Academy. “She’s pretty awesome.’’

Nerves were not much in evidence, although Spencer’s mother, Lisa, was there to watch, not jump, and she shook her head over her daughter’s determination to skydive.

“She said ‘It’s on my bucket list,’ and I said, ‘You’re too young to have a bucket list,’ ’’ Lisa Spencer said of her adventurous daughter.

Brenda Grafton, owner of Skydive Nawlins, has heard the bucket list reference many times before — although usually from jumpers who are a bit longer in the tooth than Spencer. Cristina is the oldest woman they’ve taken skydiving, but their oldest customer was a 90-year-old man who had been a pilot in WWII. He took the dive in his original flight suit.

As for Cristina, she’s mulling over how she might celebrate turning 90.

“Maybe I’ll try zip-lining,’’ she said.