TIGNES, France — In an instant, and just as it was becoming even more thrilling, the most exciting Tour de France in decades became truly bizarre, and got a new leader — Egan Bernal of Colombia — who looks all but certain to hold the yellow jersey to Paris on Sunday.

A violent hailstorm threw cycling’s greatest race into chaos on Friday, forcing organizers to cut short a nail-biting stage in the high Alps because riders were speeding, unbeknownst to them, headlong toward a road that had suddenly become covered with ice and giant puddles and cut in half by a rockslide.

Concerned for riders’ safety on mountain roads that can be dangerous at the best of times, race organizers made an on-the-spot and extremely rare decision that the stage couldn’t continue.

The shockwave was immediate and heavy in repercussions. Unable to reach the planned finish at the ski station of Tignes, organizers decided that riders’ placings would instead be based on their time at the top of the highest mountain pass of this Tour — the Iseran, at 2,770 meters (9,090 feet) above sea level — which leading riders, but not all, had just scaled when the race was stopped.

And just like that, Bernal found himself in the yellow jersey.

He flew away from Julian Alaphilippe on the climb and reached the top 2 minutes, 10 seconds ahead of the Frenchman, who had held the race lead for a total of 14 days.

Not only is Bernal the new leader, but he also now looks almost certain to stay in yellow all the way to Paris, because Stage 20 on Saturday will also be shortened, again because of expected storms and landslides. The now truncated route of just 59 kilometers (37 miles), shorn of two of its three planned climbs, is no longer likely to be hard enough for Bernal’s rivals to make him crack.

Still, from the way he stormed up the Iseran, few could argue that Bernal would be an undeserving winner. Having powered up the climb, Bernal was speeding down hairpins on the other side, with Alaphilippe hot on his trail, hoping to save his race lead, when they received the order to stop racing.

“I don’t really know what happened. I was speeding, attacking, and everything was going well and then they told me to stop. I didn’t want to stop,” Bernal said through a translator on French television. “When they told me that I was the race leader and I had the yellow jersey, I couldn’t believe it and I still can’t believe it.”

Organizers scrambled to deal with the disarray and riders clambered off their bikes, not immediately sure what was going on. Exceptionally, there was no winner of Stage 19, because no one had reached the finish.

“This Tour is crazy,” race director Christian Prudhomme said. “We would never have imagined a day like this.”

Having made France dream of having a first Tour winner since 1985, and having contributed more than anyone to make this Tour more memorable than most with his punchy riding, Alaphillipe lost the race lead as the Champs-Elysees in Paris was almost within touching distance.

Prudhomme said the hair-raising speeds of Bernal, Alaphilippe and other riders on the downhill from the Iseran in part prompted the decision to stop the race there and then.

Bernal, who races on the Ineos team, was 1:30 behind Alaphilippe at the start of the stage. Now, the last obstacle for Bernal to negotiate is the long final climb to the Val Thorens ski station on Saturday in the shortened Stage 20, putting the 22-year-old in an ideal position to become the first Colombian to win cycling’s biggest race.

Prudhomme said riders’ timings at the top of the Iseran were taken the old-fashioned way, with a watch. Normally, organizers furnish riders’ placings almost immediately after each stage. On Friday, organizers first provided delayed provisional standings and then tweaked the results in official standings that took about three hours to finalize.

Bernal now leads Alaphilippe by 48 seconds. Defending champion Geraint Thomas is third, 1:16 behind Bernal — not 1:03 back as organizers first announced.

Alaphilippe said he’d been bracing to lose the lead on the tough Alpine stage, but no one had imagined it would happen in such dramatic circumstances.

“I gave it all, I don’t have any regret,” he said. “I’ve been beaten by stronger than me.”

The sudden storm turned summer into almost winter in just minutes, with a dusting of white covering what had been lush summer pastures of green. A snowplow driver tried to clear away the slush, throwing up waves of water, on the road flooded with torrents of water and ice.

It wasn’t the first time that Alpine weather had thrown Tour organizers’ plans into disarray. At the 1996 Tour, what had been planned as a 190-kilometer (118-mile) stage from Val d’Isere to Sestrieres was slashed to just 46 kilometers because of snow, with both the Iseran and Galibier passes not climbed as planned.

Black storm clouds could be seen looming on the horizon as Bernal went over the top of the climb.