Twister

In June 1969, 5-year-old Jo Thornton (Alexa Vega), her parents, and the family dog Toby seek shelter in their family storm cellar as a powerful F5 tornado hits their farm in Oklahoma. This tornado is so strong that the storm cellar door is ripped off, causing Jo's father (Richard Lineback) to be pulled into the storm to his death. Jo, her mother (Rusty Schwimmer), and Toby survive.
27 years later in 1996, 32-year-old Jo (Helen Hunt), now a meteorologist, is reunited with her estranged husband, Bill "The Extreme" Harding (Bill Paxton), a former weather researcher and storm chaser, who has since become a weather reporter. He is planning to marry reproductive therapist Melissa Reeves (Jami Gertz). They need Jo's signature on the divorce papers and have tracked her down during an active bout of stormy weather. Jo has built four identical tornado research devices called DOROTHY, based on Bill's designs. The device is designed to release hundreds of sensors into the center of a tornado to study its structure from the inside, with the purpose of creating a more advanced storm warning system. Bill and Melissa join Jo and her team of storm chasers, and the team encounters Dr. Jonas Miller (Cary Elwes), a smug, corporate-funded meteorologist and storm chaser. When Bill discovers that Jonas has created a device based on DOROTHY, called DOT-3, he vows to help Jo deploy DOROTHY before Miller can claim credit for the idea.
During the first tornado, Jo's truck and DOROTHY I are both destroyed (the truck is picked up and lands upside-down). They continue storm chasing in Bill's truck, with Melissa in the back seat. They find a second tornado, a confirmed F2, and head off on a back road when the twister shifts its track. They soon find themselves driving through heavy rain, and instead collide with a cluster of two violent waterspouts, one of which splits, spinning them around on the highway until the waterspouts dissipate. They're fine, but Melissa becomes hysterical from the ordeal, and Bill has to calm her down.
The team visits Jo's Aunt Meg in Wakita, Oklahoma, for food and rest. While Jo is busy showering, Bill tells Melissa about his relationship with Jo and reflects on the day Jo's father died.
They soon learn that an F3 tornado has formed near some hills, but they have trouble finding it. Jo drives ahead of the team to intercept the oncoming tornado, but a telephone pole falls on the back of Bill's truck and knocks DOROTHY II out onto the road, scattering its mini sensors all over the road. As Jo jumps out to gather the dozens of mini sensors, the tornado lifts and touches down closer. Bill pulls the upset Jo into the truck and moves to safety. The two confront each other over their marriage and Jo's obsession with stopping tornadoes, due to being devastated about her father's death.
The following night, an F4 tornado obliterates a drive-in cinema during the showing of the 1980 horror film The Shining, forcing everyone to take shelter in a pit in a car repair shop warehouse, which is severely damaged by the winds and flying debris hurled at the structure by the tornado, by which the shop very narrowly misses a direct hit. By this time Melissa has been deeply traumatized by the experiences and recognizes the unresolved feelings between Bill and Jo. In desperation for her own safety, she peacefully breaks off the engagement with Bill. The tornado continues on to Wakita, devastating the town and severely injuring Meg while damaging her house to the point of collapse. Bill and Jo arrive in an obliterated Wakita and find Meg in the basement of her perforated house, and although severely injured, she is still alive. The two successfully rescue Meg and her dog and escape from the house merely moments before it collapses. Meg's injuries are not serious, but she is taken to the hospital for treatment and her safety while her dog stays with the group. The team is then horrified to hear that an even stronger storm, an F5, is forming 25 miles south of their position. Inspecting Meg's wind chime sculptures, Jo realizes that the most likely method to successfully deploy DOROTHY's sensors into a tornado would be to add additional body surface to catch the wind.
As they reach the F5, the team adds aluminum from soda cans to work as wind flaps, but the deployment of DOROTHY III is a failure (DOROTHY III is T-boned by a tree branch). Meanwhile, Jonas attempts to deploy DOT-3 by driving ahead of the storm parallel-wise, and in the process ignores Bill's warnings to turn around. Moments later, the tornado shifts course towards Jonas's truck, causing a piece of a nearby cell phone tower to come flying through the windshield and impale his driver Eddie (Zach Grenier). Their truck is then caught by the tornado and it is slammed into the ground, and both men are killed instantly in the ensuing explosion. Jo and Bill set out on their own and are able to deploy the last DOROTHY successfully, using Bill's truck as an anchor. From miles away, the research team immediately sees results on their computers from the sensors. Bill and Jo's celebration is cut short, however, as the tornado shifts course toward them. They take shelter in a pump house where they anchor themselves to the pump plumbing with leather straps. The tornado destroys the pump house and Jo and Bill find themselves in the vortex of the super-massive funnel, securely floating inverted in air.
After the F5 dissipates, Jo and Bill find themselves alone on the floor of the former shed. They decide to run their own lab and rekindle their marriage and the team celebrates their accomplishment.

Question: WHO BREAKS THEIR ENGAGEMENT WITH BILL UPON REALIZING HIS FEELINGS FOR JO?
Answer: MELISSA