On the morning of July 3, the sky was a pure blue and the waters sparkled on Geneva Lake, a summer playground in southeastern Wisconsin.

Just after noon, the sunshine was overtaken by a violent thunderstorm, surprising even seasoned boaters as the winds grew as strong as a hurricane’s and whipped the waters on the lake into a turbulent froth.

The storm’s path was about 26 miles long, and within that path the impact was immense. Meteorologists reported how, over several minutes, it generated three downbursts of winds that dropped to the ground like a waterfall and radiated out, each stronger than the last. It knocked over hundreds of trees that fell onto homes, roads and parked cars around Geneva Lake, Lake Como and Delavan Lake and the surrounding towns and rural areas. Power outages were widespread.

On Geneva Lake, the sudden storm brought tragedy. Waves crashed into a 25-foot-long powerboat, capsizing it and sending all 10 occupants into the water, according to the City of Lake Geneva Police Department. Six adults, ranging in age from 44 to 75, and a 12-year-old boy were rescued. Three children, a 6-year-old boy and two girls, ages 7 and 10, drowned.

The day started out beautiful.

Forecasters had warned of a slight risk for thunderstorms across southern Wisconsin on July 3.

“A few severe storms with gusty winds and hail are possible,” the National Weather Service office in Milwaukee said in its early morning forecast.

That morning, not a storm was in sight on Geneva Lake, where boaters and sunbathers soaked up warm, sunny weather at the start of the long Fourth of July weekend.

Just after 9 a.m., some 200 miles to the west, a handful of severe storms blossomed in eastern Iowa. Through the morning, the storms moved into northwestern Illinois and into southern Wisconsin, merging into a single line that straddled both states as it moved to the east-northeast.

Meteorologists took note. The Weather Service issued a severe thunderstorm watch at 11:52 a.m. for southeastern Wisconsin, including Walworth County, where Geneva Lake is located. A watch is designed to give people notice that the conditions are in place for weather to turn dangerous — which in this case happened almost instantly.

A minute later, the Weather Service issued a more urgent alert, a severe thunderstorm warning, specifically for Walworth County. It warned that a storm capable of causing considerable damage to trees with winds up to 70 miles per hour was possible.

Less than half an hour later, the storm swept across Geneva Lake.

The rain came down sideways.

People on the lake said the storm approached suddenly, with blue sky appearing one minute and sheets of rain falling the next.

“I saw the wall of water coming, and I turned around to talk to the team, and all of a sudden it was on top of us,” said Ken Martzke, an owner of Elmer’s Boat Rentals on the lake. He said he had called his fleet of 20 rental boats to shore and was standing on a pier when the storm hit.

Pat Hodgman, who was on the water, said he and his brother were unable to bring their boat in before the storm arrived. He said they saw the dark clouds in the distance as Geneva Lake Water Safety Patrol boats raced along the shore with flashing lights and flags.

Ted Pankau, the director of the water patrol, also described the storm a surprise. “It was the fastest-moving storm I’ve seen here in watching 35 years of storms going through,” he said.

“It was intense,” Mr. Hodgman said. “Visibility was low, and a combination of large waves coming over the bow and the rain — there was no chance to stay dry. You could feel the wind rapidly intensify, but never imagined it would get to the magnitude it was.”

At about 12:10 p.m., 911 calls began alerting the authorities to the fatal boat accident, Tom Hausner, the Walworth County undersheriff, said at a news conference that day. The circumstances surrounding the accident were still under investigation, the police department said on Monday, noting that the boat was operated by a 47-year-old man with “extensive boating experience.” All the children aboard the boat that capsized were wearing life jackets, the authorities said.

The storm lasted only about 30 minutes and the blue sky returned soon after as people around the lake took in the tragedy. Meteorologists estimated its winds to be as strong as 80 m.p.h. over Geneva Lake and 100 m.p.h. in spots to the north, including over Lake Como.

“This storm was unusual in its strength,” said Steve Vavrus, the state climatologist for Wisconsin. “Fewer than one percent of thunderstorms in Wisconsin generate winds this high.”

Thunderstorms occur at any time of the year in the Midwest, but they are most common in June and July, when the battle between warm, humid air from the Gulf of Mexico and cold, dry air from Canada intensifies in the middle of the country and creates instability in the atmosphere. Forecasting them with precision can be tricky.

These storms come in all shapes and sizes and are considered severe when they drop hail larger than quarters, generate winds of 58 m.p.h. or higher, or spawn a tornado. Southern Wisconsin averages 50 days each year with some risk for severe storms, according to Bill Bunting, the deputy director of the Storm Prediction Center, an arm of the Weather Service focused on forecasting severe thunderstorms.

He said that it’s not unusual for a severe thunderstorm to occur somewhere in Wisconsin, but when you consider a small geographic location such as Geneva Lake, thunderstorms are “exceptionally rare.”