US spring storm
A satellite image shows the large storm moving across the north-central United States early Thursday, April 11, 2019.
Pack away the sidewalk tables and flip-flops; break out the boots and shovels.

Nature was showing its fickle side on Wednesday, with blizzard conditions, heavy snow and frigid air pounding parts of the Rockies and the Plains, just a day after the weather was sunny and idyllic. Schools and highways were shut down, hundreds of flights were canceled, and some communities braced for floods.

The storm, caused by a low-pressure system moving east from the Pacific Ocean, dropped temperatures by up to 50 degrees in places like Denver, where it was sunny and in the mid-70s on Tuesday but reached the mid-20s by Wednesday night. The low-pressure system was affecting areas from Colorado to Michigan, with heavy snow and thunderstorms, and even down into Texas, where dry conditions and high winds led to wildfire warnings.

While the whipsawing forecasts drew groans, they did not come as much of a surprise to those familiar with springtime in the Plains and the Rockies.

"In Colorado, that's not uncommon at all," said Natalie Sullivan, a meteorologist with the National Weather Service in Boulder, Colo., noting that April is historically the state's second-snowiest month of the year, behind March. "We have warm conditions, and then a weather system will come through and bring cold air from a different region, and then - boom - you have snow coming down."

Ahead of the storm on Wednesday, Gov. Jared Polis activated the Colorado National Guard, and about 50 soldiers were prepared to respond to stranded drivers. State officials also shut down a highway in the eastern part of the state and part of a highway that cuts through the mountains, citing numerous accidents.

"Reopening is up to Mother Nature," the Colorado State Patrol said on Twitter.

Even before the low-pressure system reached the Rockies, it wreaked havoc on the West Coast, knocking out power in Los Angeles and kicking up dust storms in Nevada, according to AccuWeather.

Officials in states across the Midwest watched anxiously for weather dangers.

Much of the region is still reeling from severe flooding brought on by storms and rising rivers last month. The floods inundated small towns and created a humanitarian crisis on the Pine Ridge Native American reservation, where tribe members found themselves trapped in their homes with little access to food.

Officials in Hamburg, Iowa, worked quickly to add a temporary five-feet-tall levee to the town's current levee system before the Missouri River crests by Sunday or Monday, as they expect. Floodwaters inundated much of the town of 1,100 people last month after heavy rain accumulated on frozen ground in the region.

"We'll build that levee and vigilantly watch throughout the night in case flood does come," said Cathy Crain, the Hamburg mayor.

More pressing than Hamburg's weather, Ms. Crain said, was the snow and rain in cities to the north, such as Yankton, S.D. When precipitation hits those areas hard, it swells the Missouri River and flows down into Hamburg.

"We are so busy we don't have emotions," she said. "We are just moving to recover and to protect our town."

Meteorologists predicted as much as two and a half feet of snow in parts of eastern South Dakota.

Forecasters were hoping that the long-term effects of this storm would not be as severe as those that set off the flooding last month. A heavy snowfall takes time to melt and run off into rivers and streams, reducing the chance of flooding, meteorologists say, while heavy rain brings faster runoff and greater danger. Warmer temperatures in recent days have also thawed the ground, meaning it should better absorb moisture.

Still, Gov. Kristi Noem of South Dakota ordered state offices closed on Wednesday. Schools also closed in Rapid City, S.D., where several inches of snow fell and the streets became icy. State officials closed a roughly 125-mile stretch of Interstate 29 on Wednesday, from Brookings to the North Dakota border.

It was not all that unusual to have a heavy snowfall in Rapid City in April, said Domico Rodriguez, the general manager of Hotel Alex Johnson. But this was different, he said, because the two-degree wind chills sweeping through the city came just three days after he was out playing softball in 70-degree weather.

"It's disheartening," he said, adding that it was difficult to believe the forecast.

In downtown Minneapolis, where snow was falling at a brisk pace, Chameera Ekanayake spent his lunch break huddled beneath a small ledge, trying without much success to keep the flakes out of his hair.

The snow was an unwelcome development in a weather-weary city. After a harsh winter, he said Minnesotans had "been sick of it for a while" and hopeful that the snow was done for the season.

"We had some pretty good days in the last couple weeks and so were like, 'Yeah, this is the end of it,'" said Mr. Ekanayake, 32, who works in local government.

He said he planned to work from home on Thursday, when conditions were expected to worsen.

In Denver, the storm canceled more than 700 flights into and out of the city, postponed baseball games and sent businesses into shelter mode.

The turn in the weather had Mitchell Carroll, the general manager at a Denver restaurant called Illegal Pete's, longing for the sunny skies last week that had helped put the restaurant far ahead of its sales totals from a year ago.

Prospects were not looking good for Illegal Pete's 130-seat patio this week. "You're not really sure because Coloradans aren't really fazed by weather," Mr. Carroll said.

"But if it's a blizzard warning," he added, they might stay in.

Mitch Smith contributed reporting from Minneapolis and Julie Turkewitz from Denver.