From the mid-Atlantic to Maine, across the Great Lakes region, and throughout the West to California, public officials cautioned residents about the dangers of excessive heat and humidity. In Oklahoma, the heat index — what the temperature feels like to the human body — was expected to reach 107 degrees (41 degrees Celsius) on Sunday.

In the Midwest where South Dakota, Iowa and Minnesota meet, floodwaters rose through the weekend. In northwest Iowa, 13 rivers flooded the area, said Eric Tigges of Clay County emergency management. Entire neighborhoods — and at least one entire town — were evacuated, and the town of Spencer imposed a curfew Sunday for the second night in a row after flooding that surpassed the record set in 1953.

“When the flood gauge is underwater, it’s really high,” Tigges said in a news conference organized by Spencer officials.

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Gov. Kim Reynolds declared a disaster for 21 counties in northern Iowa, including Sioux County. In drone video posted by the local sheriff, no streets were visible, just roofs and treetops poking above the water.

Reynolds told reporters Sunday that more than 1,000 displaced residents slept in shelters Saturday night. National Guard troops were helping with water rescues and transporting needed medications lost in flooding.

“Businesses are shuttered. Main streets have been impacted,” Reynolds said. “Hospitals, nursing homes and other care facilities were evacuated. Cities are without power, and some are without drinkable water.”

National Weather Service meteorologist Donna Dubberke said parts of northern Nebraska, southeastern South Dakota, southern Minnesota and northwest Iowa received eight times the typical average rainfall. And more heavy rain was expected this week.

In South Dakota, Gov. Kristi Noem declared an emergency after severe flooding in the southeastern part of the state bordering Nebraska. Several highways were closed. Sioux Falls, the state’s largest city, had more than 7 inches (17.7 centimeters) of rain in three days. At least one person died in the floods, Noem said Sunday, without providing details.

Several rivers, including the Big Sioux, James and Vermillion, were expected to peak sometime Monday through Wednesday night, the governor said at a news conference.

“I want to remind everybody to remember the power of water and the flow of water, and to stay away from flooded areas,” Noem said. She added that “we’ve got a few days in front of us here that’ll be a little rough, but we’ll get through it together and do the very best to protect infrastructure, homes and families as we possibly can.”

Emergency management officials in the small South Dakota community of Dakota Dunes on Sunday issued a voluntary evacuation order for the area’s roughly 4,000 residents. Dakota Dunes is near the Nebraska and Iowa borders and is sandwiched between the Missouri and Big Sioux rivers, both of which are expected to crest in the coming days. Emergency management in Dakota Dunes warned residents that a mandatory evacuation could come quickly if flood barriers are breached.

But elsewhere, the heat was the biggest worry.

“It’s more important for people who are going to be outside to stay hydrated, because heat, humidity and low winds, even if you’re in good shape and not really acclimated to it, it could be a danger, ” said Bruce Thoren, a National Weather Service meteorologist in Oklahoma. “It happens quickly.”

The cities of Washington, D.C., Baltimore and Philadelphia all saw record heat on Saturday, with more high temperatures expected again Sunday.

Lamont Cousins, who owns the Ampersea restaurant on Baltimore’s waterfront, said business had been slow this weekend. The 40 outdoor dining seats at the restaurant, usually packed this time of year, were empty until around dinnertime Saturday.

“I think it’s affected us because people are staying home scared,” he said.

On Saturday, when he went to put umbrellas on the tables, it was already over 90 degrees. But Cousins said he’s not too worried about the lost business – and he expected Sunday would be better.

“Yesterday, it was nobody walking around. It’s hotter today, but there’s a breeze going. Yesterday, it just felt like I was being punished,” Cousins said.

Last year the U.S. experienced the most heat waves since 1936, experts said. An AP analysis of data from the Centers for Disease Control and Prevention found that excessive heat contributed to more than 2,300 deaths, the highest in 45 years of records.

The National Weather Service warned of the potential for rare tornadoes in the Northeast later Sunday. Tornadoes on Saturday struck in Wisconsin, leveling the historic Apple Grove Lutheran Church, founded in 1893 in the town of Argyle.

“The good news is we are all safe,” Dan Bohlman, pastor of Apple Grove Lutheran Church, said on the church website.

Marvin Boyd, meteorologist at the National Weather Service in Burlington, Vermont, said a severe thunderstorm warning was issued for parts of northern New York as a storm with wind gusts exceeding 60 mph (95 kph) and the threat of tornadoes head toward Vermont near Lake Champlain. It is one of several expected to pass through the region Sunday afternoon.

“It’s an unusual alignment of ingredients for Vermont and northern New York to produce a threat of tornadoes,” Boyd said.

___

Associated Press journalists Summer Ballentine in Columbia, Missouri; Leah Willingham in Charleston, West Virginia; Julie Walker in New York; Ron Todt in Philadelphia; and Michael Casey in Boston contributed reporting.

Both mean it’s really hot — and people will hear those terms a lot more as the world heats up. What’s the difference?

Here’s what to know:

What’s a heat dome?

It’s helpful to think of a heat dome as what’s happening in the atmosphere. A heat wave is how that affects people on the ground, said Ken Kunkel, a research professor of atmospheric sciences at North Carolina State University.

When a high-pressure system develops in the upper atmosphere, it causes the air below it to sink and compress. That raises temperatures in the lower atmosphere.

Because hot air expands, it creates a bulging dome.

The boundaries of this week’s heat dome are not well-defined, Kunkel said, but the National Weather Service has said that the most extreme heat is expected in the Ohio Valley and the Northeast.

The eastern heat dome follows an earlier-than-usual one this month in the Southwest. Last year, there were 645 heat-related deaths in Phoenix.

What is a heat wave?

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A heat wave is defined by how intense the heat is, how long it lasts and where it occurs, said Jeff Masters, a meteorologist with Yale Climate Connections.

In general, several days of 90-plus degree temperatures in Texas are “no big deal,” Masters said. Farther north, it is forecasted to be in the mid- to high-90s over the Midwest and Northeast this week, with heat indices of 100 F (38 C) or greater.

“The population’s just not conditioned to that sort of heat,” he said.

The National Weather Service said that some areas likely will reach daily records, with the heat wave lasting all week and into the weekend in some places.

The combination of clear skies and the higher summertime angle of the sun can result in high heat index readings, a measure of temperature combined with humidity. Humidity makes the weather feel hotter because the body cools itself by sweating and has to work harder when the air’s already moist.

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The Detroit area will be in the mid-90s, with a heat index around 100 Fahrenheit (38 C) in some urban areas for the next few days. The normal high temperature for this time of year in Detroit is in the low 80s. Specifically, June 18th’s normal high is 81 F, meteorologist Brian Cromwell said.

Chicago broke a 1957 temperature record on Monday with a high of 97 degrees F (36.1 degrees C). Hot and muggy conditions will continue this week, with peak heat indexes near 100 F (38 C), said the National Weather Service in Chicago.

In Cincinnati, Ohio, Tuesday’s high will be around 96 F (36 C), but will feel like 104 F (40 C), according to the weather service. The high heat will continue through the weekend.

Albany, New York, will see temperatures of 95 F (35 C) or hotter from Tuesday through Thursday, when it will peak at 97 F (36 C), with heat indexes at 100 F (38 C) or more, the weather service forecasted. New York Gov. Kathy Hochul said Tuesday that she has activated the National Guard to assist in any heat emergencies.

The U.S. last year experienced the most heat waves since 1936, experts said. An Associated Press analysis of Centers for Disease Control and Prevention data found that the excessive heat contributed to more than 2,300 U.S. deaths, the highest number in 45 years of records.

Who’s under the heat dome?

The heat dome will affect a broad swath of the eastern half of the country, from roughly the Great Plains states up through Maine.

Some locations could see their hottest temperatures on record for any month, Masters said. A new study found that climate change is making giant heat waves move more slowly and affect more people for a longer time, with higher temperatures over larger areas.

Almost 77 million people in the United States were under extreme heat alerts Tuesday.

Another excessive heat warning, caused by a heat dome, is expected in the Phoenix area on Thursday and Friday, when the highs could reach 114 F (45.5 C) and 116 F (47 C), respectively, said National Weather Service meteorologist Ted Whittock. Tuesday’s forecasted high of 105 F (40.5 C), meanwhile, is normal for this time of year.

He said his office has issued two excessive heat warnings in the past few weeks because of higher-than-normal high pressure that created heat domes.

Dos and don’ts in extreme heat

The U.S. last year saw the most heat waves, consisting of abnormally hot weather lasting more than two days, since 1936. Officials again warned residents to take precautions.

Much of the Midwest and Northeast were under heat warnings or watches.

The heat has been especially dangerous in recent years in Phoenix, where 645 people died from heat-related causes in 2023, which was a record. Temperatures there hit 112 degrees Fahrenheit (44.4 Celsius) on Saturday. Weather service forecasters say the first two weeks of June in Phoenix have been the hottest start to the month on record there.

A meteorologist with the National Weather Service in Phoenix, Ted Whittock, advised reducing time outdoors between 10 a.m. and 6 p.m., staying hydrated and wearing light, looser fitting clothing. More than 100 cooling centers were open in the city and surrounding county, including two new overnight ones.

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In neighboring New Mexico, the high in Roswell was expected to hit 107 F (41.6 C) on Monday, while temperatures in southern Colorado were expected to surpass 100 degrees (37.7 C).

In Southern California, firefighters increased their containment of a large wildfire in mountains north of Los Angeles on Monday after a weekend of explosive, wind-driven growth along Interstate 5.

The warming temperatures come amid growing concern about the effects of extreme heat and wildfire smoke. The nonprofit Center for Biological Diversity on Monday sent a petition to the Federal Emergency Management Agency asking it to recognize extreme heat and wildfire smoke as major disasters.

The agency did not immediately issue a specific response to the petition. A FEMA spokesperson for the western U.S. states said there was nothing that would preclude an emergency declaration for extreme heat, but noted that there would need to be an immediate threat to life and safety that local authorities could not respond to.

While much of the U.S. swelters, late-season snow was forecast for the northern Rockies on Monday and Tuesday. Parts of Montana and north-central Idaho were under a winter storm warning. As much as 20 inches (51 centimeters) was predicted for higher elevations around Glacier National Park.

Meanwhile, a fresh batch of tropical moisture was bringing an increasing threat of heavy rain and flash flooding to the central Gulf Coast.

Hurricane season this year is forecast to be among the most active in recent memory.

The chaotic weather map includes the potential for severe thunderstorms developing in between the hot and cold fronts. Forecasters said the colliding fronts could lead to areas of flash flooding between eastern Nebraska and northern Wisconsin on Saturday night, as well as strong storms across parts of eastern Montana into North and South Dakota.

Meanwhile, a plume of tropical moisture will reach the central Gulf Coast during the next couple days, with heavy rain expected to start Monday morning, according to the National Weather Service.

Forecasters said the threat of heavy rains in Florida continues to dissipate, but some thunderstorms could cause local flooding given the already saturated soil. Some areas between Miami and Fort Lauderdale were left underwater in recent days as persistent storms dumped up to 20 inches (50 centimeters) in southern parts of the state.

The damaging no-name storm system coincided with the early June start of hurricane season, which this year is forecast to be among the most active in recent memory amid concerns that climate change is increasing storm intensity.

With flood waters receding in Florida, temperatures were rising Saturday across much of the southern U.S.

In Atlanta, where temperatures were forecast to near 100 degrees Fahrenheit (38 degrees Celsius) on Saturday and Sunday, city officials opened a cooling center to provide relief from the heat. The city announced that a “Family and Friends Field Day” had been postponed because of the high temperatures forecast.

And in the west Texas city of El Paso, Saturday highs were expected to approach 105 F (40.6 C) and the National Weather Service issued a heat advisory through Monday morning for the region. The city has opened five cooling centers that will operate daily until further notice.

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The National Weather Service said temperatures in Phoenix, where an excessive heat warning was in effect, were forecast to reach 113 F (45 C) on Saturday afternoon. That would be short of the record set for June 15, when in 2021 the high reached 115 F (46 C).

Though Arizona is entering its three-month monsoon season — when a shift in wind patterns pulls moisture in from the tropical coast of Mexico — no rain is forecast for Saturday and most of next week.

“No chances of rain across the state,” said National Weather Service meteorologist Ted Whittock, noting however there’s a 30% chance of showers in southeastern Arizona on Friday, June 21.

Temperatures in the Mid-Atlantic and New England will likely peak in the mid to upper 90s next week, which is “nothing to sneeze at even in the middle of the summer, let alone this early in the summer,” said National Weather Service meteorologist William Churchill.

“That’s what’s particularly remarkable about this,” he said, noting that high humidity will also make it feel even hotter in many places.

Last year, the U.S. had the most heat waves — abnormally hot weather lasting more than two days — since 1936. In the South and Southwest, last year was the worst on record, according to the National Oceanic and Atmospheric Administration.

Next week’s heat wave will ramp up Sunday in the center of the country before spreading eastward, the National Weather Service said, with some areas likely to see extreme heat in reaching daily records. The heat wave could last all week and into the weekend in many places.

While most of the country experiences the season’s first stretch of hot weather, parts of Montana have been placed under winter storm watches with a potential for wet snow falling Monday night.

Churchill said the northwestern cold front is connected to the heat wave because one extreme is often accompanied by the other.