• The National Weather Service has issued a heat advisory for Chicago, St. Louis and much of the South.
• Blame a high pressure ‘dome’ over the eastern part of the U.S. that is trapping the heat.
• Thunderstorms over parts of South Dakota are expected to develop into a derecho forecast.

A swath of the nation will be dealing with dueling weather threats this week as intense heat, powerful storms and a possible derecho sweep across much of the Midwest, East and South.

Almost 200 million Americans, nearly two-thirds of the population, was under an extreme heat warning, watch or advisory July 28.

The National Weather Service has issued a heat advisory for Chicago, St. Louis and much of the region, where heat and humidity will collaborate to push the temperature index near 110 degrees July 28. Blame a high pressure “dome” over the eastern part of the U.S. that is trapping the heat. The heat index measures how hot it actually feels when factoring for humidity.

But the weather service also published a hazardous weather outlook, saying storms could cause flooding and drive wind gusts of up to 60 mph. Thunderstorms over parts of South Dakota are expected to develop into a derecho forecast to move east-southeastward into parts of southern Minnesota and northern Iowa.

“Several wind gusts in excess of 75 mph, a few line-embedded tornadoes and instances of severe hail are also probable, according to the Storm Prediction Center,” weather service meteorologist Mussie Kebede said. “There’s also a heavy rainfall component to the derecho threat today.”

Late July’s oppressive heat forecast isn’t normal

What is a derecho?

A derecho is a widespread, long-lived wind storm associated with a band of rapidly moving showers or thunderstorms, according to the National Weather Service. A derecho can produce destruction similar to tornadoes, and the term “straight-line wind damage” sometimes is used to describe derecho damage.

“By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its length, then the event may be classified as a derecho,” the weather service says.

Wind gusts could surpass 90 mph

AccuWeather also cited a “high risk” for severe thunderstorms from eastern South Dakota into southwestern Minnesota and northwestern Iowa. Widespread damaging wind gusts are possible, with gusts as high as 90 mph − and in isolated areas 115 mph, AccuWeather said. Flash flooding, large hail and isolated tornadoes also are possible. All of which could lead to power outages and damage to buildings and crops across the region.

“As thunderstorms initially develop in Montana and the Dakotas Monday afternoon, it is possible for storms to congeal into a powerful complex of thunderstorms,” said AccuWeather meteorologist Brandon Buckingham.

The severe thunderstorms could continue into July 29, bringing havoc to morning commutes in Chicago, Milwaukee and Indianapolis, AccuWeather said.

Hurricane season warning signs pile up. Brace for a dangerous August.

Heat advisories issued for much of the South

National Weather Service heat advisories also have been issued across much of the South, with heat index values up to 112 expected across portions of Mississippi, Alabama and Florida. A heat advisory for Nashville, Tennessee, posted through July 30, said the heat index could reach 109 “with little or no overnight relief.”

Extreme heat is expected to continue across much of the Southeast and deep into the week, with the most dangerous combination of high temperatures and humidity occurring through July 30, Kebede said.

In the Northeast, AccuWeather said daily records call fall in numerous cities over the next few days, with overnight lows only dipping into the upper 70s.

Late July’s oppressive heat forecast isn’t normal

What is an extreme heat advisory?

The weather service issues an extreme heat warning when “extremely dangerous” heat conditions are expected or occurring. People in those areas are urged to avoid outdoor activities, especially during the heat of the day, and to check on family and neighbors. 

An extreme heat watch is issued when conditions are favorable for an extreme heat event but its occurrence and timing are uncertain. People are warned to suspend all major outdoor activities if a warning is issued.

A heat advisory is issued for dangerous heat conditions that are not expected to reach warning criteria.

Tropical Storm Iona churning in the Pacific

While the Atlantic Ocean is calm, the same cannot be said of the Pacific Ocean, according to the National Hurricane Center. Forecasters are currently keeping tabs on four systems in the Pacific, including Tropical Storm Iona, the hurricane center said in a July 28 advisory.

Iona, located about 915 miles southeast on Honolulu, Hawaii, has maximum sustained winds near 50 mph with higher gusts, and is expected to strengthen some in the next 48 hours, the NHC said late Sunday night.

“Steady strengthening is forecast tonight and Monday, with Iona forecast to become a hurricane by Monday night,” the July 27 advisory said. “Some weakening is expected to begin by late Tuesday or Tuesday night.” Read more here.

− Gabe Hauari

Heat will keep on coming

The millions of Americans in the path of the heat wave should plan to take extra measures to stay cool through at least July 30. Keep things chill by staying in air-conditioned spaces, visit friends with pools, pile up the popsicles and lemonade and pull out the recipe for your favorite cold summer salad.

Extreme levels of heat stress have more than doubled over the past 40 years, according to NASA, and that trend is forecast to continue.

“We’ve always had heat waves,” said Max Holmes, president and CEO of the Woodwell Climate Research Center in Woods Hole, Massachusetts. “But it’s happening a lot more, with greater intensity, greater duration and greater frequency.” Read more here.

− Dinah Voyles Pulver

Hurricane season warning signs pile up. Brace for a dangerous August.

Hurricane season could be making noise soon

The first two months of Atlantic hurricane season that began June 1 hasn’t made a lot of news, and the hurricane map in the Atlantic Ocean may be nearly blank now. But forecasters say the tide may soon turn as several signs point to an uptick in activity across the basin.

WPLG-TV hurricane expert Michael Lowry told USA TODAY that “we’re already seeing longer-range forecast models start to perk up… and July 24’s long-range forecast models jumped from a generally quiet next 15 days to a much busier look for the first part of August.” Read more here.

− Doyle Rice