The eastern half of the US is facing a significant heatwave, with more than 185 million people under warnings due to intense and widespread heat conditions on Monday.

The south-east is likely to endure the most dangerous temperatures as the extreme heat spread across the region on Monday, spanning from the Carolinas through Florida. In these areas, heat index values (how hot it feels once humidity is accounted for) are forecast to range between 105 and 113F (40.5 to 45C).

Some locations in Mississippi and Louisiana face an even greater threat, with the heat index possibly soaring as high as 120F (49C).

Meanwhile, the midwest isn’t escaping the heat. Conditions there remain hazardous into Monday and Tuesday, after a weekend in which temperatures felt as if they were between 97 and 111F (36 to 44C) in areas from Lincoln, Nebraska, north to Minneapolis.

Cities such as Des Moines, St Louis, Memphis, New Orleans, Jacksonville and Raleigh are under extreme heat warnings. In these locations, temperatures will climb into the mid-90s and low 100s, with heat indices potentially reaching 110 to 115F.

The most dangerous conditions, classified as level 4 out of 4 on the heat risk scale, encompass much of Florida and extend north into Georgia and the Carolinas. A broader level 3 zone stretches from the eastern plains through the midwest and into the mid-Atlantic. This follows a weekend already dominated by extreme temperatures.

Tampa experienced an unprecedented milestone on Sunday when it reached 100F (37.8C). Other cities also broke daily temperature records, and more are expected to follow suit.

The dangerous heat and humidity are expected to persist through midweek, affecting major metropolitan areas including St Louis, Memphis, Charlotte, Savannah, Tampa and Jackson, Mississippi. Actual air temperatures will climb into the upper 90s and low 100s, while heat index readings are expected to remain between 105 and 115F for several days due to high tropical moisture.

Relief will be hard to find, even during the night. Overnight and early morning temperatures are forecast to dip only into the 70s or above, keeping conditions uncomfortable around the clock.

However, a cold front moving in later this week is expected to bring a drop in temperatures across the eastern US, offering a much-needed break from the extreme heat by the weekend.

Elsewhere, triple-digit temperatures will dominate the central US. The combination of soaring heat and dense humidity in the Mississippi River valley and central plains could make conditions especially hazardous, with some areas possibly seeing the heat index reach 120F.

Data suggests that there are more than 1,300 deaths per year in the US due to extreme heat, according to the Environmental Protection Agency. While no one single weather event can be blamed on the global climate crisis, the warming world is experiencing a greater frequency of extreme weather incidents.

According to the National Oceanic and Atmospheric Administration (Noaa), excessive heat is already the leading cause of weather-related deaths in the US, and the problem is only intensifying. For vulnerable populations, such as migrants, prisoners or schoolchildren in under-cooled buildings, the burden of rising temperatures is compounded.

Despite the increasingly crucial need to find solutions for the rising temperatures, many US agencies are currently understaffed due to cuts from the Trump administration and the so-called “department of government efficiency” (Doge).

Federal science agencies such as Noaa are now operating at reduced capacity despite the outsized weather threats. Hundreds of meteorologists have left the National Weather Service in recent months, and several offices, including Houston, have had to scale back the services they provide.