Heat indexes, which give a sense of how much warmer the temperature will feel because of humidity, will reach 105 to 115 degrees Fahrenheit (41 to 46 degrees Celsius) in some of the hardest-hit areas of the Midwest. (It will feel even hotter in direct sunlight, since the heat index is calculated for shade.)

HeatRisk, an experimental new tool from the NWS that combines temperature, humidity and data on when heat-related hospitalizations tend to increase in a given area, falls into the “extreme” and “high” categories, the two highest, for much of the region.

Taylor and other meteorologists stress that people should be very careful when doing anything outdoors, especially working, playing sports or participating in other strenuous activities. Prolonged exposure to such conditions can lead to heat exhaustion, the symptoms of which include fatigue, dizziness, nausea and cessation of sweating. If a person with this condition does not move to a cooler location or does not receive prompt treatment, heat exhaustion can progress to heat stroke, in which the body loses its ability to cool itself, an extremely dangerous condition.

Taylor says that if people must go outside, they should take frequent breaks in the shade or seek an air-conditioned space when they can. You should also “try to limit your time outside from late morning to early evening” and drink plenty of water, he says.

In areas of the northern Midwest, such as Minnesota, health concerns about heat are particularly high because the current heat wave coincides with the region’s busy state fair and college move-in days, along with other activities “where you have such a large group of people outside,” said Joe Calderone, a meteorologist with the NWS’s Twin Cities office. Those concerns are especially high for at-risk groups such as young children, the elderly, people with multiple health conditions or who take certain medications, people who work outside and the homeless.

“Be sure to check your backseat when you get out of a car if you have small children,” Taylor warns. According to the NWS, 27 children have died this year from being left in hot cars. Taylor also warns of the risk to pets left in cars or outdoors for extended periods of time.

Temperatures will also be higher at night, with lows only dipping to 70 degrees Fahrenheit, further increasing the risk of heat illness. “When there’s little lighting at night, there’s more heat stress,” said Ashton Robinson Cook, a meteorologist and meteorologist with the NWS’s Weather Prediction Center.

Heat waves like this have never been unheard of or even rare. But they are getting hotter and more frequent than in the past because of the extra heat trapped in the atmosphere by greenhouse gases from burning fossil fuels.

This heat wave won’t last long, though. A slow-moving cold front will move into the northernmost parts of the U.S., creeping under the heat wave late Monday night and early Tuesday. And “eventually, the bulk of the heat and the worst of the heat indices will shift eastward,” Cook said. Eastern areas will also cool off a bit later in the week as the front moves further east.