Forecasters are warning of scorching heat across a wide stretch of the U.S. South and Midwest, where it will feel as high as 115 degree in some spots.

Parts of 13 states on Monday will be under heat advisories, from Texas, Louisiana and Florida in the South to Missouri and Illinois in the Midwest, the National Weather Service reported.

Some of the most oppressive conditions Monday will be in Louisiana, Arkansas, Mississippi and Oklahoma, forecasters said. Heat exhaustion and heat stroke are among the main threats in those areas.

“You are more likely to develop a heat illness quicker in this type of weather, when it’s really humid and hot,” said Gary Chatelain, a National Weather Service meteorologist based in Shreveport, Louisiana.

Stifling humidity is hanging in the Louisiana air partly because the area has seen such a wet summer, Chatelain said.

More of the same is in store for Tuesday, when heat and humidity will again make for dangerous heat indexes. However, an approaching cool front should help ease the intense heat by Wednesday, Chatelain said.

“If you’re going out in the summer, prepare for the worst,” he said.

That means people spending time outdoors should take breaks in the shade, drink plenty of water, wear hats and light-colored clothing, among other precautions, he said. Anyone who stops sweating in the heat should be aware that it might be a sign of heat illness.