HOUSTON (AP) — Sweltering heat more commonly seen in the throes of summer than in the spring was making an unwelcome visit this week to a large portion of the U.S. – from the Dakotas to Texas and other parts of the South – and putting millions of Americans on alert for potentially dangerous temperatures.

In Austin, forecasters warned that the early heat wave could break a century-old record for May of 104 degrees Fahrenheit.

“Definitely more like August this week than May,” said Cameron Self, a meteorologist with the National Weather Service’s office in the Houston and Galveston area.

It is not unusual to have temperatures above 90 degrees Fahrenheit in May in Houston and other parts of Texas.

“But getting long stretches of temperatures well in the 90s that usually holds off till June,” Self said.

This week’s extreme heat was predicted to have some Texas cities experiencing the longest string of triple-digit days they have ever had before in June, said John Nielsen-Gammon, the Texas state climatologist.

Several cities around North Dakota set record high temperatures in recent days, with some shattering highs that stood for well over 100 years.

While cooler air is expected to provide some relief for the northern part of the country, Texas and states in the southeastern part of the U.S. were likely to suffer through this heat for at least the next week or so.

What’s causing the unseasonably high temperatures?

A very strong ridge of high pressure over the south-central United States that is centered over the Gulf of Mexico is responsible for the extreme heat. For the next six to 10 days, much of south central and the southeastern United States will be warmer than normal, with the highest temperatures occurring over parts of Texas and Florida, Self said.

The same weather system heating up Texas and other southern states was also the same one that helped spike temperatures in the Dakotas, Minnesota and parts of the Midwest, Nielsen-Gammon said.

Heat in the Dakotas, Minnesota breaks records, boosts wildfire conditions

In the last few days, the Dakotas, Minnesota and southern parts of Canada have seen unusually warm temperatures for this time of year, as much as 30 degrees above normal, said Mindy Beerends, meteorologist-in-charge for the National Weather Service in Grand Forks, North Dakota.

“This is very early for this type of heat,” she said.

Fargo on Sunday hit 95 degrees Fahrenheit, surpassing the previous high of 93 degrees Fahrenheit set in 1887. Bismarck on Monday reached 97 degrees Fahrenheit, breaking the record of 92 degrees Fahrenheit set in 1880.

The unseasonably high temperatures, along with very low humidity and spreading drought conditions have created unusually dangerous fire conditions in northern Minnesota, Gov. Tim Walz and other state officials said at a briefing Tuesday.

Weather conditions were expected to improve Thursday when rain and cooler temperatures should give firefighters a break, the officials said.