A record-breaking heat wave with a combination of humidity and temperatures well into the 90s was descending on cities in the Midwest and heading to the Northeast this week in what is expected to be a dangerous weather system days before summer officially begins, forecasters said.

“The duration of this heat wave is notable and potentially the longest experienced in decades for some locations,” the National Weather Service said on social media on Sunday.

A high-pressure weather system could bring record-breaking temperatures to central and eastern parts of the United States this week, National Weather Service forecasters said on Monday, threatening a large swath of the country with multi-day excessive heat.

Some 80 million people are currently under a heat advisory or warning as heat is expected to surpass 90 degrees Fahrenheit (32.2 degrees Celsius) and flirt with 100 degrees in some areas, which is 20 degrees above average this time of year, meteorologist Andrew Orrison of the National Weather Service said.

Tens of millions of people from the Ohio Valley to the Northeast are about to experience the first significant heat wave of the summer, and it has the potential to be not only record-breaking but also life-threatening.

The sweltering temperatures are the result of a massive ridge of high pressure that’s establishing itself over the eastern U.S., and temperatures are set to skyrocket into the 90s and approach the 100-degree mark in many cities. The humidity will make it feel even hotter, with feels-like temperatures soaring above 100 degrees.