The official start of summer is still days away, but the Midwest today was already enveloped in extreme levels of heat and humidity that could last throughout much of the week. The heat index — a measure of how the temperature feels — hit 102 degrees in Cincinnati.

Similarly sweltering conditions are expected to push into the Northeast beginning tomorrow and continuing into the weekend. The area sometimes reaches such high temperatures in late July or August, but it will be a drastic change for millions of Americans who have grown accustomed to several months of mostly comfortable temperatures. Here’s what to expect where you live.

“What makes this concerning to forecasters is that it is an early season heat wave,” The Times’s meteorologist, Judson Jones, told me. “Some places are 10-20 degrees above where it should be this time of the year.”

Judson said it was also concerning that the heat was expected in many areas to stick around. “It’s the first one of the season and it’s going to be sweltering for four or five days,” he warned. “The body hasn’t really adapted for summer heat yet.”

Environmental, labor and health care groups filed a petition today to push the federal government to declare extreme heat and wildfire smoke as major disasters, unlocking funding for cooling centers, health screenings, and other preparations.

From tomorrow to Thursday as many as 75 record highs, both daily and for the month, are expected. The June heat wave in the U.S. follows what was the hottest May worldwide on record — and the 12th consecutive month in which the average global temperature was the highest on record for that month.