Brace yourselves.

Environment Canada has issued a heat warning for Tuesday until Friday — a period which might wind up as “the longest and most significant heat wave of the summer so far.”

Tuesday afternoon will see highs in the mid-30 C range, which is not unusual for this summer. Overnight, however, a humid air mass moving north from the U.S. is expected to crank up the ‘mugginess’ dial to 11.

Starting Wednesday, Toronto could see some truly intense heat and humidity—which, for those without A/C, will feel somewhere between the high 30 C and low 40 C mark.

The coldest Toronto will feel over the next few days is somewhere in the low 20s, and perhaps only briefly during the morning chill.

According to Environment Canada’s current forecast, a weak cold front is expected to move through Toronto on Friday night, bringing with it a chance of rain and storms, along with some relief.

Environment Canada and the city both list the ill, the young, those on certain medications and those who are marginally housed as being the most at risk during heat waves.

Toronto doesn’t have a heat alert in effect at the moment. The city tends to make those announcements in the morning, so check back for more details.