An unseasonably cool June delivered a scorching weekend with temperatures in Seattle peaking at 90 degrees Sunday afternoon and forecasters predicting the heat won’t fade until Tuesday.

Before Saturday, Seattle hadn’t experienced an 80-degree day this year — only the eighth time in the last 78 years that temperatures hadn’t hit this level by this point in June, the last being in 2011, according to Matthew Cullen, a meteorologist for the National Weather Service in Seattle.

“This is quite late,” Cullen said. “The first heat event can cause problems for folks because we haven’t had time to get used to it.”

Saturday’s high reached 87 degrees at Seattle-Tacoma International Airport, 14 degrees hotter than the typical high for June, Cullen said, while the low temperature of 67 was well above the normal low of 54 degrees.

“Overnight lows will be the big story of the next night,” he added, as higher nighttime temperatures won’t allow the habitual cooling for sunstruck homes without air conditioning.

The culprit of this heat wave is a less-extreme version of last year’s deadly, triple-digit heat dome. A mass of hot air pressing down above the region, tethered to high atmospheric pressure, is resisting the marine air from the Pacific that typically circulates a cool breeze through Western Washington.

The heat is forecast to blaze into Monday, with the National Weather Service projecting a moderate risk for people who are sensitive to heat. Cullen said that clouds will start to move Monday from the coast further inland, ushering in cooler air by Tuesday.