Much of the country is used to occasional 100-degree days. Seattle, which has had just three in the past 123 years, is not.

So unaccustomed is Seattle to scorching heat that, in 2015, only one-third of the housing units in its metropolitan area had air-conditioning.

That’s going to make this week dangerous.

The National Weather Service is predicting “widespread record highs” as a heat wave engulfs the Pacific Northwest. An excessive heat warning is in effect from 2 p.m. on Tuesday through 9 p.m. on Friday. Seattleites can expect temperatures in the mid-80s to lower 90s on Tuesday. Wednesday will be in the 90s. And that three-digit barrier: Thursday may break it, with highs potentially “near 104.”

In Portland, Ore., the second-largest city in the Pacific Northwest, highs of 104 to 107 are expected on Wednesday and Thursday, threatening the record of 107 degrees, which was set in 1965 and tied in 1981. Friday, too, is expected to reach 100, which would make this week only the seventh time since 1940 that Portland has had three consecutive days of triple-digit heat. And with a forecast of 99 degrees on Tuesday, the city is flirting with a four-day streak, something that has happened only twice since 1940.