Portland’s Impending Heat Wave Includes a Sneaky Threat: Toasty Nights

The longer the heat lasts, the less the city cools down after dark.

In what is becoming an unwelcome summer tradition, Portland is facing down a heat wave early next week that could raise temperatures to triple digits for three consecutive days.

The forecast, for highs nearing 100 degrees Fahrenheit on Tuesday-Thursday, July 27-29, summons memories of last summer’s deadly heat dome, which killed 69 people in Multnomah County amid 116-degree temperatures. But the most recent forecast models used by the National Weather Service are showing Portland likely to dodge such intense heat, with the most probable high temperatures around 98 degrees.

“The good news is that the number of member [forecasts] suggesting 105 F or warmer has dropped considerably relative to 24 hours ago,” the NWS’s Portland office writes today. “For example, the NBM suggests just a 15-20% chance of 105 F or warmer Tuesday, Wednesday or Thursday next week for Portland.”

In other words, the city will most likely experience summer heat more in line with the 99-degree highs last month, rather than the 116-degree highs last year. That’s summer at Disneyland, not living in a pizza oven.

But even as meteorologists anticipate more modest highs, they warn that the heat wave may linger for a longer period than initially forecast. That matters because the longer the heat lasts, the less the city cools down at night.

“Probabilities suggest a 40-50% chance that temperatures of 95 F or warmer will occur Monday through Friday so this heat event may end up still being quite impactful due to it lasting quite a bit longer than most of our heatwaves,” the NWS writes. They project the “low temperatures to steadily climb each day next week.”

How toasty will the nights be? Jon Bonk, an NWS meteorologist, tells WW that lows will remain in the 60s all week, with the worst night on Tuesday, when the temperature won’t drop below 68.

“That in itself is pretty noteworthy,” Bonk says. “It’s certainly grabbing attention.”

Warm nights can be especially dangerous in the “heat islands” at the eastern edges of the city, where swaths of concrete expel the heat they collected during the day.

Multnomah County has not yet announced when and where cooling shelters will operate next week.