The early heat wave gripping the Northwest is rare — and worrying

Temperatures in Seattle are forecast to climb close to 90 degrees on Sunday. Historic data shows there have only been six years since 1948 when May temperatures reached this level, and they have never occurred this early in the month. In Portland, readings are expected to top 90 degrees on several days. Vancouver, B.C., could see its earliest 80-degree day on record.

Meteorologists and heat experts are warning that “subtle” early-season heat waves can cause large numbers of hospital admissions as they catch people when their bodies are least used to heat.

“Typically, unseasonably warm weather early in the ‘heat season’ can be particularly challenging to human health because the population is less acclimatized versus later in the summer,” said Zac Schlader, an associate professor at Indiana University-Bloomington who researches the impact of heat stress on the human body. “If these same conditions were experienced later in the year, they are unlikely to have as large of an effect on human health,” he added.

Acclimatization to increased temperatures requires at least two hours of exposure to heat for a period of four to 14 days. Schlader said acclimatization has two components: the body becomes better at responding to heat, and people adapt their behavior, for example by drinking more water.

Outdoor workers are especially at risk during an early heat wave when temperatures first begin to increase in the spring or early summer. The Labor Department’s Occupational Safety and Health Administration (OSHA) recommends shorter shifts and more frequent breaks. This allows workers to gradually gain tolerance to the increased heat.

Early heat events “are where we tend to see a lot of either emergency department visits or hospitalizations due to heat illness,” according to Kimberly McMahon, public weather program manager at the National Weather Service.

“Being early in the warm season and relatively mild, these events may fly under the radar of the public,” said Kevin Lanza, an assistant professor at UTHealth Houston School of Public Health. He warned that areas characterized by “urban heat islands” — neighborhoods with few trees and an abundance of heat-trapping pavement — would see higher temperatures.

McMahon said the population groups most vulnerable to extreme heat include the elderly, small children, outdoor workers and pregnant women. People who don’t have air conditioning in their homes or can’t afford to run it are also at risk.

As of 2021, only 53 percent of homes in Seattle were air-conditioned — the second-lowest share, after San Francisco, among the 25 largest U.S. metro areas. In southern cities like Atlanta, Houston and New Orleans, more than 99 percent of homes have some form of air conditioning installed.

According to the nonprofit research group Climate Central, the expected heat wave in the Pacific Northwest was made two to five times more likely by global warming. Director Andrew Pershing said the group is working to detect the “fingerprint” of climate change in an area while or even before a heat wave is unfolding, “because that’s when people will be able to make that connection that this event is made worse because of carbon dioxide in the atmosphere. These are the events that we have to anticipate and prepare for.”