Seattle heat waves are getting longer and hotter

Data: EPA ; Note: Includes 50 most populated metros; New York City, Houston, Minneapolis-St. Paul and Denver were excluded for incomplete data in this period; Chart: Axios Visuals

It's not a mirage. Heat waves in Seattle are lasting longer.

Why it matters: Extreme or prolonged heat caused by climbing global temperatures is the top weather-related killer in the U.S.

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Driving the news: The average length of heat waves in Seattle increased 2.1 days between 1961 and 2023, compared to the national average of 1.4 days, according to updated data from the EPA that shows the upward march of heat waves in the U.S.

Threat level: Seattle is now in the middle of its first heat wave of the year, with high temperatures and overnight lows that broke daily records across Western Washington over the weekend and expected highs of 96° today and 86° tomorrow.

• Prolonged, early season heat may pose an elevated health risk to people whose bodies have not yet acclimated to the warmer temperatures.
• It can also exacerbate existing drought conditions and the threat from wildfires.
• A red flag warning has been issued for parts of the Cascades by the National Weather Service due to increased risk of fire.

Context : Seattle logged a record-breaking six-day stretch of consecutive days with highs above 85° in July 2022, National Weather Service Seattle meteorologist Dev McMillian told Axios.

• 17 of the top 20 warmest nights in Seattle on record have occurred in the past 20 years, according to the National Weather Service Seattle .
• So far this month, we have had four consecutive days with highs above 85°, McMillian said, and both Tuesday and Wednesday are expected to be as hot or hotter.

What they did : The EPA tracked events of two or more consecutive days when daily, humidity-adjusted temperatures topped the 85th percentile of historical July-August temperatures between 1981-2020.

What they found: Heat waves are longer, more intense and are occurring more often than they used to in major cities across the United States with an average of six per year over the last two decades compared to two per year in the 1960s, according to the EPA data.

• The average heat wave season across the 50 cities in the EPA indicator is about 46 days longer now than it was in the 1960s.

What's happening: In addition to human-caused climate change , cities lose vegetation and gain pavement, parking lots and concrete, creating urban " heat islands " where temperatures are hotter than the surrounding areas.

What we're watching: Whether cooling coastal breezes will arrive in the Seattle area by Thursday, dropping highs to the low 80° as predicted by NWS Seattle.