The 10 US Cities Where Summer Has Warmed Most Since 1990

I pulled June through August daily high temperatures from NOAA's GHCN-D dataset for every major US city with a continuous weather station record from 1990 to 2025. That's 35 summers, roughly 3,200 daily readings per city, drawn from our database of 139 million NOAA records. I compared the 1990–1999 decade average against the 2020–2025 average to calculate the change.

A few things jumped out. First, the warming isn't evenly distributed. The West is cooking. Nevada, Idaho, Utah, and Arizona dominate the top of the list. Second, the “danger day” count — days above a city-specific heat threshold — is increasing faster than raw averages suggest. Phoenix didn't just get 3.6°F hotter on average; it added 20 extra days above 110°F per summer. That's three more weeks of life-threatening heat.

Third, some cities that feel brutally hot — Houston, Miami, New Orleans — haven't warmed as much in raw temperature. But their humidity has spiked, making the heat index far more dangerous even when thermometer readings look stable. The numbers below track dry-bulb temperature only; the lived experience in humid cities is worse than these figures suggest.

A 3.6°F increase in Phoenix's summer average doesn't sound catastrophic. But temperature distributions have fat tails. When you shift the entire curve by 3.6°F, the number of extreme days doesn't increase linearly — it explodes.

Phoenix went from averaging 21 days above 110°F per summer in the 1990s to 41 days in the 2020s. That's not a 3.6°F story. That's a “three extra weeks of unsurvivable outdoor heat” story. In 2023, Maricopa County recorded 645 heat-related deaths — more than any US county in history.

Not every city is a heat horror story. A handful of US cities have seen minimal summer warming since 1990, almost all thanks to one thing: proximity to cold water.

San Francisco's summer average has budged just 0.3°F in 35 years. The Pacific fog machine — cold upwelling water from the California Current meeting warm inland air — acts as natural air conditioning. Mark Twain's supposed quote about the coldest winter being a summer in San Francisco is still holding up.

But even these cities aren't immune to heat extremes. Portland, despite just +0.8°F average warming, hit 116°F during the 2021 Pacific Northwest heat dome — shattering its previous record by 9 degrees. The marine buffer protects average conditions but can't prevent atmospheric blocking patterns.

Climate Central analyzed 243 US cities and found that human-caused climate change accounts for at least 50% of summer warming in 91% of cities. Urban heat island effects layer on top of that baseline, adding 2–8°F depending on city size and design.

Three factors explain why the West dominates this list:

The data points in one direction. 2026 is on track to be the hottest year in US history. January through April was the warmest start to any year ever recorded. March shattered the all-time monthly departure record by 9.35°F. The Southwest hit 115°F before summer officially started.

Now add Super El Niño at 50% probability — potentially the strongest since 1877. NOAA's seasonal outlook shows 36 states with above-normal summer temperature odds. The Pacific Northwest, which already has the fastest summer warming rate of any US region (+3.9°F since 1970), faces a 90%+ chance of above-normal temperatures by August.

For the cities on this list, that means the 2020–2025 averages shown above aren't the ceiling. They're the new baseline.