What it means
An excessive heat warning is the moment the summer stops being weather and becomes logistics. The National Weather Service’s Phoenix office issues it when the forecast heat is dangerous for most people, not just the frail, and the region reorganizes around it: trail closures at Camelback and Piestewa Peak, cooling centers open at libraries and churches, landscapers starting before dawn, and everyone you know moving their walk to before sunrise.
Since March 4, 2025 the official name has been Extreme Heat Warning. The National Weather Service changed the word because “excessive” sounded like paperwork and “extreme” sounds like what it is. The change has not fully reached conversation, older signs or the habits of people who have lived through thirty Phoenix summers, so you will hear both. They mean the same alert with the same criteria.
You will hear it in June, when the first warning of the year lands on bodies that have not yet adjusted, and again through July and August, often for days at a stretch. The practical rules for hiking under one are in our summer sunrise rules; the resort-season version is in the summer playbook.
Where you’ll meet it
Phone alerts, the evening news, Phoenix’s trail gates, the county’s Heat Relief Network signs, and any local’s explanation of why the 9 a.m. tee time is a bad idea from June to September.
Where the word came from
The phrase is Weather Service vocabulary. For decades “Excessive Heat Watch” and “Excessive Heat Warning” sat at the top of the heat ladder, above the Heat Advisory. In 2025 the agency’s Hazard Simplification effort renamed the top two tiers “Extreme,” on the argument that heat is the country’s most consistent weather killer and deserves a word that registers as dangerous. The advisory stayed as it was.
What makes Phoenix different is how the warning is triggered. Instead of a fixed temperature, NWS Phoenix uses HeatRisk, a five-level scale from green to magenta that compares forecast highs and overnight lows with local climatology and was calibrated against heat mortality data. When Major (red) or Extreme (magenta) risk is forecast, the office issues a watch at about 50 percent confidence and a warning at 80 percent or more. That is why a 110-degree day in late July can pass without a warning while a 108-degree day in early June can get one: the office’s own heat page notes that 105 in May feels hotter to an unacclimated body than 105 in July.
The warning became a civic trigger in 2021. After twelve Phoenix firefighters were sent home with heat illness on June 16 of that year, following three mountain rescues in under 24 hours, their union asked the city to restrict summit trails on 105-degree days. Phoenix instead tied its closures to the warning itself, and every policy built since, from trail closures to cooling-center hours, keys off the same alert.
What it is not
It is not a heat advisory, which targets heat-sensitive people, and it is not a first 100-degree day milestone. Nor is it a promise of a dry heat: monsoon humidity makes some of the worst warning days the muggiest.
The fine print
The stakes are measured in deaths. Maricopa County confirmed 645 heat-associated deaths in 2023, a record, and 608 in 2024. Preliminary 2025 figures released in March 2026 showed 427, the second straight decline, which the county credits to expanded heat relief paid for largely with federal pandemic recovery money starting in summer 2024. Most of those deaths happen far from any trail, many among people without shelter or reliable cooling. The trail rules get the headlines because rescues are visible.
Phoenix’s numbers show how often the alert now governs daily life. In 2024 the city counted 121 days at or above 105 degrees, 69 of them above 110, and 45 days of warning-triggered trail closures between May and mid-October. Its closures now run from 8 a.m. to 5 p.m. at Camelback’s two summit trails, Piestewa Peak’s summit trail and four short routes at South Mountain.
Scottsdale handles the same alert with guidance rather than gates. The McDowell Sonoran Preserve stays open sunrise to sunset; the city asks hikers to turn around at half their water and to leave dogs home above 90 degrees, and it partners with the county’s Heat Relief Network, whose cooling and hydration sites run May 1 through September 30. The Field Kit list covers what to carry. The best local advice fits in one line: on a warning day, the desert belongs to the early.