When will Phoenix temperatures finally drop below 100 degrees?

As summer technically comes to a close in Phoenix, high temperatures continue to sit at or above 100 degrees. But, as temperatures continue to decline and the edge finally comes off the heat, Phoenix’s last triple-degree day of the year is in sight.

If you want to circle a day on your calendar when things will start to cool off, your best bet is Oct. 5. According to the National Weather Service, that’s the average point each year over the past three decades when Phoenix experienced its last 100-degree day.

“We’ll be starting to see high temperatures in the 90s by the end of this month and into October,” NWS Phoenix lead meteorologist Matt Salerno said. But just because “we’re seeing a cooling trend,” Salerno added, “doesn’t necessarily mean not heating back up heading into the end of the month and into October.”

Still, unlike last year, the last 100-degree day could be earlier. The first day that Phoenix’s average high temperature historically goes below 100 degrees is Sept. 20.

The last 100-degree day is highly variable. The earliest Phoenix has welcomed its last 100-degree day is Sept. 19, which was the case in 2019, 2017 and 1998. The latest that Phoenix's last 100-degree day in the past 20 years was in 2016. That year, residents had to wait until Oct. 27 for the triple-digit days of summer to finally end.


Last year’s heat nearly beat that record, with the final 100-degree day occurring on Oct. 26, which was “kind of unheard of,” Salerno said. That’s partly because of the angle at which Arizona is hit by the sun throughout October. As the days move away from summer, the sun angle gets lower in the sky, making it “harder to reach those extreme temperatures,” Salerno explained. “The further you get past Oct. 5, the harder it is to reach 100 degrees.”

On average, the daily high temperatures in Phoenix in late September going into October hover around 90 degrees, before dropping down to the 80s the closer it gets to Halloween. According to the NWS's Climate Prediction Center, temperatures throughout September and into October are expected to be higher than average.

This summer was the fourth-hottest in Phoenix’s history. Temperatures reached 100 degrees 107 times, with 37 of those days being above 110 degrees. However, it’s unlikely that we’ll see 100-degree temperatures late into October like we did last year, which featured the hottest summer in Phoenix’s history. Last year saw 143 days where temperatures were above 100 degrees and 70 days of 110-plus-degree days.

Residents can also expect cooler nights and mornings soon. With the year’s monsoon season coming to an end at the end of the month, the city’s moisture content is dropping significantly, leading to the atmosphere drying out, resulting in lower temperatures.