The unrelenting and unprecedented heat that scorched Phoenix all summer, setting countless records, has carried over into the fall. Now it has set another blistering milestone: the most 100-degree days ever observed in a calendar year.

On Wednesday, the mercury in Phoenix climbed to at least 100 degrees for the 144th time in 2020, surpassing 143 days in 1989 for the most instances on record.

Half of the days (144 out of 288) of the year so far, equivalent to 20.6 weeks, have hit 100 degrees. A few more such days are likely.

Part of a trend

The intensity, frequency and duration of hot weather in Phoenix this year fits into an ongoing and expected trend in a warming world.

In recent decades, Phoenix has averaged about 110 days hitting 100 degrees or more per year. That’s up from about 75 in the mid-1920s.

2020 becomes the eighth year since 2000 that ranks in the top 10 for most 100-degree days since records began in 1896.

The fall has brought little relief from the punishing heat. October averages three 100-degree days, but 2020 has already produced 11.

In recent decades, the average final 100-degree day in Phoenix occurs on Oct. 4, while the latest on record occurred Oct. 27, 2016. When Phoenix hit 100 on Wednesday, it became the 10th-latest such day on record, and more such days are expected this week.

Taking stock of the records

The overall 2020 heat records in Phoenix are too many to list. But among the more notable are new highs for the number of days at or above 110 (53) and 115 (14) degrees. Not to mention, Phoenix never dropped below 90 degrees for a record 28-night stretch during the summer.

“2020 has pretty much broken every other heat record,” wrote Amber Sullins, chief meteorologist at Phoenix’s ABC television affiliate, in an email.

Here are several more significant records of note:

  • Hottest summer: The average temperature of 96.7 degrees topped 2015′s 95.1 degrees.
  • Hottest July: The average temperature of 98.9 degrees surpassed 2009′s 98.3 degrees.
  • Hottest August and calendar month: The average temperature of 99.1 degrees blasted by 1989′s 98.3 degrees and broke the record for hottest month ever recorded, set just the month before.

Depressed monsoon set stage for heat

While Phoenix’s climate has trended hotter over time, the intensity and duration of 2020′s heat is also related to an excessively dry prevailing weather pattern.

Often, in the late summer and early fall, Pacific moisture enters the Desert Southwest, lowering temperatures some. But in 2020, the North American monsoon, which ordinarily draws this moisture inland, was unusually weak. Widespread drought resulted.

Sullins wrote: “100 percent of Arizona is in drought right now.”

According to the federal government’s latest Drought Monitor, the drought is “extreme” in 78 percent of Arizona and growing.

When the land surface is dry, it absorbs more heat and dries the ground out further in a vicious self-reinforcing cycle.

Climate change and urban warming are increasing likelihood of extremely hot years

Phoenix’s temperatures have trended upward for many decades because of urban sprawl and the associated increase in heat-absorbing surfaces such as asphalt and concrete. Simultaneously, atmospheric concentrations of greenhouse gases in the atmosphere have increased, causing temperatures around the world, and in Arizona, to rise. The background warming — which is especially significant in Arizona compared with other states — dramatically increases the odds of heat extremes.

According to an analysis from Climate Central, a climate change research and communications nonprofit, the average low temperature in the city has sprinted upward by 5.5 degrees since 1970. Phoenix ranks fourth on its list of fastest-warming cities, with average temperatures climbing 4.3 degrees between 1970 and 2018, the group found.

The outlook ahead

Heading deeper into October, the odds of 100-degree days normally shrink quickly. But in this torrid year, the decline may be more gradual.

“The outlook over the next couple of weeks is warmer and drier than normal,” Sullins wrote. She noted that this fall’s La Niña event, characterized by cooler than normal ocean waters in the tropical Pacific, tends to favor drier-than-usual conditions into the winter.

Temperatures are forecast to remain near 100 degrees through at least Friday, before the most intense heat shifts toward California this weekend. By the end of this week, the count of 100-degree days could climb to a record-shattering 146.

Long-range forecasts suggest that above-average temperatures are likely for much of the rest of the month, and perhaps beyond. But, mainly due to just decreasing daylight, there is some hope that 100-degree days are soon done for the year.

Jason Samenow and Andrew Freedman contributed to this report.