Historically, the last 100 °F day in Phoenix occurs, on average, on September 28th and in Yuma on October 4th. In Phoenix, the earliest was September 2, 1904 while the latest was October 23, 2003; for Yuma the earliest was September 5, 1976 and the latest October 25, 1959. While there has been a slight shift to a later date in Phoenix over the last 100+ years, the trend is not statistically significant. No trend exists in Yuma. Looking at the historical data for Phoenix (1896-2008), we can plot the probability the last 100 °F day will occur by calendar day (figure below). Half of the time the last 100 °F day has occurred between September 21st and October 6th. Ten percent of years it has occurred on October 11th or later.

After several days of mild temperatures, high pressure is expected to build over the southwest states later this week. This will bring temperatures well into the upper 90s and lower 100s for the lower deserts, about 10 degrees above typical temperatures for this time of year. If Phoenix reaches 100 °F it will be unusual but not unheard of as warmth this late in October has a three percent chance of happening each year (or a return period of roughly 33 years). Record high temperatures for mid-October are in the low 100s.

Forecast High Temperatures (Mouse Over)
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