September is a transitional month in Las Vegas, with average highs dropping from 99° on the 1st to 88° on the 30th.
Unfortunately for fans of cooler fall temperatures, this September has been unusually warm.
The mean temperature this month (the average of the daytime high and nighttime low) has been 88.7° through the 24th, which is 4.9° warmer than observed values from 1981-2010. The warmest September (not including this year) happened in 2015 when temperatures hovered 3.5° above-average and reached 87.3°. Last year, however, the average September temperature was 81.9° which is 1.9° cooler than average.
It's interesting to note that the temperature outlook for October, November, and December from the Climate Prediction Center suggest that Las Vegas' stretch of warmer-than-average weather will continue through the end of the year. There's a 50% chance we see above-average temperatures in that three month span, and a 34% chance that we see average temperatures.
There's only a 16% chance that Las Vegas tallies below-average temperatures during that stretch.