The National Weather Service in Riverton has put together some data around Wyoming as to when the first freeze of the season typically is.
In Riverton and Lander, the first freeze is typically mid-September.
Riverton’s earliest, ever recorded freeze was August 7th in 1912. Lander’s was August 22nd in 1904.
Riverton’s average first freeze is slightly earlier than Lander’s, September 16th compared to September 21st.
Coincidentally, looking at the extended forecast, September 16th may be the first chance at seeing temperatures around the freezing mark in the Riverton/Lander area as the low is projected in the mid-30’s.
Dubois is not listed in the study, but has already seen low temperatures over the weekend in the mid-30’s.
— NWS Riverton (@NWSRiverton) August 29, 2018