In winter through early summer of 2014–2015, temperatures in the western U.S. were substantially enhanced due to a persistent high-pressure ridge and warm water in the northeastern Pacific Ocean. Concurrently, monthly averaged maximum daily 8 h average (MDA8) O3 in June 2015 was enhanced by 3–13 ppb across a large portion of the western U.S. At the Mount Bachelor Observatory (2.8 km above sea level) in central Oregon, O3 in June 2015 was enhanced by 11 ppbv compared to the long-term mean. Some urban areas had many days in June 2015 with MDA8 values above the current air quality threshold of 70 ppbv. We show that the high O3 was associated with enhanced temperatures, reduced cloud fraction, increased stagnation, and increased biogenic emissions. The data in June 2015 show enhanced ΔO3/Δ temperature slopes at several sites, compared to previous June data, due to these multiple factors.