After Denver’s hottest September on record, Tuesday will be a cool break from the heat before record-breaking temperatures return, according to the National Weather Service. September had an average temperature — a combination of the daily highs and lows throughout the month — of 70 degrees, according to NWS meteorologists. The previous record, set in 2015, was 69.4 degrees. The top 20 warmest Septembers in Denver are all within 3 degrees of each other, according to NWS data. September typically sees about two days reach temperature highs at or above 90 degrees, according to NWS data.