Usually by this point in the fall, many Chicagoans have already taken out heavier jackets and gloves to get through the morning commute. But so far this year, those mornings have been comparatively mild. According to the National Weather Service, this September will go down as one of only four on record without a low temperature below 50 degrees.
This September was the fifth warmest since 1871, the weather service said, averaging nearly 4 degrees above normal temperatures. Chicago has also had a dry fall so far, with 2 inches less precipitation than normally recorded in September.
Rain is expected to return over the weekend, though, with scattered showers during the day Saturday, and more widespread rain over the night and into Sunday morning, the weather service said.
Temperatures will remain relatively warm throughout the weekend, with highs in the mid-80s and lows in the high 60s.
The weather service said warmer Septembers were more commonly recorded before 1942 when its observation site was in downtown Chicago.
The weather service said this can be attributed to the proximity to Lake Michigan and the urban heat island effect, wherein cities with large concentrations of pavement and buildings with less green space absorb and retain more heat. Chicago temperatures are now recorded at O’Hare International Airport, where the lake and urban heat island effect have less of an impact.