Cooler temperatures are here to stay, for now. Low temperatures have bottomed out in the low to mid-50s across northern Indiana in recent days, with isolated pockets of temperatures in the 40s. That’s 5-10°F below normal for low temperatures across the northern half of the state. High temperatures have also been below normal, with highs staying in the 70s just in time for Labor Day Weekend, despite normal high temperatures for much of the state usually still in the low 80s.
That’s welcome news for anyone who has struggled with what has been one of Indiana’s most humid summers on record. In Lafayette, the dew point temperatures rose to 75°F or above for 122 hours so far this year, the fifth most of any summer since dew point observations began in 1996. In Terre Haute, dew point temperatures at or above 75°F were observed for 464 hours—the most of any summer since records began there in 1996. The reason 75°F is chosen as a benchmark is that it is when humidity is considered oppressive.
So, after this summer of humidity, it’s worth getting a little excited about fall weather. Some folks may be wondering, though: is that all? What’s the chance of more summer heat? It’s certainly not unheard of. While oppressive dew points become considerably less likely in September and October, the actual air temperature can rise well into the 80s and even 90s in early Autumn. Indianapolis observed 90°F+ temperatures on September 19 and September 21, 2024—just in time for the astronomical Fall. In 2019, temperatures rose to 92°F from September 30 to October 2 in Indianapolis. Even as far north as South Bend, temperatures at or above 90°F have been observed in September in 18 of the past 25 years, and once in October in 2019. It’s essential to note that many of these 90°F observations in September have occurred right around or just after the Autumnal Equinox (usually September 22), which leaves plenty of time this year for such heat to occur.
Even with all of these historical data in mind, just because the temperature doesn’t reach 90ׄ°F doesn’t mean it won’t feel hot heading through September and October. The threshold that constitutes above normal temperatures—aka “heat”—changes heading into the Fall. Since the normal high temperatures in the state fall below 80°F for much of September, it only takes a few days of temperatures a few degrees above 80°F and a dew point temperature around 70°F for it to feel summery. For the folks hoping for a final stretch of summery weather, you may be in luck: the National Weather Service’s Climate Prediction Center (CPC) has a likely chance of above normal temperatures from September 6-19.
Figure 1: The CPC displays a 55-60% chance of above normal temperatures for much of the eastern and southern U.S. heading through mid-September.
