This fall has been unusually warm in Indiana. Why is that? The midwest experienced what is known as a heat dome from late July to late October. Heat domes are caused by long term and idle high pressure systems over the continental tropical, or cT, air mass.
The cT mass in question is usually situated over the mojave desert. However, due to an unstable jet stream, it was pushed further east over the heartland, which is where the high pressure system was.
High pressure systems are characterized by outward air flow and clear skies, so when a high pressure system is idle over an area for a long period of time, it causes significantly more sun exposure. That in of itself heats things up a bit, but combined with the cT air mass, it was significantly worse than other fall heat waves.
Scenarios like this one often bring more than just high temperatures. This specific heat dome brought a minor drought, which caused air quality issues in the corn belt. Harvest season was a major allergen producer this year.
That same unstable jet stream is currently causing winter-esque weather in the same area that was affected by the heat dome. Several widespread freeze warnings were issued the week of October 22. No snow is in the forecast, but relatively steady sleet and rain should be expected through early November.