Temperatures for today and the weekend will still be quite mild across the middle of the country with lots of 70s and 80s, but a big shot of cold air will enter the weather picture on Monday and that will be the start of the cold weather for really the rest of September.
By Tuesday we are looking at highs in the 50s and 60s in the Midwest even with a lot of sunshine that day, and there will be more days like that later next week. With this type of pattern, it should be just a matter of time before we see at least a part of the Midwest record their first frost and/or freeze of the season. The first morning to watch will be Tuesday, and while frost/freezing conditions that morning will not be widespread in the Midwest, I do believe that we will see a few spots in the far northwest get to 32 degrees or a little colder (with eastern parts of the Northern Plains see a lot of sub-32 degree temps that morning).
Wednesday will be cool further east; can't totally rule out a frost, but more than likely enough westerly and southwesterly winds that day to keep most everyone above 32 degrees. There should be other cold mornings later next week (maybe as early as Friday). The pattern coming up is clearly not wet for the Midwest and Plains. Each new shot of cool air that arrives will have rain with it (first one on Monday) but totals in those areas look light.
I think that most of the country, not just the Plains and Corn Belt, are in below normal rainfall for the 6-10 day period. It is a forecast that will favor rapid harvesting conditions, but at the same time is concerning in the southern Plains winter wheat areas where recent rains really did not even start to make a dent in the drought for that area.
