The descent into winter has begun. From a solar point of view, we have been headed toward winter since late June when the sun peaked in the sky.
However, actual weather tends to linger behind the peak of incoming solar radiation, so average temperatures peak in July and don't really start to drop much until around the first of September.
On balance, the average high and low temperatures for Fargo-Moorhead in September are 71 and 47 degrees. But there is a noticeable drop from early to late September. The averages for Sept. 1 are 77 and 53. The averages of Sept. 30 are 64 and 41.
Because weather is more likely to be either above or below average on any given day, the decent is not always smooth, but it is inevitable. The decrease in solar radiation is something that simply must be accounted for. It will get colder and colder, more or less, until winter arrives.