The end of summer stationary boundary seems to have appeared on the weather maps in the last week or so.
In about May, the "rainy season" stationary boundary starts off near the Philippines and moves slowly north, hitting southern Japan about early June, and central Japan about mid June(usually about 4-6 weeks duration). It eventually passes slightly further north, where it then dissipates about in early/mid July. This ended early this year, around late June.
In September, the stationary boundary and rain appears again for about a week, heading south. When it moves through, the cooler air follows. This usually happens in the 2nd or 3rd week in September, but many of these normal processes seem to have been happening earlier in the last year.
Here's the map:
You can also see it on the Korean map as a rain boundary.