As Hurricane Earl made landfall in Nova Scotia on September 4th, a large polar vortex dumped an unseasonably cold air mass into the Midwest.
The freezing line at 700 mb reached the Ohio River:
On the night of September 6th, a storm over Minnesota bottomed out at 994 mb:
Frost advisories were issued for the arrowhead of Minnesota and northern Wisconsin on the morning of September 9th, and some stations in this area dropped below freezing.
Another cold air mass on September 14th prompted frost advisories down to lower Michigan:
On September 17th freeze warnings (lighter blue) were issued for most of North Dakota, northern Minnesota, and northern Maine:
It was even cold enough for snow to accumulate in the lower elevations in central Montana.
On September 24th, a 993 mb storm prompted storm warnings (purple) over Lake Superior.
Also on the 24th, a huge Gulf of Alaska storm reached 953 mb:
On September 25th, the North Pacific jet stream had already reached 200 knots:
This morning, September 27, frost advisories went down to the county northwest of Milwaukee:
A much more extensive freeze is possible next weekend. The 850 mb freezing line is forecast by GEFS to reach the Ohio Valley by Saturday morning (12Z Oct 2).