It hit 20 degrees in Iron Mountain on September 25, 1947??the earliest 20-degree low in the record books at that location. Ironically, this occurred after the warmest August on record for the Upper Peninsula and much of the Upper Midwest (Image 1 above). September cooled back relative to average. The mean anomaly was just under a degree above average. But then October flipped in a big way. The month ended an eye-popping 9.5 degrees above average (Image 2).
When it gets down into the 20-degree range, there is no doubt the growing season is over and winter is right around the corner. It??s interesting to see when some other locations have hit this chilly milestone. It all depends on??to borrow the adage from real estate??location, location, location.
Iron Mountain is land-locked and well away from the influence of the Great Lakes. The Houghton County Airport, which is on a peninsula that sticks out into Lake Superior, receives moderation from the still warm water at this time of the year. Its earliest 20-degree low came on October 22, 1969. Ontonagon, right on the shore of the big lake, did fall to 19 on October 10, 1990, still over two weeks later than Iron Mountain. Munising residents had to wait until October 20, 1978 to feel their first 20-degree morning.
The first 20-degree low occurs even later near the shore of Lake Michigan in the Lower Peninsula. The water off Lake Michigan is even warmer and this moderating influence makes the area adjacent to the lake the largest producer of red-tart cherries in the world. In the U.P., it??s warm enough long enough along the shore of Lake Michigan to grow grapes for wine production.
The growing season has ended for much of the interior Upper Peninsula already. It appears that the coldest night in this stretch will occur tomorrow night as a large area of high pressure settles in with clear skies, dry air and little or no wind. Widespread frost and freezing temperatures are likely, though points along the shores of the Great Lakes will likely still remain above freezing.