Support the Timberjay by making a donation.

Serving Northern St. Louis County, Minnesota

September among the fastest-warming months of the year

Marshall Helmberger
Posted 9/28/22

REGIONAL— Septembers aren’t what they used to be in northern Minnesota. Forty years ago, the fall air often started creeping its way into the North Country in late August, when the …

This item is available in full to subscribers.

Please log in to continue

Log in

September among the fastest-warming months of the year

Posted

REGIONAL— Septembers aren’t what they used to be in northern Minnesota. Forty years ago, the fall air often started creeping its way into the North Country in late August, when the traditional cold spots often recorded their first frost. By early-to-mid September, widespread frost was commonplace, and the growing season for sensitive garden plants, like tomatoes and squash, was at an end. Before the end of the month, many stations occasionally recorded a dusting of snow.
Those days appear to be gone.
Now, summer seems to hang on, even in a part of the country where fall once came early. And that’s not just your imagination, according to state climatologists, who note that September, along with June, are the two fastest-warming months of the year in Minnesota.
This year has been no exception. Temperatures this month have been running anywhere from 1.5 to 2.5 degrees above normal across northern St. Louis County, and that’s based on the most recent 30-year average, which factors in much of the warming that’s occurred in recent decades.
Those months are typically the bookends of summer here in northern Minnesota and their steady warming has meant that summer temperatures arrive earlier in the spring and linger on well into what used to be autumn.
The changes in early autumn weather can be seen in the climate data in International Falls, which has the longest period of record at a single location of any place in the region, outside Duluth. Every ten years, the National Oceanic and Atmospheric Administration, or NOAA, updates what are known as monthly climate “normals,” or averages, which are determined by averaging the readings at each weather station for each month. While the variations of day-to-day weather can easily mask the effects of climate change, the trend line of average temperatures provides a far more authoritative data set for climatologists.
Climate normals are based on a 30-year average, which is updated every ten years. That update drops the data from the oldest decade as it adds the data from the most recent ten-year period. If the climate is changing, that should show up as climate normals are adjusted every ten years. To the surprise of very few climatologists, those 30-year normals have been climbing over the past few decades in most places, including International Falls, particularly in months like September.
In the most recent 30-year period, 1991-2020, the average September temperature in the border city was 54.2 degrees F. That’s a full degree warmer than the normal of 53.2 degrees F for the period from 1971-2000. And if you consider the changes seen in just the past 15 years, from 2006-2020, the differences are even more dramatic. September’s average temperature is now 54.7 degrees F, or 1.5 degrees F above the 1971-2000 normal.
The first frost of the season is also coming later in the Falls, and elsewhere around the region.
According to Boulay, the median first frost date for the Falls is Sept. 11. This year, the mercury didn’t reach the 32-degree mark in the Falls until Tuesday morning, Sept. 27, more than two weeks later than usual. And that was early compared to last year, when the border town didn’t reach 32 degrees until Oct. 17, a new record for the latest date ever.
“They don’t make Septembers like they used to,” said Boulay.
The warming in June and September isn’t limited to Minnesota. A recent study published in the journal Geophysical Research Letters, have shown that the summer is longer across almost all of the Northern Hemisphere, having increased by 4.2 days per decade on average, over a 60-year period beginning in 1952.
As might be expected, the same study found that winters have been shortening as well, as have both spring and fall. The study concluded that at the current rate of change, summers in much of the Northern Hemisphere will last up to six months, on average, while winters will last less than two months by the end of the century.
While that might please some Minnesotans, the researchers believe the impacts to agricultures and the ecosystems upon which all life depends, could be profound.