Warmest first half of meteorological fall on record

Our balmy summer-like warmth is setting more records across Minnesota.

Average temperatures for the first half of meteorological fall
Average temperatures for the first half of meteorological fall in the Twin Cities since the 1930s

The seemingly endless summer of ‘21 continues to set records.

The first half of meteorological fall is the warmest on record for Duluth, Minn., and ties the warmest on record for the Twin Cities. Minnesota is just one of the areas setting records for the warmest start to meteorological autumn (September-November) on record.

Bob Henson with Yale Climate Communications elaborates:

Here are a few of the locations that had their highest average temperatures for the first half of meteorological autumn (Sept. 1 – Oct. 16), along with the years in which the period of record (POR) began at each site:

Buffalo, N.Y.: 66.5 degrees (old record 65.8 degrees in 1881; POR 1873-)

Chicago: 69.0 degrees (old record 67.9 degrees in 1931; POR 1872-)

Milwaukee: 67.4 degrees (old record 66.1 degrees in 1931; POR 1871-)

Minneapolis-St. Paul: 64.9 degrees (tied with 1897; POR 1872-)

Duluth, Minn.: 59.2 degrees (old record 58.8 degrees in 1920; POR 1874-)

Fargo, N.D.: 62.1 degrees (old record 61.8 degrees in 2015; POR 1881-

Bismarck, N.D.: 62.1 degrees (old record 61.8 degrees in 1938; POR 1874-)

And on the record early autumn warmth that covers most of the northern United States:

The first half of autumn 2021 came in as the warmest on record for a broad set of towns and cities spanning much of the northern tier of the United States. From Bismarck to Buffalo, millions of people have experienced a September and early October milder than any observed in almost 150 years of recordkeeping.

It’s not only been mild but also strangely calm in many areas, with light winds and dry, mild air prevailing for days on end. A round of severe storms last week was the only major puncutation mark this month.

Foliage has been dragging its feet in many areas, with both temperatures and fall color running weeks behind the norm. When unusually mild weather is sustained well into autumn, especially at night, it can help lead to a dull leaf season.

The first half of October, in particular, has been stunningly warm over most areas east of the Rockies.

Minnesota is running about 10 degrees above average so far this October.

Temperature departure from average this month
Temperature departure from average this month .
Midwest Regional Climate Center

Climate change and La Niña

So why are we feeling record warmth this fall across the Upper Midwest? It’s likely the combination of well-established climate change trend of warmer fall seasons in the Upper Midwest and the La Niña event in the tropical Pacific Ocean.

Henson’s piece details the connections:

When you pump massive amounts of greenhouse gas into the atmosphere — including more than half a trillion tons of carbon dioxide in just the last 20 years — you can expect far-reaching, wide-ranging effects. The tendency of human-caused climate change to boost the odds of dangerous extremes, including summer heat waves, torrential downpours and wildfires stoked by drought and heat, is all too well documented. It’s no surprise to occasionally see more pleasant weather also getting shifted in a direction consistent with a warming planet — even if it doesn’t come close to compensating for the myriad profound global threats posed by a human-altered climate.

This autumn’s mildness has been a classic mix of an ever-warming climate modulated by the distinct impact of La Niña, which has emerged over the past month and is expected to continue into early 2022. La Niña — a semi-cyclic cooling of the eastern tropical Pacific and the counterpart to the warming of the same region known as El Niño — tends to produce unusually warm temperatures across most of the central and eastern United States during autumn, especially in October. That’s pretty much what has happened.

A cold front brings Minnesota back to reality later this week. But temperatures could push the 70-degree mark again by next Tuesday.