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.

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

Buffalo, NY: 66.5° (old record 65.8°F in 1881; POR 1873-)

Chicago, IL: 69.0°F (old record 67.9°F in 1931; POR 1872-)

Milwaukee, WI: 67.4°F (old record 66.1°F in 1931; POR 1871-)

Minneapolis-St. Paul, MN: 64.9°F (tied with 1897; POR 1872-)

Duluth, MN: 59.2°F (old record 58.8°F in 1920; POR 1874-)

Fargo, ND: 62.1°F (old record 61.8°F in 2015; POR 1881-

Bismarck, ND:  62.1°F (old record 61.8°F in 1938; POR 1874-)

Figure 1. Average temperature for the first half of autumn (September 1 – October 16) in Chicago, Illinois, for each year since official records began in 1872. (Image credit: NOAA Regional Climate Centers)

Several places have also set records for going the deepest into autumn without temperatures dropping below key benchmarks. Boston had its first reading below 50°F on October 18; before this year, the most laggard initial dip below 50°F of any autumn in Boston was 46°F on October 13, 2018.

Bismarck, North Dakota, did not get its first 32°F reading this year until October 15 (record previously October 14, 2018). Even International Falls, Minnesota, the self-proclaimed “Icebox of the Nation,” did not get down to freezing until its low of 30°F on October 17 (record previously October 8, 2016). The city may not dip below 30°F through at least October 24, already now at the latest on record for crossing that threshold.

Unusual warmth in the Great Lakes has both reflected and supported this autumn’s mildness. All five Great Lakes have pushed into record-warm territory for the season over the past few days, based on the 27-year database maintained by NOAA CoastWatch.

If there’s no denying the weirdness afoot, it’s also hard to ignore how pleasant some of these days have been, even for dedicated climate hawks such as independent meteorologist Guy Walton.

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 conditions, which have 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.

The outlook: more weird warmth to come

A couple of fast-moving cool fronts this third full week of October will push temperatures down into the normal range for most of the eastern half of the U.S. However, looking further ahead, the 6-to-10 and 8-to-14-day outlooks issued Sunday by NOAA’s Climate Prediction Center show unusual mildness resurging, with only the Pacific states projected to run below average. If this outlook pans out, the odds will rise for seeing the warmest October on record for the nation as a whole.

As for the winter to come, it’s wise not to expect October’s eerie tranquility to continue. The prototypical La Niña tends to accentuate the usual north-to-south temperature contrasts, with even colder-than-average readings typical toward the northwest and even warmer-than-average readings toward the southeast. Energetic fronts often separate these zones, with intense winter storms often followed by a heightened risk of severe weather by early spring.

As sometimes happens, the La Niña of 2020-21 departed from this script somewhat, most notably with the catastrophic February cold wave in and around Texas.

Tropical trouble is still a possibility

The Atlantic tropics have been as oddly quiet as the Midwest in recent days, but there is still a chance of some legitimately frightening tropical weather before hurricane season ends on November 30, and perhaps even before Halloween.

Figure 2.  The Global Tropical Hazards and Benefits Outlook shows the potential for tropical cyclone formation in the Caribbean and Eastern Pacific during Week 2 (October 20-26). (Image credit: NOAA/CPC)

The latest Global Tropical Hazards and Benefits Outlook, issued by NOAA and partners each Friday, indicates the potential for tropical cyclone formation in the southeast Caribbean during Week 2 (October 20-26). La Niña tends to favor tropical development in the Atlantic, and there is plenty of unusually warm water available in the Caribbean, both at the surface and at depth.

In the northeast Pacific, we will need to watch for development southwest of Mexico in about a week. Extended runs of both the European and GFS models suggest some eventual landfall potential early next week in Mexico.

Jeff Masters contributed to this post.

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Bob Henson

Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...