CHICAGO — After years of little snow across the Chicago area, recent record-breaking snowfall and below-freezing temperatures might seem to contradict scientific reports of winters getting warmer. But climate change is still transforming how locals experience the changing seasons, including this fall, one of the top 10 warmest recorded in Illinois.

“People are like, ‘Oh, look at this snow. It’s not climate change,'” said Trent Ford, the Illinois state climatologist.

But trends in recent decades point to an overall warming of average temperatures in winter as well as fall, spring and summer, from human activities such as fossil-fuel burning that release heat-trapping gases into the atmosphere. And that overall warming doesn’t rule out some occasional outliers, including the extreme cold this week.

“Winter is, in fact, starting later as well,” Ford said, even if cold and snow this year might seemingly point to the contrary.

When thinking about climate change’s impact on seasons, he said, “We talk about the winter a lot — winter warming — and certainly we talk about more summer heat, things like that. Fall is sort of forgotten about. What’s really interesting is that all seasons, all months, are warming.”

In Chicago, fall is the second fastest-warming season after winter, according to climate science nonprofit Climate Central. Preliminary reports from weather stations across Illinois show statewide average temperatures this fall were 2.5 to 3 degrees above normal, Ford said.

This might only translate to the difference between buying an ice-cold apple cider — instead of a more seasonally appropriate hot version — on a summer-like fall day at a farmer’s market.

An analysis from Climate Central found that Chicago is experiencing 13 more fall days with higher-than-normal temperatures compared with 1970, while average autumn temperatures have increased by 2.6 degrees in that same time span. It mirrors a 2.8-degree average rise in 237 of 243 U.S. cities analyzed.

The unpredictability of snow

As winters rapidly warm up, it doesn’t necessarily mean less snow — just like more snow than normal does not necessarily mean winters aren’t warming up.

At 12.3 inches, accumulation totals in Chicago this snow season— tracked from July through the following June — are quickly catching up to the 17.6 inches that fell during the entirety of last season. These totals also seem to be following a similar trend from the 1978-1979 season, which ended up being the snowiest recorded in the area with a total of 89.7 inches.

“It was remarkably not snowy for a long time,” Ford said, referring to low accumulation totals in recent years. “So this is definitely a contrast.”

Still, it’s hard to tell how much snow the rest of the winter holds, as “snow does not behave in a simple way.”

Senior NWS Chicago meteorologist Gino Izzi said making snowfall predictions is “very tricky, unfortunately.”

It’s hard to establish a direct correlation between early snow activity yielding more snowfall over the course of winter and toward the end of the season. However, seasonal meteorological outlook reports have indicated a potential for a more active winter.

“The ingredients for snow are cold air — cold enough to make snow instead of rain — and disturbances, so storm systems moving through,” Ford said.

The current pattern of La Niña— a climate phenomenon of colder-than-normal sea surface temperatures in the Pacific Ocean expected to persist through most of the Northern Hemisphere’s winter — tends to plunge northern Illinois into colder-than-normal temperatures and create more storm systems moving through the Ohio Valley that can bring winter weather for much of Illinois, Ford said.

“The range of possibilities is pretty significant,” Izzi said. “Certainly, we’ve had winters with similar patterns that have been relatively dry, but more often than not, a pattern like we’ve got coming up this winter tends to feature a little bit more in the way of precipitation. Now, more precipitation doesn’t necessarily mean more snowfall. It kind of depends on which side of the storm track we end up on.”

Another factor that could yield more snow is a warmer-than-normal Lake Michigan for this time of the year, at least until the lake freezes over.

As cold air moves across the open, warm water of the Great Lakes, it picks up heat and moisture. Now less dense, the air then rises, cools and condenses into clouds, which produce heavy snow in narrow bands downwind.

Lake Michigan’s average surface water temperature in October hovered around 4.5 degrees higher than the 30-year average of just over 57 degrees, according to data from the National Oceanic and Atmospheric Administration.

“So if you get the right pattern in place, that lake (effect) snow could really be something,” Ford said.

The snowpack on the ground could also reinforce a blast of arctic, below-average temperatures that hit Chicago with “brutal” cold over the last few days after a substantial jet stream shift, Izzi said.

“There’s kind of a feedback cycle. When you’ve got snow cover on the ground, it actually keeps temperatures cooler above it,” he said. “The snow is going to act essentially like an ice cube, and it’s going to delay … any kind of warmup.”

Falls that feel like summer

Scientists have found that climate change is making summer heat linger longer into the fall.

“What we consider summer-like temperatures are extending more frequently into September, and even in some cases, into October, more often than they used to,” Ford said.

In September, the Chicago area had 15 days of 80-degree weather, including a five-day stretch at the end of the month. The following month had four days of 80-degree weather — and a day with a high of 89 — beginning Oct. 2.

September had six days, October had seven days and November had three days with temperatures 10 to 20 degrees above normal.

According to NWS data, September this year had an average temperature of 69.3 in Chicago, which is 3 degrees above normal. October followed with an average of 58.1, which is 4.1 degrees above normal and November had an average of 42.4, which is 1.1 degrees above normal. The Chicago area had a fall average temperature of 56.6 degrees from September through November, compared to a normal average of 53.9 degrees.

October statewide tied for the ninth warmest on record since 1985. Fall temperatures statewide have been some 2.8 degrees above normal, according to preliminary data, Ford said.

Fall temperatures in some western Illinois locations, especially by the Quad Cities, were closer to 4 or 5 degrees above normal.

“We think about fall — that’s Sept. 1, all the way till (Nov. 30). That’s a pretty long period of time. And for the entire state, or even just one location, to be a full degree above normal, when you take all of the weather variability … that gets averaged out, anything above a degree above normal for a season is actually pretty significant.”

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Chicago Tribune’s Kori Rumore Finley contributed.

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