Chicagoans woke up Sunday morning to snow blanketing their cars and homes and a winter weather advisory in place. As flakes fell on one of the busiest travel days of the year, some might have assumed it’s a harbinger of tough winter conditions to come.
But scientists say that’s probably not the case.
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Measurements indicate that a “strong” El Nino is brewing through the Northern Hemisphere, with a greater than 55% chance of it continuing from January to March, according to the National Weather Service’s Climate Prediction Center. For Chicago, this means a milder and drier winter.
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“Even though El Nino does tend to bring fewer extreme cold events in Chicago during the wintertime, it doesn’t mean that we won’t have any nor does it tell us anything about individual weather events,” said Trent Ford, the Illinois State Climatologist. “You can still have a very early start to winter, a very late spring freeze even in an overall milder wintertime.”
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What is an El Nino? El Nino is a natural climate phenomenon where sea surface temperatures are warmer than average in the central and eastern Pacific Ocean near the equator, according to the weather service. Occurring on average every two to seven years, El Ninos usually begin in the summer and peak in intensity during the winter.
Scientists at the Climate Prediction Center announced that this El Nino first emerged in June. They noted at the time that not only were El Nino conditions present, but they expected them to “gradually strengthen” as winter approached.
An El Nino can have a significant effect on weather, even as far away as the Midwest. Unusually warm waters can shift jet stream patterns, Ford said, leading to drier and warmer conditions in the northern U.S. and Canada. Conditions from southern California to along the Gulf Coast are typically wetter with a heightened chance of flooding, and drier than average in the Pacific Northwest and Ohio Valley.
La Ninas, on the other hand, have opposite effects, causing cooler winter temperatures in the North and a more severe hurricane season.
A driver clears snow off a car on Nov. 26, 2023, in Logan Square. (Brian Cassella/Chicago Tribune)
“Depending on its strength, El Nino can cause a range of impacts, such as increasing the risk of heavy rainfall and droughts in certain locations around the world,” said Michelle L’Heureux, climate scientist at the Climate Prediction Center.
“Climate change can exacerbate or mitigate certain impacts related to El Nino,” L’Heureux added. “For example, El Nino could lead to new records for temperatures, particularly in areas that already experience above-average temperatures during El Nino.”
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Potential for ‘historically strong’ El Nino Earlier this month, the Climate Prediction Center said above-average sea surface temperatures were indicative of a strong El Nino, noting that there’s a 35% chance of it becoming “historically strong” for the November to January season. The last strong El Nino was the 2015-16 winter, Ford said.
Stronger El Ni？os are more likely to affect global temperature and precipitation patterns. For Chicago, Ford said it means that the average winter temperature will likely exceed 28.2 degrees and average total precipitation will probably be less than 6 inches from December to February.
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However, Ford said seasonal averages don’t tell the full story about the impacts of winter weather. For example, even though last year’s winter was relatively mild, he said extremely cold temperatures in December caused the state to lose a “significant” percent of its peach crop, raising prices.
“We could have a relatively mild winter with a two-week period of time that is just extremely cold. If it averages out to be pretty close, if not slightly warmer than normal, then the forecast verifies,” Ford said. “But it doesn’t really give us the idea of the impact that extreme cold had.”
Effects on plants, animals A milder winter can have some effects on plants, Ford said. Warmer temperatures may reduce exposure to “sensitive” perennial crops and plants that can’t handle the cold well, such as peaches, apples, blackberries, cherries and strawberries. But Ford said a warmer winter could increase the risk of spring freeze damage.
Warmer temperatures also decrease exposure risk for housing-insecure people, Ford said, including the thousands of migrants sleeping at police stations.
But as the state grapples with the lingering effects of drought, Ford said drier conditions aren’t ideal for boosting soil moisture before the spring. Even if crops in Illinois aren’t affected, people still might see prices climb at the grocery store if, for example, the pistachio crop is damaged in California due to warmer temperatures, said Rae Wynn-Grant, a wildlife ecologist who hosts a podcast on PBS.
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Wynn-Grant said El Ni？os can change the migration patterns of birds. She said in the winter, migratory bird species are typically in Central or South America for warmer temperatures, so Americans likely won’t see the impact until spring when they make their way back to the Northern Hemisphere.
“It might be that there are fewer birds because El Nino impacted their overwintering habitat. It might be that there are birds showing up in urban landscapes because for whatever reason, the resources are greater,” she said. “It is also very likely that we see absolutely no change.”
Overall, Wynn-Grant said many of the impacts of El Ni？os are difficult to predict.
“Many scientists like me remain interested in what is going to happen,” she said. “What is this going to look like, and how are we all going to feel it and experience it?”
rjohnson@chicagotribune.com