Welcome to fall, Chicago — meteorologically speaking.
Weather forecasters consider fall to begin Sept. 1 and end Nov. 30. Dividing the four seasons into three-month increments gives the experts consistent dates to track temperature and precipitation.
Here are the biggest weather stories of our summer:
Temperature: It was hot — tied for the eighth-warmest summer on record in Chicago.
The average high temperature from June through August was 84.1 degrees, which is 1.6 degrees above normal, according to the National Weather Service.
“Unsurprisingly to anyone who stepped outside the last few months, summer 2021 in Chicago was warm and humid,” Trent Ford, Illinois state climatologist, wrote in an email to the Tribune. “The average temperature was 75.3 degrees, the eighth highest on record (back to 1873). This compares to 76.7 degrees last summer, which was the warmest summer on record. Four of the top 10 warmest summers on record in Chicago have occurred since 2010 (2010, 2012, 2020, 2021).”
The average low temperature was 66.5, which is 2.4 degrees above normal.
“This summer, Chicago recorded 36 days when the nighttime low temperature did not drop below 70 degrees, the fourth highest on record,” Ford said. “Warm nights can exacerbate heat-health issues that develop during the day because they do not allow folks to recover (i.e., open a window, take a break from AC, step outside, etc.) from the daytime heat.”
Summer was also very humid, Ford noted. “The average summer dew point temperature in Chicago was 65.4 degrees, the fourth highest on record (dew point record goes back to 1947),” he wrote in an email.
Rain: Finally arrived — but more is needed.
A total of 13.19 inches of precipitation was recorded this summer, which is 1.13 inches above normal. That makes summer 2021 the 33rd-wettest Chicago summer, Ford said.
The rain was welcome — especially following the third-driest spring on record in Chicago. Yet, the area continues to experience a precipitation deficit of more than 6 inches below normal for the year so far.
“We’re still dealing with drought conditions along the Illinois and Wisconsin border, worst in Lake County,” wrote Brett Borchardt, meteorologist for the National Weather Service’s Chicago office.
“It is important to note that there are still areas, especially north/west of the city, that are still experiencing significant soil moisture deficits,” Ford said. “We will need a near to wetter-than-normal winter to recharge those soils to avoid coming into next spring with deficits. Abundant and long-lasting snow is one way to recharge soils through the winter and into next spring, so this gives folks one more reason to hope for a big, snowy winter.” (Ford noted he loves winter and snow.)
Severe weather: Lots of it — 17 total days.
Watches and warnings for severe weather were plentiful this summer.
“The Chicago National Weather Service issued 13 tornado warnings in August, the most on record for the office (since 1986),” Ford noted. “So far this year the (National Weather Service) office has issued 36 tornado warnings, the fourth most on record by Sept. 1. What’s interesting though is that all of these 36 warnings were issued in climatological summer. This year was actually only the fifth since 1986 where there was no tornado warnings issued in the Chicago area before June (other years were 1989, 1990, 1992, and 1993).”
Borchardt said the majority of severe weather experienced so far this year happened in June (nine days) and August (eight days) — after no severe weather reports issued at all in May. The severe weather season got off to a slow start, with the first “severe weather day” not occurring until April 29, he said.
Tornadoes so far this year:
June 20: Four tornadoes including the Naperville to Willow Springs EF-3.
June 25-26: Four tornadoes, and the first tornado warning to include downtown Chicago since June 23, 2010.
Chicago has not had any reports of hail larger than 2 inches in diameter yet this year, which is unusual. “We typically average three to four reports every year,” Borchardt said.
“Based on averages from 2001 to 2020, we will have two to three more days of severe weather by the end of the year,” Borchardt said.
What to expect this fall
It could be a beautiful autumn — or it could not.
Ford says the outlook depends on La Niña climate pattern.
“It is looking more and more likely we will reenter La Niña this fall/winter. La Niña falls tend to be a bit drier and warmer across the Midwest, but there is A LOT of variability between La Niña years,” he wrote in an email to the Tribune. “So, the outlooks may lean a bit on La Niña conditions, but I wouldn’t necessarily bank a warm, dry fall at this point. From an agriculture perspective, warm and dry conditions starting in about a week and going through October are very welcome to make for a quicker and less stressful harvest. Ecologically, we could use a near to wetter than normal fall to help recharge soils in the area ahead of winter.”
“The first freeze, which is when the temperature drops at or below 32 degrees, typically occurs between Oct. 11 and Oct. 12 across the Chicago suburbs and Oct. 21 to Oct. 30 in the city and along the lakeshore,” Borchardt said. “However, it has occurred as early as Sept. 22 (1995) and as late as Nov. 24 (1931).”
“The first trace or more of snow typically occurs on Oct. 30, but has occurred as early as Sept. 25 (1942) and as late as Dec. 5 (1999),” Borchardt said. “The first measurable snow (0.1 inches or more) typically occurs around Nov. 17.”
Last year, Chicago received its first measurable snowfall of the season on Nov. 24, 2020.
Sources: National Weather Service, Chicago office; Illinois State Climatologist