The Forecast for Sunday’s Chicago Marathon Has Turned Cooler

Earlier this week, the Chicago Marathon emailed its runners to say Sunday would be warm and that its Event Alert System would open at yellow, the second of four levels, a warning we reported on Wednesday alongside a forecast low of 62 F and a high near 72 F. By Saturday the outlook for race morning had become a good deal kinder. What is left of Hurricane Isaias, the first hurricane of this year’s Atlantic season, is drawing cloud and cooler air over the city, and the National Weather Service’s hourly forecast updated Saturday afternoon has it at 60 F in Grant Park at 7 a.m., half an hour before the professional race starts.

Saturday itself is still close to summer, with highs forecast between 75 F and 80 F for anyone doing a shakeout run or queuing at the expo. Overnight the temperature slides into the low 60s, and the forecast holds it at 60 F from 6 a.m. until about the start. It climbs only slowly from there, to 64 F at 9 a.m. and 66 F at 10, as the cloud thins out and a breeze of 3 to 5 mph comes in from the southeast. The last wave goes off at 8:35 a.m., so a runner in it who needs four and a half hours would cross the line shortly after 1 p.m., just as the day tops out at about 70 F.

A dew point of 55 F at the start, with relative humidity at 85 percent, takes some of the shine off those numbers, and the dew point creeps up to 58 F by late morning. On the weather service’s own comfort scale, 55 F or lower is “dry and comfortable,” and anything between 55 F and 65 F is on its way to sticky, which is where most of the field will spend the back half of the race. Sweat evaporates more slowly in that kind of air, so a 64 F morning can feel heavier than the thermometer suggests.

NBC 5 meteorologist Kevin Jeanes expects the wind off Lake Michigan to keep the shoreline in the 60s through the afternoon, he said in his race-day outlook for NBC Chicago, while neighborhoods a few miles inland could reach the 70s, and the morning may bring the odd sprinkle. The course spends its middle miles well away from the water, and organizers have found that miles 14 to 19, through the West Loop, Greektown and the Near West Side, can run 3 to 5 degrees warmer than the rest of the route, the Chicago Tribune reported this week. “It’s not the same across all 26.2 miles,” George Chiampas, the race’s chief medical and safety officer, told the paper.

Yellow was set on the warmer forecast, and the race can move the level in either direction before the gun. At yellow, runners are told to slow down and be ready for conditions to get worse, while green, the lowest level, asks them only to enjoy the event and stay alert. Whichever color is in force on Sunday morning will be announced over the public address system and shown on flags and signs at the start, the finish and every aid station.

Even the cooler version is warmer than a typical Chicago race morning, which sits at about 53 F, according to The Weather Channel. When Kelvin Kiptum ran the 2:00:35 that still stands as the course record in 2023, the start was forecast at a cloudy 47 F, LetsRun reported that week. Jacob Kiplimo, the defending champion, would need 1:59:29 to take Sabastian Sawe’s world record, and at that pace he would reach the finish at about 9:30 a.m., with the temperature somewhere around 65 F. We looked at what that attempt would take earlier in the week, when the forecast was a few degrees less forgiving.

For everyone else, our pace calculator will build a mile-by-mile plan around whatever goal the morning allows, and our tips for running in the heat still apply to anyone out past noon. The professional race starts at 7:30 a.m. CT, and our guide to watching the race has the TV and streaming details.