More than 55,000 participants, the largest field in the race's history, are expected on the streets of Chicago on Sunday, Oct. 11, for the Bank of America Chicago Marathon. Race day is shaping up warmer than normal. The National Weather Service forecast for downtown, updated Saturday afternoon, called for sunny skies, light winds and a high near 71 degrees.

That is about six degrees above the average high of 65 degrees for the date, according to the race's historical weather guide. Earlier in the week, some forecasts had called for highs near 77 degrees. The latest outlook is milder, but steady sun and rising afternoon temperatures can still catch slower runners in the warmest part of the day.

A marathon generates large amounts of body heat, and runners who trained in cooler conditions may not be fully adapted to a warm race. Slower finishers, first-time marathoners, older runners, and anyone with heart disease or a recent illness face the greatest strain as the day warms.

A Warm Day Against a 65-Degree Normal

Wheelchair and handcycle athletes start shortly after 7:20 a.m., and the three runner waves start at 7:35, 8:00 and 8:35 a.m., according to an NBC Chicago race-day guide.

NBC Chicago meteorologists expected temperatures near 60 degrees at the start and closer to 70 degrees along the lakefront, where a lake breeze may help. Inland areas could reach the upper 70s, with some places nearing 80.

Conditions may not feel the same across the entire course. Runners who finish in five or six hours will be on the road into the early afternoon, when the sun is strongest.

Flag Colors, Medical Tents and the Lessons of 2007

The race uses a color-coded Event Alert System with four levels: green, yellow, red, and black. Yellow, the moderate level, signals "less than ideal conditions" and tells runners to slow down and be prepared for conditions to worsen. Red signals potentially dangerous conditions, and black means the event is canceled.

Organizers set the alert level at yellow for Sunday and asked runners to heed it by adjusting their pace, slowing down and staying hydrated, the Chicago Tribune reported. Runners should watch course signage and race communications, since the level can change as temperatures rise.

Dr. George Chiampas, the race's chief medical and safety officer, told the Chicago Tribune that conditions will vary along the route, with the western part of the course likely to be warmer if the sun is out. "It's not the same across all 26.2 miles," he said.

Medical support is available at 22 on-course locations, including tents at all 20 aid stations, one between miles 14 and 15 and one in the final mile, according to the race's course amenities page. Organizers have reason to plan carefully. In 2007, when temperatures reached 89 degrees, the race was stopped about three and a half hours after the start.

Heat Illness Signs Runners and Spectators Should Know

Heat illness can develop gradually or quickly. Signs of heat exhaustion include headache, nausea, dizziness, weakness, irritability, thirst, and heavy sweating, according to the CDC's guide to heat-related illnesses. A runner with these symptoms should slow down, move to shade or a medical tent, and get help from medical staff.

Heat stroke is a medical emergency. Warning signs include confusion or slurred speech, loss of consciousness, seizures, hot and dry skin or heavy sweating, and a very high body temperature. Anyone showing these signs needs emergency care right away, and bystanders should alert medical staff or call 911.

Confusion is an important clue because the affected runner may not recognize it. Friends, pacers, and spectators who notice a runner weaving, stumbling, or answering questions strangely should flag down course medical staff rather than encourage the runner to push on.

Spectators also face heat exposure. Older adults, young children, pregnant people and anyone standing in direct sun for hours should bring water, use shade when possible and take breaks indoors.

Pacing, Water and Knowing When to Stop

The most effective step on a warm race day is adjusting expectations. Slowing the pace early, rather than trying to bank time, lowers the body's heat load and reduces the risk of trouble late in the race.

Runners should use aid stations steadily, follow their own tested fueling plan, and avoid trying new products on race day. People with heart conditions, diabetes or a recent illness such as a cold or stomach bug should be especially cautious and consider whether to run.

The finish area deserves attention, too. Some runners feel fine while moving and then become lightheaded once they stop. Walking slowly after crossing the line, finding shade and reporting dizziness to medical volunteers can prevent a fall.

Stopping is a valid choice. Runners who feel dizzy, confused, or unusually exhausted can stop at any medical tent. After the race, continued confusion, fainting or very little urine output warrant prompt medical evaluation.

The forecast points to a warm but manageable day for most prepared runners who respect the yellow flag, watch out for one another and slow down before the heat forces them to.

Key Questions Answered

How warm is the Chicago Marathon expected to be? The National Weather Service forecast for downtown Chicago called for a sunny high near 71 degrees on Sunday, about six degrees above the average high of 65 for the date.

Is the race under a yellow flag? Yes. Organizers set the alert level at yellow for Sunday. Runners should watch course signage and race alerts because the level can change.

What does a yellow flag mean? Under the race's Event Alert System, yellow means less than ideal conditions. Runners are told to slow down and be prepared for conditions to worsen.

What are the signs of heat exhaustion? Signs include headache, nausea, dizziness, weakness, irritability, thirst and heavy sweating. Runners with these symptoms should slow down and seek help at a medical tent.

When is it an emergency? Confusion, slurred speech, loss of consciousness, seizures or a very high body temperature can signal heat stroke. These require immediate emergency care.

Where can runners get medical help on the course? The race has 22 on-course medical locations, including tents at all 20 aid stations, one between miles 14 and 15 and one in the final mile.

Who faces the highest risk? Slower runners on the course into the afternoon, first-time marathoners, older adults and people with heart disease, diabetes or a recent illness face the greatest strain in warm conditions.

Published by Medicaldaily.com