BOSTON — With temperatures expected to rise into the high 80s during Monday’s Boston Marathon, race organizers are advising inexperienced runners and those with medical conditions to sit this one out, and are warning those who do run to be extra careful with the heat.

“This is not a day for personal bests,” said Pierre d’Hemecourt, one of the race’s medical directors. “This is a time to slow down and not take any risks.”

In a news conference to address concerns over forecasts calling for temperatures to rise to 88 degrees at the Back Bay finish line by afternoon, officials from the Boston Athletic Association offered a deferment that would allow runners who registered this year to run next year instead.

“For many people, running the Boston Marathon is a once-in-a-lifetime experience,” BAA executive director Tom Grilk said. “We don’t want people to feel they have to run because perhaps it’s not the wisest decision under these conditions.”

Last year’s race went off under perfect marathoning conditions — temperatures in the 50s and a significant tailwind — that helped Geoffrey Mutai shatter the course record with the fastest time ever run over 26.2 miles: 2 hours, 3 minutes, 2 seconds. But this year the weather is expected to slow the leaders and cause havoc for the nearly 27,000 recreational athletes who see Boston as the culmination of a running dream.

d’Hemecourt said to runners with underlying medical issues, such as a cough or a cold or a recent stomach virus that left them dehydrated: “Please don’t run the marathon on Monday.”

Those who have only trained in a cooler climate or who haven’t spent at least 10 days acclimating to the heat should also skip the race.

“This will be a day where you get hot and dehydrated and may have heat problems,” d’Hemecourt said. “If you do run, run slow. This is not a personal best day. Take breaks to walk and recognize any heat problem symptoms.”

Grilk said the course will be kept open for an extra hour — until 6 p.m. — to discourage runners from over-exerting themselves.

“It allows for an additional 2 minutes per mile to slow down and be sensible about it,” he said. “Be part of what could be a well-remembered experience, but do it in a way that is careful. Ultimately it is an individual sport and individual decision, but we want to make sure we provide people with the comfort of knowing they can make the decision.”

The BAA last offered a deferment in 2010, when the Icelandic volcano eruption stalled air traffic in Europe and prevented about 300 runners from getting to Boston.

Those who do run, race officials warned, should be alert to the symptoms of heat stroke: confusion, headaches, nausea, vomiting and excessive fatigue. d’Hemecourt said runners experiencing any of those should stop running and walk to the nearest aid station to be evaluated.

The BAA medical staff also warned runners not to drink too much water, which can be dangerous. They should drink when thirsty and take slightly more water than they normally do when they train.

“Personal responsibility has to happen on Monday,” race director Dave McGillivray said.

The Boston Marathon last dealt with hot weather in 2004, when temperatures reached 85 degrees. About 2,000 people from the field of 18,000 sought medical attention, half on the course and half at the finish line medical tent; 300 of those were foot injuries and most of the rest had heat-related problems.

McGillivray said extra water will be available at the start, on the course and at the finish.

“Our experience back in 2004, which was similar conditions to what we may see on Monday, really was an eye-opener for us,” he said. “Everything that can be done is being done.”