A surge of summer-like air promises to create challenging or even hazardous running conditions for today’s Boston Marathon. Afternoon temperatures around Boston are likely to spike into the mid-to-upper 80s, easily breaking Boston’s record for the date of 84 set in 2003. Record have been maintained in Boston since 1872. Average highs in Boston at this time of year are in the mid-50s.

Heat timeline: At 9 a.m., the temperature in Boston was already 69 degrees. By the time women start running at 9:32 a.m., it should be in the low-to-mid 70s, with mid-to-upper 70s by 10 a.m. when the men begin. At noon, temperatures will reach the low 80s with mid-80s by 2 p.m. A high temperature of 85-88 is likely between 3 and 4:30 p.m.

A model simulation shows southerly winds streaming over New England (left) and very warm mid-afternoon temperatures (right). (Penn State)

The cause: Ahead of a cold front slicing through the Midwest, southerly winds are streaming northward, tapping tropical air from the Gulf of Mexico. An area of high pressure off the Southeast U.S. coast is enhancing this warm weather feed, its clockwise circulation acting like a pump to further channel the July-like air in the direction of New England.

Impacts: Running long distances with temperatures into the 80s increases the risks of heat-related illness, especially for older adults, inexperienced runners, and/or those with pre-existing medical conditions.

Officials are urging runners with any doubts about their fitness under these circumstances to defer participation for another year. From the Associated Press:

B.A.A. [Boston Athletic Association] co-medical director Pierre d’Hemecourt warned 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 not run a full marathon before, or who have not run in the heat, should also sit this one out, race organizers said.

“Only the fittest runners should consider running. The risks that you’ll see tomorrow are simply greater than normal,” B.A.A. executive director Tom Grilk said, advising runners who do line up at the start to slow their pace and focus on finishing rather than a fast time. “You should adopt the attitude that this is not a race; it is an experience.”

Related: Boston Marathon 2012: Heat causes officials, runners to adjust