The weather in Boston is expected to reach as high as 86 degrees today, forcing the race officials to issue a warning regarding the "red-zone conditions" for this years' race. This unseasonably high heat will surely have an impact on the less experienced runners.
With the men's race starting last, the runner's will be racing during the hottest parts of the day and will be experiencing the heat's full effects. The top contenders compete in marathons across the world, including in the scorching hot Dubai, have run in conditions similar to the weather in Boston, and while the heat will have an impact, it shouldn't be expected to drastically effect the top contenders.
With the women's race starting relatively early in the morning at 9:32 a.m., Caroline Kilel, Sharon Cherop, Aselefech Mergia, and the rest of the top contenders should be finished before the truly oppressive heat kicks in.
With the weather during the first few hours of the race expected to be overcast, the women should be fine. While I don't expect any of the women to set world records, their times shouldn't be greatly affected by the heat and should be only a little bit slower than last years.
Geoffrey Mutai is set to defend his title in this year's men's race, and he will be racing in drastically different conditions than last year. Mutai ran the fastest marathon ever during last year's event, with a blistering time of 2:03.2 with temperatures around 50 degrees and a noticeable tailwind.
While the heat is going to be unseasonably high, it shouldn't have to great of an impact on the top contenders. Mutai, Gebre Gebremariam, Robert Kiprono Cheruiyot, and Wilson Chebet all post time's just slightly over the two hour mark and thus will miss the oppressive heat that is anticipated later in the day, but it will noticeably effect the slower runners. Similar to the women's race, the heat should only have a negligible effect on their times.
While the heat creating "red-zone conditions" is a problem for the less experienced runners, the top contenders in both the men's and women's races will be fine. They are professionals and have plenty of experience running in similar conditions across the globe. A slight impact is expected on the times of the top professionals, adding approximately a couple minutes to their times, anything greater then that shouldn't be expected.