With four days to go until the 46th TCS London Marathon, the race-week forecast has settled on the kind of cool, dry Sunday that makes elite organisers and record-chasers exhale at the same time. The Met Office has London's high at 15C (59F) with an overnight low around 5C, a seven per cent chance of rain and easterly winds of 8 to 12 mph — a touch of assistance, on balance, for the back half of the point-to-point course from Blackheath to The Mall. Race organisers have already begun briefing elite teams that aid-station set-ups will assume dry rather than slick tarmac, and that the usual damp-London contingency for sub-elite runners has been relaxed.
For the men's field, those numbers matter. Sebastian Sawe, the defending champion, told reporters at Thursday's pre-race press conference that anything above 13C changes the shoe choice and the pacing plan, and that his target of attacking Kelvin Kiptum's 2:01:25 course record was now realistic rather than aspirational. Jacob Kiplimo, Joshua Cheptegei, Tamirat Tola, Deresa Geleta and Amos Kipruto complete one of the deepest start lists London has ever assembled, and with three men in the line-up holding personal bests under 2:03:00 the conversation has drifted inevitably towards Kiptum's 2:00:35 world mark. The likely pace on Sunday will have halfway somewhere around 60:45 — a rhythm that demands a cool morning and a calm start.
The women's field is, if anything, better placed for the conditions. Tigst Assefa, the Ethiopian defending champion and holder of the women's-only world record of 2:15:50 set on the same course last year, will face Joyciline Jepkosgei (2:14:00 in Valencia), Sifan Hassan — whose status was in doubt for a week in March before she confirmed her start — and Peres Jepchirchir's replacement on the elite entry list. Four of the six fastest women in history are on the line. A 2:15 at 15C on flat London tarmac is no longer a ceiling so much as a launching point, and Assefa's team has publicly pointed to the sub-2:15 barrier as a genuine day-one target.
Conditions also matter at the other end of the field. The race expects more than 59,000 starters, a new record, with the mass event staggered across blue, green and red starts from Blackheath. Forecast temperatures at the back of the field will sit closer to 12 to 14C by the time runners are on the Embankment, and organisers have reiterated that the water and Lucozade Sport stations across the course will run at peak capacity. The dry forecast also reduces the usual risk of clothing and medical incident that London has learned to plan for during warmer or wetter years — and, more mundanely, means the cobbled section around the Tower of London should not throw up the usual slippery early miles.
The only meaningful caveat is the wind. Easterlies down the Thames tend to feel like a headwind through miles 22 to 25 even when the vector numbers say they are helping, and a strong east-to-west flow can turn what should be a fast closing stretch into a grind. Current guidance has the wind staying light, but elite teams will be watching Friday and Saturday's updates carefully. If the forecast holds, Sunday in London will look much like the cool, bright Patriots' Day that served up Boston's record-breaking Monday — and that is as good a setup as a spring marathon can reasonably ask for.