A Chilly London Marathon, But Will It Snow?

A chilly arctic blast is on the way for the weekend, more on this in my blog: Winter Bites Back To End April, so it looks like being a cold day for the thousands of runners and spectators at Sunday’s London Marathon.

Some media articles have suggested runners could face sleet and snow on Sunday, well, I don’t think it can be ruled out, but unlikely, as most of the wintry showers look to stay north and east of the capital – with just a 20% chance of a shower passing over the route, as it stands. The greatest chance of a shower looks to be late in the Marathon towards mid afternoon, looking at GFS below. Though the resolution doesn't show the hit and miss nature of showers.

Since the London Marathon started in 1981, no snow has fallen during the annual 26 mile run since then. Certainly the air at the surface could be dry enough below cloud level on Sunday, i.e. dew points below freezing, for sleet or even the odd wet snow flake to fall to the ground in any showers that do make across the route of the Marathon. Though only if temperatures fall enough under any showers – ideally around 4C. The air temperature doesn’t have to be close to freezing for snow to fall, but the surface air flow does need to be dry (for ‘warm’ snow) i.e. 20% humidity with dew points well below freezing. Theoretically it could snow with temperature as high as 8C, but the air would have to be very dry – which is unlikely in the UK and more likely in interior continental areas far from any maritime influence. Any wet snow on Sunday won’t settle though, as ground temperatures need to be close to freezing.

Dew points below freezing on Sunday, suggest the air may be dry enough at the surface to allow wintriness to fall from any showers in the SE

Approximately 38,000 runners will attempt the 26.2 mile course through London’s  streets on Sunday. The race starts are staggered, with the Mini Marathon going first at around 8.40am, followed by the wheelchair races, women's race and men's race. The mass race will begin shortly after the men.

The London Marathon starts in Greenwich Park, when it will likely be chilly but dry and perhaps sunny – temperature still only around 3C between 8am and 10am. By the time runners reach Tower Bridge by late morning to lunch time, temperatures perhaps rising to around 5-7C, but perhaps cloudy by then. Then those who reach the final 500m along Birdcage Walk, past Buckingham Palace and over the finish line can expect temperatures as high as 8 or 9C early to mid afternoon. It may well be cloudier through the afternoon, with that 20%  chance of catching a shower, which may fall as mix of rain, sleet, hail and even contain the odd wet snow flake.

The chilly conditions probably ideal for most runners. But spectators are advised to wrap up. The chilly and brisk north to northwesterly wind, reaching 14-16 mph, will make it feel more like -1C at the start of the race, to 3-5C towards the end of the race.

Here’s a forecast of the weather, temperature, wind, and what it will feel like for central London through the day.