Overall, it's not looking too bad for Sunday 23rd April. It looks mostly dry; any rain will be light and patchy if it does come. There will be a good deal of cloud which should break at times allowing warmer sunshine but not with the sun beating down for the whole race. The air remains cool with light winds.
If you have been looking at the weather over recent days or weeks, you may have seen the forecast vary, sometimes with rain, then more sunshine.
There several variables for the forecast for Sunday, mainly around the amount of cloud cover. This is always the forecast challenge under an area of high pressure, which is what will still govern the London weather this weekend.
It doesn't look like the race day will be hot or humid, nor soaking wet or blowing a gale. So no severe weather or extreme temperatures.
Looking at the hourly or 3 hr symbols for Sunday, you might see cloudy or bright. Earlier in the day looks grey but on the cool side. So a few extra (disposable) layers for the very beginning.
The amount of cloud cover can vary two ways. Thicker, lingering cloud could bring a little bit of patchy rain, which is why you may be seeing rain around late morning/early afternoon on the app display or website forecast. Any rain will not be heavy, just slight and will come and go quickly.
The other way sees the cloud thinning and breaking to give sunny spells; this will affect the temperatures.
Currently, it looks like pre-start will feel like 6C and feel like 9 to 12C through the race. Actual air temperatures of up to 14C. However, with thinner cloud and more sunshine, the air temp could reach 17 or 18C by early/mid afternoon. Spectators take note too.
The winds add slightly to a cooling on how the air feels. The southerly breeze should veer to a westerly and be around 10-15mph, so light but noticeable. A lot of the end section is heading west so a slight head wind by the afternoon.
UV Index- This looks at the Ultraviolet radiation from the sun, so the strength of the sun' s rays in relation to harm caused to humans on a particular day. For London, a middle 4/5 is showing. So sunscreen is recommended even if the day looks grey and cloudy early on. UV still passes through the clouds and there will be sunny spells at some point in the day.
Pollen - There are moderate levels of pollen in the air. This is mainly tree pollen, oaks and plane trees with a little grass pollen appearing.
Lots of information on the Virgin site about registration on Saturday, what documents to bring and the final preparations. @LondonMarathon
Don’t do as one of my old Met Office/BBC colleagues did and left his race numbers on another forecaster's kitchen table on the Sunday morning. He then got rugby tackled in the last half mile as an imposter. His microchip proved he’d run the race and he came back to finish with the Deep Sea Diver over a week later but gutting is not the word.