The fastest man on earth could face some very British weather as he goes for gold over 100m.
Jamaica's Usain Bolt will have fans in the Olympic Stadium screaming and on their feet shortly before 10pm - just as the rain starts coming in from the south, forecasters said.
Heavy showers and thunderstorms could hit the final straight of the women's marathon on The Mall in central London before clearing for most of the athletics at the Olympic Park.
But it will be "touch and go" whether Bolt has to face the weather as his final runs at 9.50pm.
Paul Mott, a meteorologist with MeteoGroup, the weather division of the Press Association, said: "There's a risk of some rain coming in from the south about 10pm. Most of the athletics should be dry, but it'll be a bit touch and go for the 100m final."
Britain's Andy Murray, playing on Wimbledon's centre court which can be covered by the roof, should escape the rain for his back-to-back finals in the men's singles and the mixed doubles. But fans flocking to see the women's marathon should brace themselves for heavy showers and thunderstorms.
"It's going to be pretty wet under foot for the marathon," Mr Mott said.
"We've got some quite heavy showers and thunderstorms coming in and spreading northwards across London at the moment. Those look like they'll be staying around for at least a couple of hours. It will be quite wet for the marathon runners.
"I think in the early afternoon we'll see a bit of sunshine between the showers, but pretty heavy downpours at times. These showers won't be persistent, they'll be on and off, but it'll be pretty wet under foot.
"It'll be fairly cool as well, 16C or 17C, which will be fairly good for the marathon runners, rising to about 21C this afternoon. Later this afternoon, early evening, it should be turning drier and clearer, with gentler winds as well. But it will be touch and go for the 100m final."