The Met Office warns transport could be disrupted in part of Devon as torrential rain hits the county, with up to 60mm of rain forceast for one day.
It is not yet known of the bad weather will lead to any disruption to the event.
By Tuesday and Wednesday, overall rain coverage will decrease, but it won't be completely dry. Showers will linger during the first half of the day, but once the boundary moves out, rain chances will drop.
The warning will remain in place until 9pm on Tuesday and covers a swathe of north Devon - including Exmoor.
A flood alert has already been issued for south-east London, and the Met Office issued an amber severe weather warning for heavy rain in London and the south.
Heavy rain causes flash flooding.
Forecasters have warned that parts of the United Kingdom could be inundated with more than 100mm of rain this week.
A wider yellow prolonged rainfall warning, spanning an area of eastern England from the Humber down to the south coast, was set to last until midnight.
Heavy rain and thunderstorms are on the way today, bringing an end to the longest stretch of consecutive days of dry weather in 2019. Main threat from any stornger storms would be heavy rainfall. Wind: NE 5-10 miles per hour.
Temperatures are forecast to reach a maximum of 17C. Minimum temperature 7 °C. "The showers will then gradually ease later this evening".
Chilly throughout. Wednesday unsettled with further rain and strong onshore winds.
Mostly cloudy overnight with a low near 62.