The Met Office has put a severe weather warning in place for Saturday, with thundery downpours expected to hit Cornwall.
Forecasters warn that the public should be aware of the "potential for localised flooding due to the intensity of these storms", adding that at this stage the location of the heaviest rainfall is "uncertain".
The forecast for heavy rain is due to warm, humid air likely to be pulled northwards from France late on Friday and during Saturday.
The chief forecaster at the MeT Office said: "Within this airmass, there is an increasing likelihood that heavy, thundery showers will break out. It is likely that many places within the current broad warning area will miss the worst of the storms; over the next few days it may be possible to focus on the areas most at risk."