Heavy showers, hail and thunderstorms are likely to spread from the south across Cornwall and Devon on Friday.
The yellow warning by the Met Office is valid for Thursday night, with it remaining in place through much of Friday.
A spokesman said that while many places will miss these, the public should be aware of the potential for "isolated disruption due to surface water flooding, lightning strikes and hail".
The chief forecaster said: "A warm and humid airmass covers southern Britain at present. This is already becoming increasingly unstable and producing a few thunderstorms through Thursday, but these are expected to become more widespread overnight into Friday.
"Many place will miss the worst of these - some may stay dry - but where they do occur, 20-30 mm rain may fall in a short time, leading to some surface water flooding, whilst lightning and hail may be additional hazards."
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