The Met Office has expanded a yellow weather warning for thunderstorms - with larger parts of England and Wales affected.
Forecasters say torrential downpours could cause travel disruption, and flooding is also possible.
The weather warning area has been expanded westwards and northwards, and is in force until 11.59pm tonight.
Where thunderstorms do occur, up to 40mm of rain could fall in as little as one hour - rising to 60mm over a two or three-hour period.
"These torrential downpours may be accompanied by frequent lightning, whilst gusty winds and hail may also affect a few places," the Met Office alert added.
The thunderstorms are mainly expected to happen in the afternoon and early evening, with forecasters warning their exact location "will be hard to pinpoint".
Changes to the warning area are possible in the hours.
A yellow weather alert was initially issued on Wednesday warning about the potential of torrential rain.
Revellers at Glastonbury have been told they may need to take shelter as the thunderstorms sweep the country.
Festival attendees were told to pack for all eventualities - light showers are predicted on Friday and Saturday, with heavier rain on Sunday.