The Met Office has issued a yellow weather warning for storms, with larger parts of England and Wales affected.
Meteorologists say torrential rains could cause travel disruptions, and there are also possible floods.
The weather warning zone has been extended west and north, and is in effect until 11:59 p.m.
Check the five-day weather forecast where you are
When storms occur, up to 40 mm of rain could fall in just one hour, increasing to 60 mm in a period of two to three hours.
“These torrential downpours may be accompanied by frequent lightning, while gusts of wind and hail may also affect some places,” the Met Office alert added.
The storms are expected to occur mainly in the afternoon and early evening, and meteorologists warn that their exact location “will be difficult to identify.”
Changes can be made to the warning area in the next few hours.
Glastonbury partygoers have been told they may have to take refuge while storms sweep the country.
Festival attendees were told to pack their bags for all eventualities: light showers are expected on Friday and Saturday, with heavier rain on Sunday.