Weather forecasters have said much of the UK is at risk of thunderstorms on Saturday.
A yellow warning for heavy showers and thunder has been issued by the Met Office, with up to 40mm (1.6 inches) falling in a couple of hours in some areas.
There could be flooding, power cuts and travel disruption in England, Wales and Northern Ireland.
But next week, conditions are expected to improve for the UK with drier, warmer and sunnier weather.
The Met Office yellow warning is in place from midday until 10pm on Saturday
The yellow warning lasts from midday until 10pm on Saturday and covers large swathes of the three nations, apart from some of southeast and eastern England, and some coastal areas.
The Met Office said: “Following rain overnight and during the morning, heavier showers and thunderstorms are likely to break out by late morning.
“These becoming more widespread into the afternoon, with torrential downpours possible in a few places, bringing around 20mm of rain in less than an hour and 30-40 mm in a couple of hours.
“Lightning and hail may prove additional hazards in some locations.”
Sky’s weather producer Joanna Robinson said: “Rain will move eastwards across most areas on Saturday, with heavy and thundery showers following.
“There’ll be further showers around on Sunday, perhaps thundery at first, but high pressure will settle things down after that.”
She predicted drier, warmer and sunnier conditions next week, which may last into the bank holiday weekend.
“It’s looking mostly dry next week, with more in the way of sunshine than recently,” she said.
“It will turn warmer too, with the highest temperatures in the West or North West at first, but a heatwave is not expected.
“There are signs the fine weather will last through the bank holiday weekend too.”
Some roads could flood, bringing travel disruption
Met Office meteorologist Simon Partridge predicted up to 40mm of rain could hit parts of Northern Ireland and southwest England over 24 hours this Saturday, and the same amount in the northern Pennines on Sunday.
Forecasters say there is a good chance driving conditions will be affected by spray, standing water and/or hail, leading to longer journey times by car and bus. Delays to train services are also possible.
There could also be some short term loss of power, with potential damage due to lightning strikes.
And flooding of a few homes and businesses could occur, the Met Office said.
The warning follows unsettled summer weather which has persisted across the UK throughout August.
The City of London has had 117% of its usual August rainfall already, with 60mm (2.4in) of rain falling on the capital up to 17 August compared with 51mm (2in) usually seen over the entire month, according to Mr Partridge.
Northern Ireland has had 90mm (3.5in) of rain – 90% of its 97mm (3.8in) average for the month.