Forecasters are predicting thunder, lightning and flooding as a yellow weather warning comes into force across the UK.
The Met Office has implemented the warning for Scotland and Northern Ireland until 11.59pm on Monday and 11.59pm for all of England and Wales on Tuesday after a change in air pressure led to dramatic showers.
Power cuts and delays and cancellations in trains and buses are predicted, while spray and sudden flooding could lead to difficult driving conditions for motorists.
Greg Dewhurst, a meteorologist at the Met Office, said the week would start off quite humid before cooling down later on.
He told the PA news agency: “We’ll start off initially quite humid, particularly across the south and the east of the UK, with thundery showers and sunny spells, but it will gradually become cooler and fresher as the week goes on.
“First half of the week, we’re looking at some heavy downpours and thunderstorms developing.”
Flood alerts in Pembrokeshire
Flood alerts have been issued by Natural Resources Wales (NRW) across the Pembrokeshire coast.
NRW said: "The combination of forecast tides and meteorological conditions gives a risk of flooding around the high tide.
"High tide at Milford Haven is predicted to be at 9pm. Flooding of low-lying land and roads is expected.
"We will continue to monitor the situation. You can monitor the situation in your area by using our 'River Levels, Rainfall and Sea Data' on our website.
"For more information on our Flood Alerts and how we issue them, please visit the ‘How we forecast floods, issue warnings and assess flood risk’ page on our website."