A yellow weather warning of heavy, persistent rain and possible flooding has been issued for parts of Wales and northern England on Monday.
After the downpours, however, there should be sunshine, with temperatures expected to reach 30C (86F) in southern England by the weekend, the Met Office said.
The warning of heavy rain was issued on Sunday morning and was due to last from midnight on Sunday until 9pm on Monday.
Forecasters said there was a small chance some homes and businesses could be flooded, and that some communities could be cut off.
Spray and flooding is likely to lead to road closures and difficult driving conditions, and there is a chance of transport delays and power cuts.
The Met Office said the rain would spread from south-west England to the north-east, with the heaviest, as much as 60-80mm in a few places, most likely over north-west Wales and north-west England.
The Met Office forecaster Greg Dewhurst said temperatures on Monday would be between 18C and 23C.
“The good news is that low pressure moves out of the way as we go into Tuesday, so it should be a drier day on Tuesday.
“A better chance of seeing some sunny spells particularly across eastern parts of the UK.”
Temperatures could then start to rise in southern parts of the UK as the week progresses, he said.
“An area of low pressure to the west of the UK and the Atlantic slowly starts to move towards us,” he said.
“What that allows to happen is for the winds to turn southerly, and we start to import some higher temperatures from the near continent.
“So, staying into Friday, we’re likely to see those temperatures rising, potentially getting towards the high 20s and then possibly by Friday and into the weekend, depending on cloud and shower distribution, we could locally see temperatures around 30 degrees by the weekend.”
