Thunderstorm alerts could be issued this week - with forecasters anticipating "intense rainfall" across parts of the UK.
Some areas could see up to 40mm of rain in a few hours when the deluge hits tomorrow evening.
Temperatures are also expected to rise as the week progresses - to highs of 27C (80F) on Wednesday and Thursday, and 29C (84F) on Friday.
That could hit heatwave thresholds in the northwest Midlands, northeast Wales and the North West of England - but "it is not a certainty" as this depends on cloud cover.
A plume of warm air from Iberia and France is to blame for the chance of thunderstorms.
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Met Office deputy chief meteorologist, Mike Silverstone says the downpours could cause disruption.
He added: "While there are no severe weather warnings issued at the moment, it is possible thunderstorm warnings may be issued this week."
Although last month was dominated by fine weather, the forecaster says high levels of humidity will make this warm spell feel uncomfortable.
"While in May the nights were still fairly cool, overnight temperatures this week are forecast to remain fairly warm, which can disrupt people's sleep," Mr Silverstone said.