Scattered storms are expected across Wales, the West Country, the Midlands, and London, with some areas potentially receiving over half a month's worth of rain.
Heavy downpours could bring 20-30mm of rain in an hour and 40-50mm in one to two hours, increasing the risk of flooding and localised power cuts.
While Northern Ireland, northwest England, and southern Scotland might experience showers, the impact is not expected to be as severe as in the warning zone.
The weather is predicted to stabilise from Tuesday, with temperatures reaching the low to mid-20s, potentially marking the hottest days of the year so far for Northern Ireland and Scotland. This follows a hot and sunny weekend across much of the UK, and a tragic incident where a swimmer went missing in Loch Ard.