Thunderstorms and torrential downpours could cause flooding across parts of southern England and Wales.
The Met Office has issued a yellow weather warning for storms on Monday which covers areas including Cardiff, Gloucestershire, Hampshire, Oxfordshire and London and is in place until 11pm.
Forecasters are predicting up to 50mm of rain could fall in the space of just a few hours.
Nicola Maxey, a spokesperson from the Met Office, said: “This can lead to surface water flooding as drains cannot get rain away fast enough and spray on roads.
“However, not everyone within the area will see storms or rain, but the risk is there.
“More rain will be coming through this week, with the potential for thunderstorms on Wednesday.”
Flooding has already affected the Isle of Wight, with the Met Office reporting that up to 120mm of rain had been recorded near the town of Ventnor.
South Western Railway urged customers to allow extra time for their journeys, while Hampshire and Isle of Wight Fire and Rescue Service advised those affected by flooding to move their valuables upstairs and do what they can to keep their electronics away from water.
As the week progresses, there will be a mix of sunny spells and showers, according to the Met Office.
Marco Petagna, from the Met Office, said: “Today and tomorrow, there will be a mixture of sunshine and showers for most across the UK.
“The weather will be more unsettled from Wednesday onwards. There’s showers or longer spells of rain for most areas as we go from Wednesday into Thursday and potentially Friday as well.”
However, forecasters are predicting that weather conditions should improve by the middle of the month.
Conditions look to be drier and warmer by then, according to Mr Petagna – though there is still no sign of another heatwave on the way.