Thunderstorms and heavy rains could cause flooding in parts of southern England and Wales.
The Met Office issued a yellow weather warning for storms on Monday covering areas such as Cardiff, Gloucestershire, Hampshire, Oxfordshire and London and is in effect until 11 p.m.
Meteorologists predict that up to 50mm of rain could fall in the space of a few hours.
Nicola Maxey from the Met Office said: “This can lead to surface water flooding as the drains cannot wash off the rain fast enough and spray onto the roads. However, not everyone in the area will see storms or rain, but the risk is there. More rain will come this week, with the possibility of thunderstorms on Wednesday. “
Floods have already hit 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 more time for their travels, while the Hampshire and Isle of Wight Fire and Rescue Service advised those affected by the floods to bring their valuables upstairs and do whatever they could to keep electronic devices away from water.
As the week progresses, there will be a combination of sunshine and rain, according to the Met Office.
Marco Petagna from the Met Office said: “Today and tomorrow, there will be a mixture of sunshine and rain for most of the whole UK. The weather will be more unstable from Wednesday. There are showers or longer periods of rain in most areas as we move from Wednesday to Thursday and possibly Friday as well. “
Forecasters say weather conditions should improve by the middle of the month. Conditions are expected to be three and warmer by then, according to Petagna, although there are still no signs of another heat wave on the way.