Forecasters warned of possible flooding later this week as England and Wales are likely to see heavy rainfall.
The Met Office said warm air moving in from the south on Thursday is expected to make it feel very humid across central and southern areas of the UK.
A spokesman said: "Warmer air can carry more moisture, and as such some very heavy rain is possible.
"Current indications are that central and southern England and Wales are most likely to be affected. We may also have hail and thunder, this is most likely early on Thursday and then again Friday afternoon.
"As we have already seen this summer, this type of weather situation has the potential to cause sudden, localised surface water flooding and hazardous travelling conditions."
Met Office severe weather warnings have been issued for Wales and all English regions except the North East on Thursday, and Wales and all English regions on Friday.
The weather system is expected to clear away to the east during Saturday, with drier, fresher weather following everywhere, with some sunshine.