More than half a month's rain could fall across the south of England today, which could lead to "homes and businesses" being flooded, the Met Office has warned.
Between 30mm to 40mm of rain is expected to fall in some spots in just six hours.
In a typical August, 63mm of rain falls across the southeast of England over the entire month.
Buildings and structures could be damaged by flooding and lightning strikes while homes may lose power "for a short time".
There is also "a good chance" drivers could be delayed by spray and standing water, and of rail passengers facing delayed journeys.
Holidaymakers beginning their getaway are advised to check the forecast and social media channels for updates and brace themselves for extended journey times.
A yellow weather warning for thunderstorms has been issued by the Met Office for large parts of the South and the West Midlands between 6am and midday on Friday.
The area affected covers London as well as the whole of the South East and the Solent area, stretching north all the way to Birmingham.
A yellow warning for wind is in place across coastal areas of north and mid-Wales between midnight and 6pm on Friday - and gusts could reach up to 60mph.
The Met Office said trees could fall down leading to train cancellations and likely delays.
Marquees and tents that may be used by campers in Snowdonia could also fall down in the gusts while caravans and lorries may struggle on some roads.
Homes could also lose power for a short time in affected areas, the forecaster warned.
Met Office meteorologist Rachel Ayers said showers will begin to fall in the southwest of England and South Wales before growing in intensity as they move eastwards.
Heavy showers could hit areas unaffected by the weather warnings but they are not likely to put people in any danger.
Ms Ayers said: "Given it is summer holidays, people are more likely to be wanting to be outside and travelling for holidays.
"There is a chance we could see driving conditions affected, leading to longer journey times.
"Take the advice from our partners such as the RAC with regards to driving in heavy rain if you do make that journey."
She said conditions should brighten up across most of the UK on Saturday - with sunshine and showers likely amid "blustery" conditions.
Heavy rain on Saturday is likely to be confined to the northeast of Scotland, she added.