“There is a quite heavy burst of rain coming across Ireland and Northern Ireland and it will move across Scotland, Wales and England,” Met Office forecaster Marco Petagna said.
Forecasters have issued a yellow warning for between 8am and 10pm, with transport likely to be affected and flooding possible.
The warning covers Wales, the North West and South West of England, with high ground expected to see the worst weather.
Mr Petagna said up to 80mm of rain is possible in some areas.
The average August rainfall in Wales is 107mm and the average for August in the south-west is 90mm, meaning that almost a month’s worth of rain could fall on Friday alone.
“August actually can be a fairly wet month,” he said.
Strong winds are also expected, with 30mph to 40mph gusts likely around exposed coasts and hills.
Heavy rain may coincide with peak travel times during the afternoon and early evening, leading to difficult driving conditions on major routes, the warning says.
Mr Petagna said places outside the warning area will not escape the rain.
The forecaster said it will become brighter across Scotland and Northern Ireland on Friday afternoon and the weekend will bring sunshine and blustery showers, adding that things are looking more positive next week.
“Especially come mid-week, pressure’s building a bit,” he said.