The Met Office has issued a late night weather warning as torrential rains threaten to spark floods in some areas.
The national forecaster has warned that the vast majority of England, Wales and Northern Ireland will be battered by downpours tomorrow.
It has issued yellow weather warnings for most of the country for the rest of Saturday, with a more localised alert in place tomorrow.
"Following rain overnight and during the morning, heavier showers and thunderstorms are likely to break out this evening," the forecaster warned.
"Many parts will miss these, but some torrential downpours are likely in a few places, bringing around 20 mm of rain in less than an hour and 30-40 mm in a couple of hours.
"Lightning and hail may prove additional hazards in some locations."
The Met Office changed the focus of the warnings late on Saturday away from thunderstorms to rain, and also extended them to run throughout the remainder of the day.
The warning is in place for all of England but the east coast south of Yorkshire, the eastern and central portion of Wales, and the southern half of Northern Ireland.
Tomorrow the alert continues 9am, but just for England aside from the South West.
Rain modelling shows a weather front moving across the country from west to east, dumping upwards of 32mm of rain in an hour in the worst hit areas.
Met Office forecaster Greg Dewhurst said the city of London has had 117% of its usual August rainfall already, with 60mm of rain falling on the capital up to August 19, compared with 51mm usually seen over the entire month.
Northern Ireland has had 91mm of rain - 93% of its 97mm average for the month.
Meanwhile, other areas have been exceptionally dry, including North Yorkshire which has had less than 50% of its usual rainfall.
Mr Dewhurst said overall UK temperatures and rainfall for the time of year are "about average".
Five day forecast
This Evening and Tonight:
Heavy rain and showers becoming confined to northern and eastern England, where they may persist into Sunday morning. Turning drier elsewhere with broken cloud and some clear spells.
Sunday:
Rain soon clearing from eastern England. Showers developing across some northern, eastern and central parts. Some sunny spells, especially in the west where it will feel fairly warm.
Outlook for Monday to Wednesday:
A mainly fine and dry spell, though a few light showers are likely in places on Monday. Warm, perhaps very warm for some northwestern areas, otherwise temperatures close to normal.
