The Met Office is forecasting relentless torrential rain for South Yorkshire on Friday, with more than the entire average amount for August possible.
Forecasters today updated their severe weather warning for heavy rain which will sweep across the country, beginning tomorrow.
The worst of the deluge is set to hit South Yorkshire early on Friday.
On average, Sheffield has 58mm of rain in the month of August, but this could be eclipsed in just a 24 hour period as the storm system moves northward.
The storms, which are forming in the Bay of Biscay, could include hail and lightning.
In his latest assessment, issued today, the Met Office's chief forecaster said: "An area of low pressure pushing into southern UK early Thursday continues slowly northwards to be centred near or just east of Scotland by Friday afternoon.
"With very warm air entrained into the system, further outbreaks of locally heavy rainfall are likely during Friday, particularly during the early hours and morning, with locally in excess of 50 mm by the end of Friday.
"Localised thunderstorms may also develop across the south and east of England during Friday afternoon, these potentially giving 15-25 mm of rain in a few hours.
"The development and subsequent track of this system is very complex, therefore the exact locations of where the heaviest rainfall will occur remains uncertain."