Parts of Britain could be battered with more than a month's worth of rainfall on Thursday and Friday.
The weather forecasting service has issued a yellow weather alert for London and the South East, South West, East, the Midlands, Yorkshire and Humber and the North West of England starting early on Thursday morning and continuing throughout Friday.
“The rainfall is likely to be heavy and thundery at times, with the potential for torrential downpours in places,” the Met Office said.
“The public should be aware of a significant risk of very localised disruption to travel due to surface water flooding. Lightning may be an additional hazard.”
Between 15 and 25 mm of rain is expected to fall in just 6 to 12 hours, the Met Office said, adding that as much as 50mm of rain could fall on Thursday alone. A further 50mm is expected on Friday.
To put that into context, the average rainfall for August is 48mm, according to official statistics. The wettest month of the year is usually October, when as much as 71mm falls on average.
“The development of this system is very complex, and uncertainty in the details of this event is relatively high,” the forecasting service added.