Heavy rain is expected across Sussex and the rest of the south of England on Thursday morning (August 13) before pushing north during the day.
The warning is active from 6am on Thursday (August 13), through until 11.50pm, and the rain is likely to be accompanied by thunder and lightning, with potential torrential downpours.
The Met Office is warning the public to be aware of a significant risk of very localised disruption to travel due to surface water flooding.
An area of low pressure is expected to develop just to the north-west of Spain on by Wednesday, heading north across Biscay towards the UK.
Around 15 to 30mm of rain is possible in in 6 to 12 hours, with as much as 50mm possible very locally by the end of Thursday.
A further Met Office warning is in place for the whole of Friday, as the area of heavy rain is expected to push across the rest of England during the morning.
The rain is likely to be persistent, and accompanied by hail and thunder in the south and east.
Rain will ease from the west during the evening, probably confined to eastern areas by the end of Friday.
Localised thunderstorms may develop across the south and east of England during Friday afternoon, these potentially giving 15-25mm of rain in a few hours.
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