The warnings now cover the northwest and the Midlands. London and southeast England, northwest England, southwest England, Wales and the West Midlands are also likely to be affected by the extreme weather. The storms are set to begin this morning and continue until this evening.
The Met Office warned that 20-25mm of rain could fall in less than an hour in areas affected by the storms.
Some places could see up to 50mm over the course of two or three hours.
The country is also likely to see hail and lightning.
In an update, the forecaster said: “While some areas will remain dry, slow-moving, heavy showers and thunderstorms are likely to develop from mid to late morning, before decaying during the evening.
“Where thunderstorms occur, 20-25 mm of rain is possible in less than an hour, and in one or two places 40-50 mm could fall in 2 or 3 hours.
“Hail and lightning are potential additional hazards.”
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“With harvest underway, we now have rain for parts of Britain.
“The main area on the radar is over the north Midlands into NW England with patchy light rain from the thick cloud, to the north and east.
“There is rain for mid-Wales and a few bright echoes in the English Channel. Everything is edging northwards.
“The UK Met Office has a thunderstorm warning for Friday over SW Britain.
“It is the slow-moving nature of these storms which could cause impacts when heavy rain falls onto the dry, hard ground.
“Also, there would be lightning.”
But she added: “Thunderstorm warnings are always tricky.
“Today the Met Office have an area highlighted in yellow but within that many, even most people won’t see a thunderstorm.
“The convective forecast from Netweather shows risk areas, but both are highlighting that atmospheric ingredients are coming together and could produce a storm with heavy rain.
“That then could lead to flooding or lightning strike disruption.
“Meanwhile, other areas outside of that zone could still see a thunderstorm.”