The warning covers much larger expanses of Northern Ireland, Wales and England – and is set to last from midday until 10pm.
Up to 40mm of rain is expected to inundate the areas covered by the yellow warnings on Saturday, with the exception of some coastal areas, Met Office forecaster Greg Dewhurst said.
“On Saturday we will see a band of heavy rain push north-eastwards across the UK, followed by sunshine and heavy showers,” he told PA news agency. “Sunday will see the heavy rain clearing to bright and sunny spells and heavy showers in the east.”
It comes after areas including London have already seen a month’s worth of average rainfall for August with 12 days still to go.
Mr Dewhurst said the City of London has had more than 100 per cent of its usual August rainfall already, with 60mm of rain falling on the capital up to 19 August, compared with the 51mm usually seen over the entire month.
Northern Ireland has had 91mm of rain – 93 per cent of its average 97mm (3.8in) for the month, he added.
Meanwhile, other areas have been exceptionally dry, including North Yorkshire which has had less than 50 per cent of its usual rainfall.
Mr Dewhurst said overall UK temperatures and rainfall for the time of year are “about average”.
Have your brollies at the ready this #weekend with rain and cloud spreading east across many areas on Saturday ☂️☂️
Warnings issues for #thunderstorms Saturday afternoon ⛈️
Drier but cloudy on Sunday, perhaps with some brighter spells in the west later
Stay #WeatherAware ⚠️ pic.twitter.com/DVjiuDfiYo
— Met Office (@metoffice) August 20, 2021
On Saturday, the mercury is set to rise to around 21C in London, 20C in Cardiff, and 18C in Edinburgh and Belfast.
The Met Office said in a tweet that much of the UK would see a change in weather on Friday evening, in time for the downpours expected throughout Saturday.
“Friday evening is almost here and the weather is changing in time for the weekend,” the weather service wrote. “Turning wet this evening in many northern and western areas with outbreaks of rain, heavy in places. Largely dry elsewhere with some bright spells at first.”
On the Met Office’s website, the extent of England’s weather warning can be seen by a huge yellow graphic which covers much of the nation from Newcastle, in the northeast, down to the outskirts of Plymouth in the southwest.
“There is a good chance driving conditions will be affected by spray, standing water and/or hail, leading to longer journey times by car and bus,” the warning outlines.
It finishes by saying: “Delays to train services are possible, [as is] some short-term loss of power and other services – with possible damage due to lightning strikes. Flooding of a few homes and businesses could occur.”
Head to the Met Office’s website for travel and flooding advice.
Additional reporting by PA