The warning now will be in place from 10am on Friday (August 6) until 9am on Saturday (August 7).
It was initially set to end at 11.59pm on Friday.
Forecasters have warned the thundery showers will be slow moving, bringing the risk of localised flooding and difficult driving conditions.
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Where flooding or lightning strikes occur, there is a chance of delays and cancellations to train and bus services
A spokesman for the Met Office said: "Rainfall totals will vary considerably and not all locations will be affected by the heaviest showers.
"However, where the showers do occur there is potential for 20-30mm of rain in a short space of time.
"[D]uring the course of the day there is a chance that some places could see as much as 80-100mm build up."
Here's what to expect:
There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater.
Spray and sudden flooding could lead to difficult driving conditions and some road closures
Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services
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