The alert runs from 10am on Friday, August 6, until midnight and predicts slow-moving rain and thunder, with a chance homes and businesses could be flooded quickly and damaged through floodwater.
It says spray and sudden flooding could cause difficult driving conditions and road closures.
Where flooding or lightning strikes occur, there is a chance of delays and cancellations to train and bus services.
The Met Office said: "Within the warning area these showers and thunderstorms will be slow moving and may cause localised surface water impacts.
"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 and during the course of the day there is a chance that some places could see as much as 80-100mm build up.”