The warning is in place from 11 am until midnight on Sunday.
The Met Office said on Sunday 30 August:
Showers, heavy and thundery, will reach southwest England and Wales later Sunday morning, before spreading and developing more widely during the afternoon.
Whilst some areas will miss the heaviest showers, where they do occur 10-15 mm of rain is possible in less than an hour, with 30-40 mm possible over several hours, these larger totals most likely for parts of north and northwest England and southwest Scotland.
Hail, frequent lightning and, in southern parts of England and Wales, wind gusts of 35-45 mph will be additional hazards. Heaviest showers and thunderstorms will become more isolated later in the evening.
