The Met Office has issued weather warnings for thunderstorms and has forecast potential flooding and strong winds in some areas.
The first yellow warning covers much of Wales and the west of England, including Bristol, and will be in place from 1.30pm today (Tuesday, June 16) until 6am tomorrow morning.
A second warning will then be in effect from 12pm tomorrow (Wednesday, June 17) until 9pm in the evening, with storms set to hit Bristol in particular at around 1pm.
Forecasters have warned the turbulent weather could cause flooding, damage to buildings, hazardous driving conditions and power cuts.
A spokesperson for the Met Office said: "Flooding of homes and businesses could happen quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds.
"Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services.
"Spray and sudden flooding could lead to difficult driving conditions and some road closures. Some communities might become cut off if roads flood.
"Power cuts might occur and other services to some homes and businesses could be lost."
Tomorrow's forecast for the south west says: "After patchy fog clears, Wednesday morning will be largely dry with bright or sunny spells.
"The afternoon will see a mix of sunshine and heavy, possibly thundery showers. Feeling warm. Maximum temperature 22C."
The outlook for Thursday to Saturday then states: "Remaining unsettled Thursday with locally heavy, perhaps thundery rain or showers.
"Somewhat drier Friday with a mix of sunny spells and showers. Thicker cloud and some rain likely on Saturday."
