It comes as Merryfield in Somerset recorded highs of 36.7C – the hottest June day on record and the hottest day of the year so far for the UK.
Exeter meanwhile saw highs of 33C on Thursday while temperatures of 29C were recorded in Bodmin and 28C in Plymouth.
The Met Office said scattered showers and thunderstorms were expected to move northeastwards during Thursday evening and overnight.
While many areas will miss them, forecasters noted the storms could produce sudden winds of up to 96.56kmh (60mph), frequent lightning, hail and heavy rain in Cornwall and Devon.
Jersey Met, external issued a yellow thunderstorm warning from 17:00 BST for the Channel Islands. In Devon and Cornwall, the worst of the heavy downpours is expected between 20:00 and 22:00 BST.
RNLI regional water safety lead Steve Instance said the charity was concerned about the potential for dangerously strong winds.
He said: “It is likely that these winds will develop suddenly, bringing with them stormy conditions which may not always be obvious from the beach and catch water users off guard.
“Our advice to those using watercraft such as kayaks and paddle boards is to be aware that conditions can change quickly and to carry a means of calling for help.
“Equally, during periods of lightning, it is not safe to enter the water, instead people should exit the water and seek safe shelter immediately.”
