A heatwave could hit parts of the UK this week, but heavy rainfall and thunder are expected to follow.
Temperatures are set to increase on Thursday, Friday and again on Saturday when they will potentially hit 29C, the Met Office said. But cloud, rain and thunderstorms will sweep in over the weekend, and the nights will become humid.
The Met Office added it expects the hottest temperatures of the year so far to be reached by the weekend.
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The threshold for a heatwave is different across the UK, but in some areas, it is classified as hitting highs of 25C to 28C across a three-day period. Simon Partridge, forecaster for the Met Office, said: "It will get warmer but there may well be more cloud with heavy, thundery showers mixed in as well.
"There will still be plenty of sunshine around, but it will come with much muggier nights."
There will not be a significant change in the weather over the next few days, Mr Partridge said, but from Thursday Storm Oscar, which is currently across the Canary Islands, will push the high pressure further east, causing the mercury to rise to 25C or 26C.
From Friday it will get warmer again. The warmest temperature expected on Friday is 26C or 27C, likely in central and southern England.
Saturday will be the warmest day, with forecasters expecting it to reach 27C or 28C. However, the Met Office said they are unsure as to whether this warm weather will continue.
They said: "We have to acknowledge that the chaotic nature of our atmosphere means it is not possible to predict the weather on any particular day months ahead.
"With many variables there are unavoidable limitations to what we can predict, particularly during the summer months when the influence of global weather drivers on the UK is small so confidence in outlooks at this time of year is lower than, say the winter....
"The elevated risk of a warmer than average summer this year, is consistent with wider global warming trends and the UK’s warming climate, after all, four of the five warmest summers on record for England have occurred since 2003.
"Nevertheless, looking at the figures you can see a near average summer is also still a possibility. "
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