"Although temperatures are more likely to be warmer than average over the three months, this doesn't mean wall-to-wall sunshine, it can still be warm and cloudy".
Met Éireann has dismissed a prediction that the current spell of warm weather could last into October.
The Met Office said: "While many places will miss the heaviest rain, thunderstorms are likely to affect parts of east and south east England during Tuesday afternoon and particularly into the evening".
And temperatures could edge a degree higher on Tuesday, potentially peaking at 34C (91.4F) in Essex or Norfolk, the Met Office said.
While Wednesday in Brighton looks set to remain dry, Met Office forecasts suggest that there is a significant likelihood of rain on Thursday, Friday, Sunday and Monday.
The three-month outlook shows an increased chance of stable weather conditions, bringing warmer than average temperatures and below-average rainfall.
The outlook - used by planners, businesses and government to start plotting weather-related decisions - shows an increased chance of high-pressure systems close to the United Kingdom, meaning settled conditions are more likely than unsettled weather from our typical Atlantic weather systems.
The mercury is set to rise past 30C (85F) in parts of the United Kingdom on Monday, with 33C (91F) possible in London and to the east of the capital, the Met Office said.
Roku's free, ad-supported streaming service expands to the web
The service is also available on tablets and smartphones, but it appears you have to access it using the mobile browser. Now you can enjoy all the same free movies, TV shows, and live news you get on your Roku player on other devices.
However much of Scotland and Northern Ireland will start the week with some rain.
Maximum temperatures are likely to dip to around the mid-20Cs from tomorrow.
Things are finally returning to normal.
The outlook comes amid Europe's heatwave, with several countries, including Portugal, seeing record-breaking temperatures in recent days.
Thermometers in some areas of the country passed 46C (114.8F) on Saturday.
The Met Office has issued another yellow thunderstorm warning.
The stifling conditions played a part in the deaths of two men, one in Barcelona and the other in the southern Spanish region of Murcia, according to Spanish authorities.