Heavy rain is forecast and a thunderstorm warning has been issued but the weekend will be brighter with some areas potentially hitting 30C (86F), the Met Office has said.
A yellow thunderstorm warning has been issued for Northern Ireland on Monday between 1pm and 8pm as 15 to 20mm of rain could fall in less than an hour.
Elsewhere, sunshine is expected for most of Monday but with some cloud and scattered showers in the North West, said Stephen Dixon, spokesman for the Met Office.
Overnight into Tuesday there will be heavy rain moving from the south of England to the North East, and southern and western parts of England could see 30mm of rain which would make for “uncomfortable driving conditions”, Mr Dixon added.
Wednesday will be more of a showery day rather than persistent rain, but drier in the South East.
As of Thursday there will be some showers but temperatures will start to rise towards the weekend.
Mr Dixon said: “By Friday there’s some possibility of rain in Northern Ireland, the west of England, it will be generally drier in the South East.
“The weekend could get up to the high 20s or low 30s, the South East will see the warmer weather.
“The week will be sitting relatively warmer for this time of year but more subdued than we’ve seen, but areas will hit heatwave criteria as we get to the weekend.”
The hottest temperature of the year so far was 32.2C (89.9F) recorded on June 10 in Surrey, but the forecaster said the weather is not likely to reach that level this week.
As of Monday morning, one flood warning is in place for the River Cole at Coleshill, Birmingham, from Cole End to Coleshill Industrial Estate.
And a total of six flood alerts have been issued, meaning flooding is possible.
