Temperatures are set to hit as high as 25C in parts of Northern Ireland today - and the warm weather is set to last.
It'll be dry with bright spells on Monday, feeling very warm.
The Met Office says: "Any low cloud along the east coast will clear during the morning.
"Otherwise, a dry and very warm day with bright spells but also a lot of high cloud at times. Freshening southerly winds along the Down coast later. Maximum temperature 25°C."
Tomorrow, will be "a mostly cloudy morning with outbreaks of rain, drier and brighter conditions reaching western counties later in the morning then spreading to all parts in the afternoon."
It's set to feel cooler with a maximum temperature of 20°C.
And for Wednesday to Friday in Northern Ireland, the Met Office says: "Bright spells and showers on Wednesday with brisk westerly winds. Mainly dry and bright on Thursday with lighter winds. A chance on further showers on Friday. Rather warm."
Elsewhere, highs of 33C are expected in some areas on Monday afternoon, with central, southern and eastern England all experiencing the rising temperatures during the heatwave.
It could mark the hottest day of the year so far, with the UK's record high for this year currently standing at 32.7C. It was recorded at Heathrow on June 17.
The warm weather is expected continue through the week in the high 20s for most until the weekend, when the mercury may rise again to 31C in places such as London, Reading and Oxford.
Met Office forecaster Greg Dewhurst told the PA news agency that England and Wales will experience sunshine on Monday, while Scotland and Northern Ireland are expected to have a cloudy day with warm temperatures.
"For most of us it will be a dry and sunny day, well into the high 20s from the morning, with central, southern and eastern parts of England to possibly see maximum highs of 33C," he said.
"So it's very possible tomorrow will be the hottest day of the year so far, it will certainly be on par with the existing record, maybe more.
"It will also be very warm overnight going into Tuesday, remaining in the low 20s in cities, so many may experience an uncomfortable night."