Thursday could be the warmest day of 2017 so far, with temperatures of 22C (72F) forecast, double the average for March.
The south and east of the country will see some sunshine and highs of 22C (72F) forecast in the London area, 20C (68F) in Norwich and 18C (64F) in Bristol and Nottingham.
However, it will be cooler and rainier in the north and west, with a split across the country running from south-west Yorkshire.
Belfast, Glasgow, Aberdeen and Edinburgh will peak at 14C (57F), while the mercury will top out at 13C (55F) in Newcastle.
Met Office forecaster Alex Burkill said: "We are dragging up really warm air from Iberia and with relatively calm conditions in the South East that's what is allowing temperatures to climb quite high."
The warm air mass will edge south and eastwards on Friday, but temperatures are expected to remain well above average for southern parts before a wet start to the weekend.
"Saturday is looking quite showery across the whole of the UK, although Sunday will be a pretty decent day for most with calm and bright conditions," Mr Burkill predicted.
In the sunshine, highs of 16C (61F) and 17C (63F) could be seen in the south on Sunday, while further north highs between 11C (52F) and 14C (57F) are more likely.