Britain will endure another unsettled day of showers today before sunshine takes hold and temperatures rise into the weekend with 30C (86F) highs expected.
The Met Office said cloud, rain and drizzle will spread north-eastwards throughout today with some sunshine in central and eastern areas but muggy weather overall.
Temperatures of 22C (72F) are expected in the East, but it will be fresher in the North with 20C (68F). Glasgow will be 18C (64F), Belfast 20C (68F) and Cardiff 22C (72F).
Conditions will then dramatically improve for much of the UK from tomorrow with sunnier and more settled weather as the South East reaches 25C (77F).
Similar highs are expected on Saturday before the influence of Tropical Storm Debby in the US brings very hot weather on Sunday and Monday with 30C (86F) both days.
The Met Office says a warm snap is considered a heatwave if a location records at least three days in a row with maximum temperatures exceeding a designated value.
This is 25C (77F) for most of the UK – rising to 28C (82F) in London. Given that highs of over 28C are expected in London only on Sunday and Monday, before a return to 25C on Tuesday, the warm period can unofficially be defined as a ‘mini heatwave’.
Met Office meteorologist Clare Nasir said cloud and rain will ‘slowly encroach northwards and eastwards’ today and conditions would turn ‘muggy yet again’.
She said low pressure will drift to the north of the UK and eventually clear towards the far South East and the far North East into Friday.
Ms Nasir continued: ‘For the bulk of Scotland it’s a fine start to the day with some sunshine extending towards eastern counties of England, the Midlands as well as the South East.
‘Lots of low cloud for Northern Ireland, some patchy rain here towards the South and more persistent rain just moving in across Wales as well as the West Country.
‘A bit of a breeze to start the day across Irish sea coasts as well as the South West and eventually we’ll see this rain move northwards as well as eastwards.’
The best conditions this morning will be in central and eastern areas, with Ms Nasir warning: ‘Make the most of the sunshine – eventually you’ll see some cloud here.’
She added: ‘But through the morning it is wet across Wales as well as the West Country. Lots of low clouds shrouding the hills and mountains – poor visibility here.
‘That rain will just continue through the morning as it pushes towards southern parts of Scotland, moving in across the Midlands, central southern England.’
Ms Nasir said the rain ‘could pep up around lunchtime across northern England’ and ‘it remains rather murky across the West as well as the South as we head through the afternoon’.
Looking ahead to this evening, she said: ‘The rain could become more intense and a bit more persistent across the central and southern swathe of Scotland.
‘Still rather murky across Northern Ireland – lots of low cloud and some patchy rain across England and Wales. We’ve lost the persistent rain though – it’s clearing towards the North Sea. A bit of a breeze still across the South West as well as the West.’
Through tonight and into early tomorrow morning, a mass of cloud and outbreaks of rain will continue travelling eastwards and northwards.
Ms Nasir said: ‘You will see some wet weather overnight across Scotland as the wind picks up some strength and it’s murky and humid across England and Wales with that low cloud and patchy rain.’
But she added that a ‘clearance’ will be coming in early tomorrow morning, and the wind then will pick up strength across the North and the West of Scotland, clipping the far north of Northern Ireland with some showers.
Ms Nasir continued: ‘We hang on to some dampness first thing towards the South East but the sun will come out. So a brighter afternoon to come on Friday.
‘Breezy in the north with frequent showers and some drier skies further South and East with some sunshine. Temperatures on the up here, 21C to 25C.’
The Met Office said an ‘increasingly hot and humid air mass’ will then push across the UK’s southern half this weekend.
Tropical Storm Debby, which is currently hitting eastern states of the US, is helping to strengthen the jet stream and make it ‘meander’ over the Atlantic which will allow hot air to move into the UK from Sunday.
Temperatures are expected to get to at least 30C (86F) in the South and South East of England and there is even a 30 per cent chance of the mid-30Cs (mid-90Fs).
This would bring the hottest day of 2024 so far, which is currently July 30 when 32.0C (89.6F) was recorded at both Kew Gardens and Heathrow Airport in London.
Ms Nasir said: ‘Through Saturday, there will be a few showers in the mix across the North and the West but still the clouds will be broken with some sunshine and temperatures of 21C to 25C.
‘Turning very warm on Sunday from the South and again with some sunny spells coming through although we could see some clouds and also some outbreaks of rain across the far North and North West.’
Forecasters warned of thunderstorms or heavy showers falling further north and west within the warm air mass, and in eastern areas on Monday.
A very warm night is expected on Sunday night into Monday – although by Tuesday the hot air mass will likely become displaced by fresher conditions.
The weather will then become unsettled again, with occasional Atlantic frontal systems or showers – but it will also feel pleasant in between these systems.
Met Office chief meteorologist Dan Suri said: ‘Tropical Storm Debby in North America is helping to strengthen the jet stream, and is causing it to meander over the Atlantic.
‘This will allow hot air to move into the UK later this weekend, and early next week. Temperatures are likely to reach the low 30Cs in places, notably in the South and South East.
‘Further north and west within this warm air, outbreaks of heavy showers or thunderstorms are possible. As well as the hot daytime temperatures, a very warm night is also expected on Sunday night into Monday.’
Debby made landfall along the Gulf Coast of Florida on Monday morning as a Category 1 hurricane and has already dropped up to 400mm (15in) of rain over parts of the state.
At least six people have died due to the storm , five of them in traffic accidents or from fallen trees.
Debby will continue to impact eastern and southeastern parts of the US over the coming days, with areas of Georgia and North and South Carolina set to bear the brunt.
Debby is likely to weaken and gradually break up over the weekend and into the start of next week, but its influence will play a part in the UK’s conditions.
Met Office deputy chief meteorologist Nick Silkstone said: ‘While Debby itself won’t reach the UK, the system and its remnants does play a role in the UK forecast next week, but chiefly as background influence on the shape and strength of the jet stream, which often plays a significant role in the weather we experience in the UK.
‘Debby in North America will help to strengthen and veer the direction of the jet stream and this means this ribbon of air is likely to shift further to the south.’
‘This southwards dip in the jet stream will likely be located across the mid-Atlantic this weekend and early next week, allowing south-westerly flow across the UK.
‘During this time a hot and perhaps very hot airmass could develop and be advected into the South East of the UK.
‘On balance it now looks likely that temperatures will reach 30C, with around a 30 per cent chance that temperatures could reach the mid-30Cs early next week. Debby plays a role in developing this forecast hot spell.’
Bookmaker William Hill is offering odds of 9/4 for the UK to record its hottest day of the year so far this month.
Its spokesman Lee Phelps said: ‘It looks like we’re set for another fiercely hot weekend in the United Kingdom and the odds suggest the current temperature record for 2024 could be broken.’
And rival bookmaker Labrokes is offering 10/11 that this month will be Britain’s hottest August on record.
The top temperature in the UK yesterday was 23.9C (75.0F) at Manston in Kent.