Torrential rain is set to cause flooding, travel disruption and could even leave some areas 'cut off' this weekend as a month and a half's worth of rain falls in just a few hours - but not before another blast of heat today.
The Met Office is predicting as much as 4in (100mm) of rain in parts of southern England, the Midlands and Wales during tomorrow and Sunday - while in comparison, the UK average rainfall for all of August is 2.8in (69mm).
It comes as sweltering temperatures will only fall slightly today despite England's unprecedented extreme heat warning coming to an end - after six days in a row of the mercury getting above 88F (31C) somewhere in the UK.
Temperatures in the UK are expected to be highest today in Northern Ireland, which has now broken its all-time heat record three times in a week and could do so again today, after Armagh hit 88.5F (31.4C) yesterday.
That made it the hottest place in Britain - and similar highs are expected again there today, as well as in North Wales and North West England. The UK's hottest day of 2021 so far was Tuesday when London got to 90F (32.2C).
The RAC predicts today will be one of the busiest on the roads this year, with an estimated 2.3million leisure trips being made by drivers as 'staycation getaways' begin, warning: 'Journey times are likely to be severely affected.'
The morning's sun bursts through the clouds above the church at Dunsden in Oxfordshire today
A golfer playing from a bunker at the Cazoo Wales Open at the Celtic Manor Resort in Newport, South Wales, this morning
As a result, call-outs to breakdown services are likely to 'rocket'. The RAC points to data from transport analytics experts Inrix, which predicts a 29 per cent increase in delays today compared with typical July travel times.
Looking ahead to this weekend, the Met Office's rain warning covering the whole of Saturday and Sunday said flooding of homes and business could happen as heavy rain falls on ground baked hard by days of warm sunshine.
The warning extends across most of England and Wales, except north-west England and north Wales, Cumbria, North East England and northern parts of Yorkshire.
The Met Office said: 'There is a small chance that some communities will become cut off by flooded roads.' Forecasters also warned of 'power cuts and loss of other services to some homes and businesses'.
Meanwhile, spray and standing water could lead to 'difficult driving and some road closures', while bus and train services could be hit by 'delays and cancellations'.
The Met Office said today is likely to be mostly fine, dry and warm. It said: 'Low cloud is due to clear to leave another very warm or hot day with sunny spells. Isolated thundery showers may develop in the west before rain arrives into the southwest later.'
Speaking about the extreme heat warning that was in place in England, Met Office meteorologist Alex Burkill said: 'It's an amber warning so it's quite an extreme one, and it's for extreme heat. So whilst temperatures aren't plummeting tomorrow, they are still going to be on the high side, it's less likely to cause significant impacts like we've seen over the past couple of days.'
The heat warning remains in place in Northern Ireland, and speaking last night, Mr Burkill said: 'Temperatures there could be even higher than today. So for a few days we've had their highest ever temperature.
'It was beaten yesterday, it was beaten again today, and it could well be beaten again tomorrow. So I think it could be four days within a week where they've recorded their highest ever temperature.'
Northern Ireland yesterday broke its all-time highest temperature for the third time in six days, with 88.5F (31.4C) in Armagh beating the 88.3F (31.3C) at Castlederg on Wednesday and 88.2F (31.2C) at Ballywatticock on Saturday.
Before then, a high of 87.4F (30.8C) had stood for decades after being reached on both July 12, 1983 and June 30, 1976.
But in a grim outlook for the weekend, forecasters added: 'Heavy showers and thunderstorms developing across southern parts over the weekend, then edging further north by Monday.'
In its outlook for the first week of the school holidays, an 'unsettled start' is predicted by the Met Office, 'with most places seeing showers developing by day on Tuesday, these possibly thundery at times'.
Its long-range forecast said: 'Conditions remain changeable mid-week with outbreaks of rain moving in from the west, further thundery showers in between any drier interludes and strong winds for the time of year.
'From the end of next week until early August, we will likely see changeable conditions with a mix of sunny spells and heavy showers, perhaps with some more prolonged rain at times.'
There is unlikely to be any immediate return to heatwave conditions, with temperatures only reaching the low to mid 20s Celsius (70Fs) over the weekend and through next week.
But the Met Office said it could become 'warm' again into August. Yesterday, the warmest place in England was 87.9F (31.1C) at Ross-on-Wye, Herefordshire, not quite breaking the country's record for 2021.
But yesterday was the hottest day of the year so far in Scotland, Wales and Northern Ireland. The highest temperature in Scotland was 84.7F (29.3C) at Threave, Kirkcudbrightshire; 88.1F (31.2C) at Gogerddam in Dyfedd, Wales, and 88.5F (31.4C) at Armagh, Northern Ireland.
The conditions have been marred with at least 18 deaths involving people cooling off in seas, lakes, rivers and reservoirs.
In a further incident involving people diving into water to cool off, a search was underway yesterday evening in Chester for a teenage boy who failed to surface after jumping into the River Dee off the city's Chain Bridge.
The Royal Life Saving Society UK (RLSS UK) said it was aware of 17 incidents of accidental loss of life in the water between July 17 and 20.
