Large parts of Britain, including East Anglia, face a summer soaking on Saturday with some areas expected to see nearly half of an entire month's rain fall in just an hour.
Severe weather warnings have been issued covering much of the country amid fears of flooding.
The Environment Agency has issued a total of 104 "low risk" flood alerts.
The Met Office said thundery rain is expected to start falling early on Saturday morning, with showers unlikely to ease off until the evening.
- Click below to see the predicted rainfall forecast sequence
Some places could see between 0.8in (20mm) and 1.2in (30mm) of rain fall in an hour - almost half of the UK monthly average for the whole of June of 2.9in (73.4mm).
Met Office spokesman Dan Williams said the downpours will be "pretty heavy".
He said: "We are looking at the potential for some localised flooding because they are potentially heavy enough that we could see so much rain fall in a short space of time that it can't drain away fast enough."
Regions where yellow "be aware" warnings for rain are currently in place include the East Midlands, the East of England as well as South East England.
The predicted deluge is down to warm, humid air moving in from Europe which creates a risk of downpours when it mixes with cooler air over the UK.
East Anglia has just suffered its **sixth wettest May since records began in 1910. According to Met Office figures 88.0 mm (3.5 inches) of rain fell in the region in May 2014 the most since the wettest May ever in 2007 with 118.0 mm (4.6 inches).
The Met Office said the mercury could rise as high as 25°C (77°F) on Friday and Saturday in London and the South East - not far short of the warmest temperature of the year so far, 26.3°C (79.3°F).
East Anglia just just enjoyed its second warmest spring on record during March, April and May.
Mr Williams said: "It is going to be one of those warm but humid days where you could see some bright spells but also some heavy showers as well."
He said the downpours will ease by Sunday when there will be some "good, dry and bright spells", particularly in southern and eastern parts of the country and only a risk of light rain in some areas.
Next week is likely to be "slightly unsettled" with some dry and bright conditions along with light showers, he added.