More than a month's rain is expected to fall over just two days in parts of the UK this week.
A severe weather warning has been issued by the Met Office, with fears of flooding and disruption on Friday and Saturday. Central and northern England is expected to be worst affected, with an amber alert - the second most severe category that can be given - in place for north-east England.
Up to 100mm (3.9 inches) of rain could fall in 36 hours during the downpours. The average UK rainfall for July is 69.9mm (2.8 inches), and 64.4mm (2.5 inches) for the north of England.
And thundery showers are expected to affect large parts of eastern, central and northern England on Thursday.
Met Office chief forecaster Martin Young said: "We are expecting outbreaks of heavy rain across a wide area of the UK, with worst affected spots likely to be in central and northern parts of the country.
"Rainfall totals could be 40-60mm (1.6-2.4 inches) widely across warning areas, but some places could see up to 100mm (3.9 inches) of rain through Friday and into Saturday. Given the saturated ground from the record rainfall in June, this could cause disruption - including difficult driving conditions and flooding in some areas."
People living in the affected areas have been warned to be prepared for flooding.
An Environment Agency spokesman said: "Heavy rain on Friday and Saturday may lead to significant flooding of properties across parts of northern England. We urge people to remain vigilant and prepared for flooding by checking the Environment Agency website and signing up for localised river flood warnings.
"The Environment Agency has mobilised teams across the country to check flood defences, clear any river blockages and closely monitor river levels to help reduce the risk flooding."
In Northern Ireland parts of Co Down were flooded by torrential rain. Down District Council said it had declared an emergency in the area. Newcastle SDLP councillor Carmel O'Boyle said flood water was "several feet deep" in some areas of the town.