Parts of the UK have had more than fortnight's rainfall in the past 24 hours and there is more to come as heavy thundery downpours sweep across the country, forecasters said.
Traffic disruption is possible as low pressure moves west across England, the Met Office said, after the South West, Wales and Northern Ireland saw a day of dismal weather.
Katesbridge in Co Down was the wettest part of the country with 56.6mm falling in the 24 hours to 5am, the Met Office said. Its monthly average is 97mm.
There were also heavy rain in Exeter (33.2mm), Bournemouth (26.4mm) and Gogerddan in south west Wales (25.8mm) as a heavy band of thundery showers moves west.
The south east of England and East Anglia are likely to suffer the worst of the rain today, a Met Office spokesman said, as the front bringing it stalls because of a lack of wind.
A yellow warning for rain is in place across the south of England valid until 10pm and the Environment Agency has 10 flood alerts in place, five in the south west and five in Wales.
Possibility of disruption
A Met Office spokesman said an area of low pressure moving through the south of the England and would affect an area south of a line between the Wash and Cardigan Bay in Wales.
"It is already bringing heavy showers to the south coast from Portsmouth westwards," he said.
"It is heavy rain that will become thundery in nature through the course of the day.
"We could see another 20mm of rain, so for a lot of areas that is a third of a month's rain. There is 40mm to 60mm possible in some places."
He added: "There is the possibility of disruption."
The rain will not move west into the North Sea until around midnight, he said.
Away from the south, most other parts of Britain should have a mostly clear day with some sunshine apart from parts of north east Scotland.
More rain is forecast for southern Britain on Tuesday and Wednesday before the weather improves into the bank holiday weekend, the Met Office spokesman said.