The agency advised people on the roads this weekend to keep at least double the usual separation distance from other vehicles and reduce their speed.
It comes as more bad weather is expected the day after parts of the southeast suffered a month of 24-hour precipitation.
The Met Office predicted that rain outbreaks would hit the south late Saturday afternoon through Saturday evening, with the Environment Agency issuing 25 flood alerts, where flooding is possible.
These alerts apply to London and surrounding areas, Kent, parts of the Home Counties, the West Midlands and Wiltshire.
It will be largely dry in most of the UK during the day as temperatures are expected to range from 19 ° C (66.2 ° F) in London and Cardiff to 18 ° C (64.4 ° F) to Belfast and 17 ° C (62.6 ° F) in Edinburgh. A few sporadic showers are also expected in Scotland.
“Big thunderstorms will erupt in northern France this afternoon, and it is possible that they will cut across south-east England on Saturday and overnight through Sunday,” said Steven Keates, meteorologist at Met Office, at the Telegraph.
“Monday is shaping up to be humid, especially in light of further thunderstorms across France which could return to southwest England.