The Met Office fears up to 1.5 inches could fall in some areas, potentially closing roads and leaving communities marooned.
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The impending deluge has triggered two yellow warnings for rain across the South East and South West of England this weekend.
South London, Kent, Dover and Hastings are covered by one of the alerts, in place from 3pm on Sunday until 9am on Monday.
Exeter, Devon and Plymouth have also had a warning issued from 6am to 6pm on Sunday.
The Environment Agency had issued 62 flood warnings and 168 flood alerts in England by Saturday evening.
And the Met Office’s Greg Dewhurst warned of an “unsettled” day as an area of low pressure moves across the south of the UK on Sunday and into Monday.
He added: “It will bring some heavy rain and some strong winds as well.
“This rain is falling on saturated ground and could lead to some localised flooding problems.
“Take care if you are travelling through the day on Sunday across southern counties.”
Sunday’s temperatures are set to reach around 7C-9C and there should be generally light winds but it will feel chillier.
Temperatures plunged to minus 6C on Friday night leaving much of the country waking up to freezing conditions with “patchy mist and fog in places“.
Chilly conditions settled in from the north leaving frost on the ground – with early risers forced to de-ice their cars due to the cold blast.
What to do when the road is flooded
If the road is flooded, turn around and find another route
The number one cause of death during flooding is driving through flood water, so the safest advice is turn around, don’t drown.
Although the water may seem shallow, just 12 inches (30cm) of moving water can float your car, potentially taking it to deeper water from which you may need rescuing.
Flood water also contains hidden hazards which can damage your car, and just an egg-cupful of water sucked into your car’s engine will lead to severe damage.