The Met Office has issued a yellow warning for the city with the skies to open from around 9am and continue to pour until well into the evening.
A weather warning has been issued for West Kent and East Sussex as the thunder rolls in.
The Met Office warning of rain for Northern Ireland is valid until 8pm on Sunday.
“A low pressure system is expected to run into the UK from the south on Monday morning, bringing areas of heavy rain particularly across some southern areas of England”.
It comes after heavy overnight and morning rain in places. In addition to this there is the potential for strong winds, particularly around exposed coastal areas. There is 40mm to 60mm possible in some places.
Traffic disruption is likely across the south and motorists are urged to take extra care on journeys.
Radar maps show an area of rain moving in from the south.
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.
Firefighters wearing rescue lines and carrying buoyancy aids swam out to the child and brought him safely back to the riverbank.
The day would have a high of just 16 degrees.
More rain is forecast for the south on Tuesday and Wednesday – but the weather is set to improve in time for the Bank Holiday weekend.
A Coral spokesman said: ‘The odds now make for grim reading, with bets flooding in on this year being the wettest in history’.