The government alert affects the Ravensbourne area in the boroughs of Lewisham, Bromley, Greenwich and Croydon as river levels are set to rise this afternoon (Tuesday, October 8).
The rainfall is predicted to clear in the early hours of the morning on Wednesday (October 9).
This includes the River Quaggy, Kyd Brook, Chaffinch Brook and Pool River.
Flooding is forecasted to impact low lying land near the river this afternoon and overnight.
While property flooding is unlikely, the government is prepared to issue warnings is needed.
It is recommended that motorists plan their driving routes to avoid low lying roads near rivers, which could be flooded.
People should not try to walk around or drive through flood water.
A spokesperson said: “We are closely monitoring the situation.
“Our incident response staff are out on the ground checking and clearing any blockages and debris from the river.
“Please stay aware in case further warnings are issued.”
The Met Office forecasts that weather will be dry with sunny spells at first, but showers will soon spread northwards across all parts from late morning.
The showers will become heavy and possibly thundery in the afternoon and evening.
There is a risk of hail.
Hour-by-hour forecast for the next two days
Tuesday, October 8
1pm – 17 degrees, 50 per cent chance of rain
2pm – 18 degrees, 70 per cent chance of rain
3pm – 17 degrees, 70 per cent chance of rain
4pm – 17 degrees, 90 per cent chance of rain
5pm – 16 degrees, 90 per cent chance of rain
6pm – 15 degrees, 9- per cent chance of rain
7pm – 15 degrees, 90 per cent chance of rain and thunder
8pm – 14 degrees, more than a 95 per cent chance of rain, thunder and lightning
9pm – 14 degrees, more than a 95 per cent chance of rain, thunder and lightning
10pm – 14 degrees, more than 95 per cent chance of rain
11pm – 14 degrees, more than 95 per cent chance of rain
Wednesday, October 9
1am – 13 degrees, 60 per cent chance of rain
3am – 13 degrees, 30 per cent chance of rain
4am – 13 degrees, less than five per cent chance of rain
5am – 13 degrees, less than five per cent chance of rain
6am – 13 degrees, less than five per cent chance of rain
7am – 13 degrees, less than five per cent chance of rain
8am – 12 degrees, less than five per cent chance of rain
9am – 13 degrees, less than five per cent chance of rain
10am – 13 degrees, less than five per cent chance of rain
11am – 14 degrees, less than five per cent chance of rain
12pm – 15 degrees, 10 per cent chance of rain
1pm – 15 degrees, 30 per cent chance of rain
2pm – 16 degrees, 40 cent chance of rain
3pm – 16 degrees, 30 per cent chance of rain
4pm – 16 degrees, 30 per cent chance of rain
5pm – 16 degrees, 30 per cent chance of rain
6pm – 15 degrees, 40 per cent chance of rain
7pm – 14 degrees, 30 per cent chance of rain
8pm – 13 degrees, 20 per cent chance of rain
9pm – 13 degrees, 20 per cent chance of rain
10pm – 12 degrees, 20 per cent chance of rain
11pm – 12 degrees, 20 per cent chance of rain
