A flood alert has been issued for Ifield Brook, the Upper River Mole and Gatwick Stream.
The Environment Agency issued the alert at 4.30pm and said river levels were expected to rise throughout this evening in response to persistent and heavy rainfall throughout this morning and this afternoon.
Between 15 and 20mm of rain has fallen today and there are further scattered showers forecast for the rest of the evening.
An Environment Agency spokesman said: “Flooding of low lying land and roads is expected.
“River levels are currently expected to peak at Horley late Friday evening.
“If the rainfall this evening is heavier, some property flooding could be possible.
“We are closely monitoring the situation and have teams on the ground working to reduce obstructions and maintain flood defences.”