Flood warnings have been issued for homes and roads in parts of Crawley.
Properties in Ifield and Lowfield Heath could be affected because of rising nearby river levels following heavy rain.
The alert is being sounded by the Environment Agency which says that flooing is expected at both Ifield Brook and the River Mole at Ifield, and at the River Mole at Lowfield Heath, near Gatwick Airport.
Officials say that flooding can be expected throughout the day because of rapidly rising river levels following this morning’s heavy rainfall.
An Environment Agency spokesman said: “Flooding is expected to affect properties around Rusper Road and Hyde Drive, with flooding to low lying land and roads also anticipated.
“Further rain showers will continue this afternoon causing the river to remain high, and could cause flooding to continue until the showers clear tonight.”
Residents and businesses are being urged to move possessions from the ground and avoid contact with flood water.
The spokesman added: “We have our staff responding in the area to clear river blockages and reduce water levels around Ifield.
“Our incident room is open and managing the situation.”