Four flood alerts have been issued across the county.
Reisdents have been told to "be prepared" by the Environment Agency as "flooding is possible".
This means people should have a bag of medicines and important documents ready in case they need to leave their home.
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The areas where flooding is possible includes Patcham, Aldingbourne and Barnham Rifes, West Dean, Singleton, Charlton, East Dean, Chilgrove, and the Western Rother.
The Met Office have warned that between 15 to 20mm of rain could fall within a three hour period today.
A yellow weather warning for wind and rain is in place until later this evening.
People have been warned to expect groundwater flooding and travel delays.
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