Showers are forecast from around early evening in Horsham, Crawley and Chichester.
Further towards the east of the county, the weather is expected to be cloudier but dry towards the end of the day with a ten per cent chance of rain eastwards from Worthing.
Temperatures are set to stay warm at around 19 to 20 degrees, falling to a low of ten degrees at night.
The Met Office is advising of a ‘medium’ pollen count today.