Chichester will start with hail showers changing to light rain by lunchtime, the Met Office said. The city will see highs of 18 degrees Celsius and lows of 14.
Worthing starts with heavy rain changing to cloudy in the afternoon with a high of 18 and a low of 15.
Horsham will start with heavy rain changing to cloudy in the afternoon peaking at 18 before dropping to 14 overnight.
Brighton and Eastbourne will see heavy rain changing to sunny intervals in the afternoon with highs of 18 and lows of 15.
Hastings is set to start cloudy changing to heavy rain by lunchtime with a high of 18 and low of 15.