Chichester will start cloudy changing to light rain by lunchtime, the Met Office said. The city will see highs of 11 degrees Celsius before dropping to 3 overnight.
Worthing will see light rain changing to cloudy by lunchtime with a high of 10 and a low of 6.
Horsham will start cloudy changing to light showers in the afternoon with a peak of 10 and a low of 4.
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Brighton and Hastings are forecast to start cloudy changing to sunny intervals by early evening, with highs of 10 and 9 respectively before dropping to 7 and 8 overnight.
Eastbourne will start with sunny intervals changing to cloudy by lunchtime with a high of 10 and a low of 7.