Thunderstorms are expected to develop across the south coast from Tuesday before continuing overnight and into Wednesday.
The Met Office said the storms could bring torrential rain, hail and lightning, with potential disruption to travel and flooding.
They were rated on the third level out of four on the Met Office's impact scale.
Sussex residents can expect several thunderstorms to merge into areas of 'prolonged and widespread rain' on Tuesday night and Wednesday morning.
Predominantly around 15 to 30mm, with up to 50mm in a few locations.
After a drier interlude, the Met Office said, further thunderstorms may develop during Wednesday afternoon, with rain and flooding likely to be the largest impact but hail, lightning and gusty winds could also make an appearance.
The Met Office has issued the following guidance on what to expect:
- There is a small chance that homes and businesses could be flooded quickly, with damage to some buildings from floodwater, lightning strikes, hail or strong winds
- Where flooding or lightning strikes occur, there is a chance of delays and some cancellations to train and bus services
- Spray and sudden flooding could lead to difficult driving conditions and some road closures
- There is a slight chance that power cuts could occur and other services to some homes and businesses could be lost