Homeowners could be at risk of sudden flooding and power cuts today.
The Met Office has issued a yellow weather warning for thunderstorms.
Heavy showers and thunderstorms will roll in through the day with a risk of hail and thunder amid the rainfall.
Some places may see up to 40mm in two hours, according to the Met Office.
The warning covers most of Sussex, all coastal areas, and will continue until 7pm this evening.
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A spokesman for the Met Office said: "If you find yourself outside and hear thunder, protect yourself by finding a safe enclosed shelter, such as a car.
"Do not shelter under or near trees, or other structures which may be struck by lightning. If you are on an elevated area move to lower ground."
The weather service also warned of spray and sudden flooding which could lead to difficult driving conditions and public transport cancellations.
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