A yellow weather warning has been issued across the county.
The Met Office yellow warning for thunderstorms will be in place from 2am to 8am tomorrow morning, August 27.
The alert is in place for both East and West Sussex.
Residents are warned to expect some disruption due to the weather, including flooding and lightning strikes.
The Met Office said there is a chance of delays and cancellations to public transport and power cuts may occur.
Those at risk of flash flooding are advised to consider a flood plan and the public should check road conditions before driving, amending travel plans if necessary.
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The forecaster said: "An area of thunderstorms and heavy rain will move north into southeast England later Wednesday night and into Thursday morning.
"Accumulations of 20-30mm in an hour are possible locally, while 30 to 40mm in 6 hours can be expected fairly widely.
"There is a small chance that some places may see more than 50mm in three hours. Lightning and hail of 1-2cm will also be hazards and, in a few places, strong gusts may also present a hazard."
It comes after weeks of dry and hot weather which earlier in the year saw a rare red weather warning for heat.
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