The warning was put in place at 4pm and lasts until midnight.
The Met Office said: “Many places staying dry, but isolated thunderstorms may cause some travel disruption.”
It comes after temperatures of around 27 degrees were reached around lunchtime in many parts of the county.
Both sun seekers and traffic wardens were kept busy in Worthing – see more here
In Hastings visitors and residents flocked to the beach – see more here.
Meanwhile Brighton and Hove council said it was ‘concerned’ about the number of visitors who had arrived in the city for the day, urging anyone who was considering visiting not to travel there.