Today will see clear blue skies and sunshine for the majority of the day, with temperatures reaching up to 23C.
However from around 4pm, light winds and some cloud is forecast.
From 6pm, a yellow warning is in place for thunderstorms across the area.
Some light rain showers can be expected from around 8pm, and from 9pm Sussex is expected to see thundery showers.
It will remain warm and there will be high humidity.
Hail could even accompany gusty winds across the area before midnight, according to the Met Office.
The yellow warning is in place until the end of Thursday.