According to Windy, the most severe system of the week should arrive on July 22 on the south coast.
Charts from the weather service show a red dot covering Chichester, Portsmouth and Worthing from 8am.
There, rain totals could exceed 20mm (0.7 inches) for three hours until 11am, potentially emptying up to 2.1 inches of rain onto parched land.
The system will gradually move east, becoming larger and thundery as it passes over London and the southeast.