West Sussex County Council is warning residents of intense thundery downpours overnight on Wednesday (22 June).
Residents are advised to be prepared for localised flooding as well as frequent lightning.
Disruption to travel is likely with journey times expected to be increased due to the intense weather.
The amber weather warning from the Met Office advises that torrential rain should be expected, with a chance of hail.
30mm or more of rain could fall in an hour with rainfall in excess of 50mm possible in two to three hours.
The weather is forecast to improve on Thursday.
For more information follow the West Sussex County Council Resilience and Emergency Team on Twitter or register for flood warnings on the Environment Agency website.