A total of 15-25mm of rain is likely across the warning area, but 40-60mm may fall in some places, most likely over the Downs, according to the Met Office.
Rain will clear to the east by the end of the day, it added.
“Give yourself the best chance of avoiding delays by checking road conditions if driving, or bus and train timetables, amending your travel plans, if necessary,” a Met Office spokesperson said.
“People cope better with power cuts when they have prepared for them in advance. It’s easy to do; consider gathering torches and batteries, a mobile phone power pack and other essential items.”
For those at risk of property flooding, the weather service recommends preparing a flood plan and an emergency flood kit.
It also said weather warnings may change quickly and recommended staying up to date with the weather forecast in your area.
