Flooding has begun to impact areas of the county as a yellow weather warning is under way.
Worthing has been hit by floods this evening as the heavy downpours begin.
Pictures show water overflowing rapidly from drains with burst lids, while police have closed Newland Road which the AA has reported as "impassable".
One video shows a man wading through flood water with a walking aid and wellington boots.
The Met Office weather map showed that Worthing and Chichester could be the hardest hit this evening, with as much as 16mm per hour potentially hitting Worthing.
Surface water has also been reported by the AA on the A27 Sompting Bypass westbound at Upper Brighton Road, with drivers warned to "approach with care".
A Met Office spokesman said: "Outbreaks of rain, heavy at times with a risk of thunder, are expected to develop later on Monday afternoon then move northwards through the evening and overnight.
"15 to 20 mm rain may fall in less than an hour, with a few places seeing up to 40 mm in 2 hours but perhaps more widely within a longer period overnight.
"There is a slight chance of power cuts and loss of other services to some homes and businesses.
"There is a small chance that homes and businesses could be flooded, causing damage to some buildings."
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