Greg Dewhurst, a Met Office meteorologist, urged people to “take care” if travelling through the day on Sunday across southern counties.
The weather agency warned there is a chance of delays or cancellations to train and bus services and that driving conditions could be difficult.
A separate yellow warning is in place across parts of the southwest, including Exeter and Truro, until 6pm on Sunday.
The Met Office said flooding is possible, which could cause damage to buildings.
“We are keeping an eye on this area of low pressure as it moves into southern counties of England and Wales over the course of Sunday and into Monday,” Mr Dewhurst said.
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“It will bring some heavy rain and some strong winds as well. This rain is falling on saturated ground and could lead to some localised flooding problems.”