The national weather service has said that many areas of the country will see between 10 and 15mm of rainfall in less than six hours, while some locations could experience as much as 50mm over the same period.
The Met Office’s yellow weather warning for rain, which will last from 3am until 2pm, stretches from Torquay in the south-west to Leicester in the Midlands, covering all of the south-east too.
Luke Miall, a meteorologist at the Met Office, said: “On Wednesday, there will be a lot of heavy rain around – particularly central, southern and eastern parts of England.
“Where we see the rain, that could give the risk some pretty difficult travelling conditions and the risk of some flooding.”
The Environment Agency has issued four flood alerts in England, meaning that flooding is possible in those locations, two of which are in Cornwall.
As well as experiencing heavy showers, coastal areas in Hampshire, Kent, Sussex, Essex and Suffolk will be hit by strong winds, which could reach speeds of 65mph.
The Met Office has put in place a yellow wind warning for parts of these counties from 7am until 5pm, as transport delays are likely and coastal communities could be affected by spray and large waves.
Storm Barbara, which was named by Spanish meteorologists, will pull away from the UK by Wednesday evening, heading off into the North Sea towards Scandinavia.
Although there will still be showers on Thursday, conditions will be generally brighter and will give a “bit of respite” after a “fairly unsettled” start to the week, according to Mr Miall.