Dozens of flights have been cancelled and a number of schools are expected to close as Callum – the third named storm of the season – barrels up the UK coast.
Forecasters issued an amber “danger to life” warning as the weather front is expected to bring in 70mph winds from the Atlantic.
The Met Office says Wales will bear the brunt of the storm with heavy rain causing fast-flowing and deep floodwaters, which are likely to make conditions difficult.
Yellow warnings are in place for North East England, North West England, South West Scotland, Lothian Borders, South West England and Yorkshire & Humber.
Meteorologists say a “zone” of heavy, slow-moving rain will affect South Wales and the south west of England and those areas could see up to 4in by the end of the day.
Powerful gales are set to batter the coast in the early hours of this morning, before easing off and then strengthening again to 70mph in the afternoon.
The upgraded Met Office warning means that flooding caused by torrential rain and leaves blocking drains is more likely than first thought.
Flybe has already cancelled 25 flights in and out of Belfast City Airport due to weather conditions and all passengers are advised to check before travelling.
They include planes setting off from London, Exeter, Manchester, Birmingham, Newcastle, Southampton, Leeds, Cardiff and Edinburgh.
A spokesperson said: “Due to the high winds forecast for Friday 12 October, all Flybe operated flights departing from Belfast City Airport from 6am to 9.30am have been cancelled, apart from flight BE731 to Leeds Bradford and BE680 to Edinburgh which have been delayed until later that day.
“Customers affected by the cancellations should not travel to the airport.
“The safety of its passengers and crew is our number one priority and Flybe will, as always, do everything possible to minimise any disruption.”