Dozens of flights have been cancelled and a number of schools are expected to close as Storm Callum - the third named storm of the season - barrels up the UK coast.
Forecasters have 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."
Meanwhile the Department of Education has urged schools, colleges and universities to "err on the side of caution" if there is an apparent risk to safety.
What is an Amber weather warning?
Amber means "be prepared".
There is an increased likelihood of bad or extreme weather, potentially disrupting plans and causing travel delays.
Road and rail closures, interruption to power and the potential risk to life and property are also considered probably in an Amber warning.
Be prepared to change your plans and protect you, your family and community from the impacts of the severe weather.
It is more serious than a Yellow warning which means "be aware" and less serious than a Red warning meaning "take action".
Up to six inches of rain could fall on Northern and Western areas will fall at the same time before the deluge tracks westwards into Saturday and beyond.
The wet and windy weather will continue well into the weekend for much of Scotland and Wales as weather warnings remain in place for Saturday.
Strong south-easterly winds will also add to the stormy feel — with a "medium" level of impact expected.
Some rural communities could even become cut off as the floods block smaller B-roads.
Met Office chief meteorologist Frank Saunders, said: “Strong winds at this time of year can increase the rate of leaf fall which can potentially block drains or culverts and, with the heavy rainfall expected over Friday and Saturday, could well heighten the potential for flooding.
"There is also a risk that the high winds associated with Storm Callum, combined with high tides, may lead to some coastal impacts due to large waves.”
It comes after Wales saw the worst of the rain on Thursday, with 25mm in just one hour - equivalent to a little under what would normally fall in an entire week.
Pembrey Sands in South Wales saw a total of 32mm over 12 hours while Scolton Country Park in Pembrokeshire was not far behind with 30.2mm.
Wales usually sees an average rainfall in October of 169.6mm and a monthly average of 121.7mm.
The wet weather is set to come on the tail-end of Tropical Storm Leslie — which is currently helping sweep up warm air from the Canary Islands to Britain.
Despite the storms, temperatures have been much higher for this time of year than usually expected during Autumn.
The last time temperatures exceeded 24C at this time of year was in October 2011 when a staggering 29.9C was clocked.