The Met Office has issued rare red weather warnings, meaning there is a danger to life, as Storm Éowyn batters the UK - see a map of affected areas here
The worst disruption is due to hit Northern Ireland and parts of Scotland, with gusts of up to 100mph (161km/h) expected in some coastal areas
Millions of people are being urged to stay at home and not travel during the storm
Widespread travel disruption is expected, with some flights and ferry crossings already cancelled
In the Republic of Ireland, a record gust of of 114mph (183km/h) was recorded at Mace Head, while 560,000 premises are without power
Have you been affected by Storm Éowyn? Send us your stories, pictures and videos if it is safe to do so - details on how to get in touch are here
Watch: Friday's forecast as Storm Éowyn hits the UK
How have you been affected by Storm Éowyn?
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What is a red weather warning?
As we've been reporting, red weather warnings for wind are in place for Northern Ireland until 14:00, and Scotland's central belt from 10:00 until 17:00 today.
Red is the most serious weather warning the Met Office can issue, meaning dangerous weather is expected and people are urged to take action to keep themselves and others safe.
A red weather warning has been issued for the Isle of Man, which lies between England and Northern Ireland.
A post on its government website says the warning for "violent storm force winds" is in place until 14:00 GMT today.
The storm will peak on Friday morning, with wind gusts up to 85mph (137kmh) possible.
The statement adds "widespread disruption and damage in places" is expected.
Northern Ireland
All schools closed across the country.
Scotland
More than 120 schools are closed in Aberdeenshire., external
In the Highlands, there are six secondary schools, 20 primary schools and 17 nurseries closed - here's a full list, external.
Glasgow City Council says all of its school and nurseries are closed on Friday. Edinburgh Council have also closed all of the schools in the area.
All schools and nurseries are closed in Argyll and Bute, as well as in the Scottish Borders.
England
There are 30 schools closed in Northumberland , externaland seven in Cumberland, external.
Wales
Elfed High School in Flintshire is closed. More than 30 schools in Anglesey, external and two in Gwynedd, external say they will stay closed on Friday.
Simon King
BBC weather presenter and meteorologist
The centre of Éowyn is currently to the north-east of Northern Ireland and is moving in a north-easterly direction.
The strongest winds around a storm system like this are on the southern side, which is why wind gusts across Ireland and Northern Ireland are increasing now up to 90-100mph (144-161kph).
The power of this storm can’t be underestimated and so the advice is to stay at home - if you can - where the red warnings are in force today.
It will be a day of very strong winds for many, with dangerous conditions around coasts and hills as Storm Éowyn hits.
Watch our latest weather forecast to see how the storm is expected to develop where you are:
Storm Éowyn: Friday's forecast as red warnings in place for UK
With strong winds hitting the UK and the Republic of Ireland, dozens of flights have been cancelled and delayed already this morning.
Dublin Airport is the worst affected, with at lease 32 flights cancelled so far today, according to UK Flight Aware.
At Heathrow, one of the UK's busiest airports, 31 flights have been cancelled, and 28 are halted in Edinburgh.
Glasgow International sees 18 cancellations and George BestBelfast City has 13, representing nearly a third of today's flights.
It's expected more will be cancelled as the winds pick up speed.
As we’ve been reporting, there are currently no rail services in Scotland and some parts of the north of England this morning, with disruption expected to creep further south amid high winds.
Jake Kelly, managing director of Network Rail’s eastern region, tells our colleagues on the Today programme it’s “a decision we took very reluctantly and our advice to customers there is do not travel”.
He reiterates that the rest of the railway in the UK is currently working, but says customers should check before they travel.
“We have already got reports of trees coming down on the railway,” he says, and adds that his teams are unsure what damage they might encounter as winds increase through the day.
In Northern Ireland, a red warning for wind has now come into effect and will be in place until 14:00 GMT.
A major incident has been declared by police as the country is expected to be battered by the strongest winds since Boxing Day in 1998.
Schools, colleges and universities are closed and most hospital appointments have been cancelled as significant disruption is expected as a result of the storm.
The Met Office has said there is a danger to life and political leaders have urged people to stay at home.
A second red warning for Scotland will come into force at 10:00.
If you're just waking up and intending on travelling today, here's how Storm Éowyn might affect your travel plans.
Planes: A scan of Edinburgh and Glasgow airports show almost all flights are cancelled until at least this evening. Belfast City Airport is also under a red wind warning, and is showing most flights as cancelled. Flights further south in the UK are mostly unaffected at this time.
Trains: The National Rail website, external lists a large number of service operators expected to be impacted by Storm Éowyn, including:
Roads: There is less disruption on the roads so far, with only four travel alerts reported by National Highways, external:
Simon Calder, a travel journalist, says his cycle to King's Cross this morning was "windy enough".
If the severe weather continues, there could be disruption to the UK's "two busiest airports, London Heathrow and London Gatwick", because they're so frequently used, Calder tells BBC News.
But meanwhile, it's "extreme disruption" in Northern Ireland and Scotland with "no public transport whatsoever in Northern Ireland... all Scottish trains cancelled," he adds.
Calder says from King's Cross, you cannot travel further than Newcastle and on the west coast mainline you're advised not to try and travel north of Preston.
Elsewhere, at the airports there are "hundreds of cancellations in Edinburgh and Glasgow, the Belfast airports in particular", he adds.
Calder says he's counting around 200 cancellations so far.
"Staff are being told stay at home, it's too dangerous for you to get to work, so I'm afraid it's going to be very messy for tens of thousands of people," Calder says.
Let's bring you another update on the Republic of Ireland, after that record gust of 114mph (183km/h) was recorded overnight.
A red warning is in place for 25 counties, and will take effect in County Donegal and Northern Ireland from 07:00 GMT.
There are already more than 560,000 premises without power, according to the Electricity Supply Board (ESB), which details "unprecedented, widespread and extensive damage to electricity infrastructure".
People have been urged to stay at home during Storm Éowyn, with schools and colleges closed and public transport services not operating while the red warning is in place.
Simon King
BBC weather presenter and meteorologist
The centre of Storm Éowyn is situated just to the north-west of Northern Ireland.
A core of very powerful winds are now battering the west coast of the Republic of Ireland, where a gust of 114mph (183km/h) was recorded at Mace Head at 05:00 GMT.
This makes it the strongest recorded gust of wind in Ireland, exceeding the previous record set in 1961 during Hurricane Debbie.
The wind is really starting to ramp up in Northern Ireland now as well. Kilowen, County Down has recorded a gust of 92mph at 06:00 GMT.
Ferry operators have had to cancel numerous Irish Sea crossings on Friday because of Storm Éowyn.
Irish Ferries have cancelled crossings in both directions between Dublin and Holyhead, Rosslare and Pembroke, and Dublin and Cherbourg.
Stena Line has also cancelled some crossings between Belfast, Cairnryan and Liverpool, as well as on the Dublin-Holyhead route. Ferries between Rosslare and Fishguard are also cancelled.
Both operators have given passengers the option to travel on alternative sailings.
All Calmac's Friday services on Scotland's west coast have been cancelled, as have Steam Packet Ferries services between Heysham and the Isle of Man.
Image source, PA MediaStorm Éowyn is set to bring very strong winds to Northern Ireland and central Scotland which will have a significant impact on airports in the affected area.
Glasgow and Edinburgh - Scotland's primary airports - have both said they will be limiting operations. When we checked their websites this morning almost all departures and arrivals through Friday morning and afternoon were listed as cancelled.
Belfast City Airport said there "may be some disruption to flights" and advised passengers to check the status of their flight with their airline. A significant number of its flights were also showing as cancelled into Friday evening.
Belfast International Airport said operations would be limited for the duration of the red weather warning, which is currently scheduled to be in force between 07:00 and 14:00 GMT.
Image source, PA Media
Image source, Noreensireland/BBC Weather WatchersOne of the BBC's Weather Watchers snapped this image in Whitehead in Country Antrim as Storm Éowyn approached
Given the warnings about the potential dangers of Storm Éowyn, public services and some businesses in Northern Ireland will be closed:
The first of the two red weather warnings issued for today is due to come into force in the next 90 minutes.
But the Met Office is expecting the impact of the storm to be widespread across the UK and has issued a series of other warnings in place for most of today:
Rob Corp
Live reporter
Image source, PA MediaMillions of people in Northern Ireland and parts of Scotland have been advised not to venture out this morning as Storm Éowyn heads east across the UK.
Schools are closed and road, rail, air and sea travel faces serious disruption as a rare Met Office red weather warning is set to come into force. Forecast gusts of up to 100mph (161km/h) in some coastal areas mean there is a danger to life from flying debris.
The red warning for the whole of Northern Ireland will be in effect from 07:00 GMT to 14:00 GMT. Bus and train services have also been suspended.
Then as the storm moves east, a red warning is in place across Scotland's central belt, including the country's biggest cities Glasgow and Edinburgh, from 10:00 GMT to 17:00.
We'll bring you live updates as Storm Éowyn makes its way across the UK, with the latest weather forecasts and travel updates. Do stay with us.