Red warnings have been issued for Northern Ireland and parts of Scotland on Friday, as dangerous Storm Éowyn heads towards the UK.
The rare warnings for wind mean flying debris could cause a danger to life, with gusts up to 100mph (161km/h) along some exposed coasts.
Millions have been asked to stay at home – schools in a large part of Scotland and all schools in Northern Ireland have been advised to close and people have been told not to go out in both nations.
The Met Office warned people to expect damage to buildings with roofs blown off and significant disruption to travel.
Weather says it could be the storm of the century for the Republic of Ireland.
In the UK, the Met Office warns it is very likely there will be a risk to life and people should avoid travelling where possible. 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.
The red warning for the whole of Northern Ireland will be in force from 07:00 GMT to 14:00 on Friday, affecting the morning rush hour.
First minister Michelle O’Neill told reporters people can expect to receive an emergency alert on their phone this afternoon and “only to travel where it’s absolutely necessary”.
Stormont education minister Paul Givan said the decision to advise schools to close was taken to avoid any potential risk to life for children.
“Schools should put plans in place today for remote learning so that pupils can study at home,” he said.
Then as the storm moves east, a red warning is in place across Scotland’s central belt, including Glasgow and Edinburgh, from 10:00 to 17:00.
First Minister John Swinney announced in the Scottish Parliament people should not travel in areas covered by the red warning.
Schools in at least a dozen Scottish local authorities will be closed on Friday, including Glasgow, North and East Ayrshire, East Lothian, North and South Lanarkshire, East Renfrewshire, Midlothian, East and West Dunbartonshire, Inverclyde, and Argyll and Bute.
In Edinburgh, the Scottish Parliament will also be shut to all but essential staff due to the severe weather warning.
Across the UK winds will rapidly increase from west to east on Friday morning into the afternoon with peak gusts of 80-90mph (129-145km/h).
There will likely be a large number of trees blown over with widespread disruption to travel with roads badly affected, and flights, trains and ferries will be subject to cancellations.
Power cuts are also likely, some of which could last for a number of days.
Belfast City Airport warned there may be disruption to flights.
Train operators Avanti, LNER, Lumo, CrossCountry, and Grand Central and Northern have issued warnings not to travel in the north of England, Scotland and north Wales on Friday.
The AA urged drivers travelling in red weather warning areas to consider whether a journey is necessary, and if not to postpone it.
“If you do need to travel, make sure you’re prepared for what you may encounter. Expect to come across fallen branches and other debris on the roads, especially in rural areas,” roadside technician Chris Wood said.
Two amber warnings are also in place for parts of Scotland and the north of England on Friday. The entire rest of the country is under at least one yellow warning as Éowyn brings strong winds, rain and snow.
The Irish weather forecaster Met Éireann has issued blanket red weather warnings covering all of the country – with widespread gusts in excess of 80mph expected.
Forecaster Eoin Sherlock warned of “hurricane force winds”, adding that gusts in coastal areas may even exceed these already “extraordinary” levels.
Meanwhile, an amber warning for wind is in place on Friday from 06:00 to 21:00, for the north of England, north Wales and Scotland’s central belt.
Another amber warning warning for wind on Friday starts later at 13:00 to 06:00 on Saturday, for parts of Scotland.
However, the big change to the UK’s weather begins on Thursday, as heavy rain and strong and gusty winds move across the country.
Parts of the south coast of England, South West and much of the Welsh coast are covered by a yellow weather warning for wind until 18:00 GMT on Thursday.
It is likely sea fronts will be affected by spray and large waves and power and travel disruption.