Gale-force winds of up to 100mph have battered the UK as Storm Hector hit, with the Met Office warning of a threat to life.
Much of Britain is covered by a yellow warning, but the north and west of Northern Ireland - which will see the first winds hit - was subject to a more serious amber warning.
The Met Office said injuries and a danger to life was "likely" in coastal areas in Northern Ireland, with the chance of large waves and potential for beach debris to end up on roads, sea fronts and properties.
It said winds had reached 100mph at the very exposed Cairngorms in Scotland.
People have been advised to take care during rush hour, with potential disruption due to fallen trees.
The storm follows a weeks-long spell of sunshine and warmth, as Britain basked in an unusually hot start to summer.
The storm could disrupt royal proceedings; the Queen and the Duchess of Sussex are due to have their first official engagement together as they open Cheshire's Mersey Gateway Bridge.
Although there is not a weather warning yet for that area, they could face some stronger winds than usual.
Winds have reached 100 mph at the very exposed Cairngorms in #Scotland. Here are some low level #wind gusts which are unseasonably high considering we're in June #StormHectorpic.twitter.com/2luXbnKMWG— Met Office (@metoffice) June 14, 2018
The Met Office said in a statement: "The strongest winds will reach the west coast during the early hours of Thursday before spreading eastwards during the day.
"Westerly winds are likely to gust between 50 and 60mph in many areas and possibly around 70mph in some exposed locations. Winds should gradually ease from the west during Thursday afternoon.
"In addition, a spell of heavy rain will accompany the wind with the highest rainfall totals over parts of western Scotland."
The yellow warning remains in place until 3pm.
Met Office forecaster Sophie Yeomans said: "Over the rest of England and Wales it will just be a strong, blustery day.
"With any outdoor events planned, it may be something people are unprepared for or come as unexpected compared to the weather we have had recently."
Travel disruption is expected, and road, rail, air and ferry services could be badly affected.
It is also possible some buildings may be damaged, with a slight chance of power cuts in some parts - with the strongest winds expected in Argyll, near Oban.
Weather map for Thursday and week ahead
While the storm may dampen some outdoor plans, the Met Office is confident the hot, sunny weather should return before long, with the end of June set to be drier.