People on their way home for Christmas face "quite severe" disruption as strong gales and torrential rain are set to cause travel chaos.
Severe weather warnings are in place for much of the UK, threatening to cause localised flooding across southern England and Wales.
Strong winds are expected, with speeds of up to 80mph in coastal areas of southern England, while Scotland is set to see some snowfall, the Met Office said.
Emma Compton, a forecaster at the Met Office, said this morning that the disruptive weather is already under way, and will be widespread in just a few hours, adding: "The disruption to transport could be quite severe."
She said the South West and Wales have already been hit by rain, with heavy rain battering Cornwall, and said eastern parts will experience the same as the morning goes on.
"By lunchtime it will be everywhere," she said. "It's going to be fairly persistent."
Ms Compton warned that south west-facing places will see the worst of the rain, adding that it is "already falling on quite saturated ground", with the risk of flooding.
"There will be snow for a short time in Scotland but it will turn back to rain," she said.
Ms Compton said 0.8in (20mm) to 1.2in (30mm) of rain is expected in some places, but southern facing slopes may get more.
"Places like Dartmoor in Devon could see 60mm (2.4in)."
In terms of wind, the forecaster said it is already beginning to pick up in the West with gales at 30mph-40mph.
The worst place will be the south coast and any higher ground, which could see "severe gales" at 70-80mph, Ms Compton warned.
"But more places inland could see wind speeds up to 60mph, so that's going to be strong throughout the course of the day and ease this evening."
Ms Compton warned that the wind will pick up again during the night and Kent faces particularly strong gales.
Train operators are warning of widespread delays, disruptions and cancellations, and have made contingency plans with bus firms.
Network Rail said speed restrictions are likely on some routes and many train companies are advising passengers with tickets for today to travel earlier than their booked time, or to wait until tomorrow.
The AA has also warned that disruption is likely on the roads, and advised drivers to avoid any flood water.
The Environment Agency urged people to check its flood forecasts and warned people to keep away from coastal promenades because of wet and windy conditions.
Six flood warnings were in place this morning, along with 74 less serious flood alerts, the majority in southern England.
The worst of the weather is expected to be clear by the end of Christmas Eve, the Met Office said.
Chris Burton, from weather forecaster MeteoGroup, said the combination of wind and rain could cause flooding, bring down trees and leave widespread disruption to rail and road travel.
He said: "The rain is going to be heaviest and most persistent across southern areas of England, the Midlands and Wales, with up to 2in (50mm) of rain falling.
"There has been plenty of wet weather in the last week or so. There is a risk of localised or even widespread flooding if the rain continues overnight tonight, as expected.
"Strong winds will continue throughout today, with gusts of 60mph across England and Wales, Northern Ireland and western Scotland. Overnight they will be even stronger, with gusts of 70mph or 80mph possible across south east England and East Anglia.
"But by first thing tomorrow morning the weather should have turned a bit less wet and windy."
Rail operators are bracing themselves to deal with disruption from debris on tracks and high winds, and many are removing ticket restrictions to allow passengers to avoid the worst of the weather problems, Network Rail said.
Passengers are advised to check with their rail operator for the latest updates on travel restrictions and delays.
South West Trains said it would be running an amended timetable from 1pm until at least 6am tomorrow, with significantly reduced speeds across the whole network, while First Capital Connect will be running reduced services on its Great Northern and Thameslink routes.
Virgin Trains said tickets for specific trains today can be used at any time, and that expected 50mph speed limits between London Euston and Birmingham New Street and between Rugby and Nuneaton this afternoon mean it will not be able to run a full service, instead operating with fewer trains.
Greater Anglia and Stansted Express services will run at a reduced speed from 6pm until the end of the day, which will limit the number of trains that can run.
Trains are not expected to run tomorrow before 10am while engineers check for damage to overhead wires, and there will be no replacement bus services.
Robin Gisby, from Network Rail, said yesterday that hundreds of engineers would be out across the rail network today and tomorrow to react to problems, remove debris and repair damaged equipment
Visitor attractions have also had to put in contingency plans due to the weather.
Winter Wonderland in Hyde Park, central London, is closed today.
A spokesman said last night: "Due to the severe weather conditions of high winds and extreme rain predicted for London tomorrow, PWR Events and the Royal Parks have taken the decision to close Hyde Park Winter Wonderland on Monday for the day.
"All ticket holders for the Lycamobile Ice Rink, Zippos Circus, Magical Ice Kingdom and the Giant Observation Wheel will be refunded for Monday.
"We are very sorry for the inconvenience caused, but Hyde Park Winter Wonderland will be open as usual again on Tuesday at 10am. For further updates please follow us @hydeparkwinter."