People on their way home for Christmas face "quite severe" disruption as strong gales and torrential rain are set to cause travel chaos - with one transport boss saying "It won't be an easy task".
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.
The Department for Transport said the unsettled spell of weather will continue for the rest of the week.
Emma Compton, a forecaster at the Met Office, said: "The disruption to transport could be quite severe."
Robin Gisby, managing director of operational services for Network Rail, said the next couple of days will be a challenge.
"Our fundamental concern over the next 48 hours is to get everyone where they want to be for Christmas.
"It won't be an easy task given the forecast conditions and disruption is likely - but I want to be very clear that our priority is running as many trains as we can safely, rather than worrying about hitting the timetable bang on," he said.
He said h undreds of engineers and other staff will be across the network over the next two days, ready to react quickly to any problems, removing debris and fixing equipment where necessary.
Meanwhile, airports are advising passengers to check with their airlines before travelling and to leave additional time for their journey to the airport.
Forecaster Ms Compton said the South West and Wales were hit this morning, with heavy rain battering Cornwall, and she said eastern parts will experience the same as the day goes on.
The bad weather has resulted in hundreds of homes left without power in Cornwall.
A spokeswoman for Western Power Distribution said high winds brought down overhead power lines.
"Approximately 360 customers are without power off the Lizard in Cornwall because of overhead lines being down due to the high winds," she said.
"It started at approximately 8am and it should be resolved soon."
UK Power Networks said they were monitoring the weather closely, and said they have "robust emergency plans in place to cope with severe weather", adding that they have brought in additional staff covering engineer, technical and call centre roles.
The Tamar Bridge in Plymouth, between Devon and Cornwall, was closed to high-sided vehicles, motorbikes and caravans due to "high gusting winds".
"By lunchtime it will be everywhere," Ms Compton said this morning. "It's going to be fairly persistent."
The highest wind speed was recorded in Capel Curig in Wales, with gales blowing at 75mph.
Not far behind was Berry Head in Devon with gusts of 73mph, and St Mary's Airport in the Isles of Scilly with winds of 71mph.
A Met Office spokesman said: "The strongest winds will move across the country."
The largest amount of rainfall so far has been recorded in Whitechurch, Dyfed, where rainfall has reached 19.6mm (0.77 inches).
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, t he 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.
The QEII Bridge at Dartford was closed at lunchtime as winds were expected to exceed 60mph, the Highways Agency said.
The closure is expected to remain in place until approximately 4am tomorrow when it is predicted that the high winds will ease.
Michael Roberts, director general of the Rail Delivery Group which speaks on behalf of the rail industry, apologised to passengers whose journeys are affected by the weather.
He said rail staff are "doing everything they can", adding: "Where services are affected, operators are advising passengers to consider changing their travel plans and have already lifted ticket restrictions to help them do so.
"Passengers have been advised to travel earlier to take advantage of the greater flexibility being offered by some operators and anyone still looking to travel should check the National Rail Enquiries website for the latest information."
The AA's specialist severe weather team has been deployed across southern counties.
Darron Burness, head of the AA's flood rescue team, said: "We are working with the Environment Agency and the emergency services to provide support in the South West and elsewhere in southern counties.
"However, the situation is fast developing and we will change our plans to ensure we offer support where it is most needed. We expect the worst of the winds tomorrow to be in Northern Ireland, south and central Scotland and northern England."
The Environment Agency has warned communities across the South East to be vigilant and prepare for possible flooding.
There are 25 flood alerts in force across the region and the Environment Agency is asking people to stay away from promenades along the south coast because of the wet and windy conditions.
It is also asking people to takes steps to protect their homes from flooding and not to drive through dangerous floodwater.
Simon Beavan, regional flood and coastal risk manager, said: "With more unsettled weather forecast for the South East, following recent heavy rainfall, there could be a lot of water in low-lying lands and on roads.
"Those who live in an area vulnerable to flooding should think about the precautions they can take to limit the damage a flood can cause to themselves and their property. Be prepared: check the Environment Agency website for the latest situation and advice."
Train operators have been 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 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 are in place, along with 104 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.
A spokeswoman for Western Power Distribution said the number of homes and businesses without power in Cornwall has increased to 3,000, with a further 2,000 properties also without power across the South West.
The spokesman for the Met Office said a new wind speed high had been recorded at Capel Curig in Conwy, North Wales, with gusts blowing at 87mph.
Not far behind was Berry Head in Torbay, Devon, with gusts of 76mph.