The Met Office said there would be a "multi-pronged attack" of wind, rain and snow striking the country tomorrow.
The heavy rain could lead to more flooding as downpours of up to 40mm (1.6 inches) could fall in just six hours, a spokesman said.
The weather has already caused major problems for Glasgow rail travellers after Network Rail was forced to close part of the West Coast Maine Line yesterday due to high winds.
More bad weather is likely to affect roads, rail and ferry journeys.
In the highlands of Scotland, Wales, northern England, Northern Ireland and Scotland, the rain could turn to snow, with 5cm to 10cm (2-4 inches) falling above 300 metres and perhaps as much as 20cm (8 inches) above 400 metres.
Met Office forecaster Steven Keates said coastal regions could see gusts of 70mph.
He said: "By the start of Friday morning, the next band of heavy rain will move towards the south-west of England and this is going to push its way northwards across much of the UK throughout Friday, it will reach Scotland by the evening.
"It's a multi-pronged attack. First of all the rain, we're looking widely at 15mm to 25mm across the southern half of the UK, with 30mm to 40mm expected in parts of the south west and south Wales.
"This could fall in a six-hour period so there is concern that it could well lead to some surface water flooding, but that could also have knock-on effects into the rivers systems as well.
"The weather warning has been upgraded to amber now for rain for south west England which are valid from 6am tomorrow morning to 2pm tomorrow afternoon.
"But there is a wider yellow warning across a good part of southern England and Mid and south Wales as well.
"As the rain trudges its way further north, it's going to encounter some quite cold air so then we get a snow risk developing."
He said the areas most at risk were the highlands of Scotland, Wales, northern England, and Northern Ireland.
"We're looking at 5cm to 10cm above 300 metres and perhaps as much as 20cm above 400 metres," he said.
"Given the strong winds it is going to be pretty miserable tomorrow.
"There is a yellow warning out for strong winds across the south coast of England from Cornwall to Kent then up the eastern coast to Norfolk.
"It's a renewed risk of severe gales. In terms of wind strength there could be 60mph to 70mph gusts are quite possible along the coasts but not explicitly.
"Finally, with the strong winds along there shore there will be some big waves again. There is a risk of some localised coastal impacts, not helped by the fact that defences have been breached and damaged from previous events."