Forecasters have issued an amber weather warning over heavy rain for part of Scotland on Friday.

The Met Office alert, which covers Dumfries and Galloway and the Scottish Borders, says that 40-50mm of rain is expected quite widely and warns that fast flowing or deep floodwater is likely, causing danger to life.

The alert is valid from 3am until midday on Friday and also warns of the potential for difficult driving conditions and delays or cancellations to train and bus services.

The Met Office said that the deadly bomb cyclone that has sent temperatures plunging in the US is also causing the UK to experience wet and windy weather.

Meanwhile, the forecaster has updated its yellow weather warning for heavy rain on Friday which now runs from midnight on Thursday rather than 3am on Friday and expires at 2pm rather than 6pm on December 30.

The warning covers Central Scotland, Tayside and Fife, south west Scotland, Lothian and Borders and Strathclyde.

A yellow warning of snow and ice covers northern Scotland, apart from Orkney and Shetland, from midnight on Thursday until 9pm on Friday.

Met Office Meteorologist Simon Partridge said the wet and windy weather was being caused by the bomb cyclone in the US.

“The UK weather is going to remain unsettled with further spells of wet and windy weather due to the strengthening of the jet stream because of the weather in the US,” he said.

Mr Partridge added that the impact on the UK would be “nowhere near” as significant as it was on the US.

“The effect it’s having on the UK is nowhere near as dramatic because that system has brought up a lot of cold air further south, across the US,” he said.

Indeed, the cyclone is only having an effect on the UK due to its impact on the North Atlantic jet stream.

“What effect (the bomb cyclone) has had is to strengthen the jet stream, because the jet stream is basically driven by temperature differences.

“So the starker the difference in temperature between the northern edge of it and southern edge, the stronger the jet stream becomes.”

He said the knock-on effect for the UK is spells of wet and windy weather over the next seven to 10 days.

Thursday is forecast to be colder than Wednesday, with sunshine and some heavy showers in northern Scotland and western England, as well as a risk of hail and thunder.

Mr Partridge said that Thursday will be a “cooler feeling day” but “still rather windy and with showers” across the UK, while Friday to Sunday is forecast to be unsettled, with snow over the Highlands, showers and rain in southern England, and frosts and fog overnight.

He added: “And then on Friday, we’ll see another spell of wet, windy weather with milder temperatures and then similar sorts of patterns to that over the next few days.

“So the general sort of knock-on effect of the weather in the US is that in general the UK is going to be a little bit milder than it would normally be at this time of year.”