After a relatively quiet and cold first half of December, the weather is going to turn much more unsettled from Wednesday onwards – bringing heavy rain and an increased risk of flooding.

As always, the cause of this change is down to the position and speed of the jet stream. During the next five days, it is going to largely sit to the south of the UK, which means that low pressure systems will be steered towards us.

It’s not just the jet stream’s position that will play a part in our weather fortunes in the coming week, but also its speed. At times, it will reach around 200mph, which will increase the frequency at which areas of low pressure arrive on our shores.

swollen river g wp Flood risk increases as heavy rain returns

Autumn and winter is a time of year when much of the UK experiences greatest amounts of rainfall, so this in itself is not unusual.

However, this year, the problem is that that it has been very wet. We’ve had the wettest April and June on record, followed by the wettest summer in a century, with downpours continuing into the autumn.

The abundance of rainfall has led to the ground in many areas becoming sodden with water. It simply doesn’t have the capacity to soak up more, if any, water at all. So rather than water going into the ground, it stays above it, causing surface water or river flooding.

You only have to take a look at the Channel 4 News flood story archive to see how widespread and frequent flooding has been during the last six months.

Rainfall during next five days

The first area of low pressure will bring heavy rain and brisk winds to many places during Wednesday, Thursday and Friday.

umbreallas rain g wp 300x199 Flood risk increases as heavy rain returnsWhile there is a little uncertainty in exactly where the greatest amounts of rain will be, England, Wales and south east Scotland look most prone.

Latest predictions from the weather computer models have 30-50mm falling widely in these areas over the next five days, with some places seeing as much as 70mm.

With so much rain expected to fall, it is worth keeping an eye on the latest flood warnings from the Environment Agency and Scottish Environment Protection Agency.

Is Christmas still looking green rather than white?

There is still a trend for Christmas day to be unsettled with brisk winds, rain or showers and average temperatures, which means away from mountain tops in Scotland, snow is looking unlikely.

However, some of the weather computer models have shown hints of colder air arriving later on Christmas day, but it’s not a strong signal and can’t be trusted just yet.

As ever, you can get the latest five day forecast on the Channel 4 Weather website and follow my regular updates on Twitter – @liamdutton