Evening rush hour snow for south east England on Thursday

Since the weekend, it’s been clear that it’s going to turn much colder across the whole UK later this week, as Arctic air arrives from Canada.

However, one thing that has evolved day by day has been the detail of where and when it is likely to snow.

The main change to the forecast in the last 24 hours has been the increased chance of snow affecting southern parts of the UK during Thursday afternoon and evening – especially south east England and East Anglia during Thursday’s evening commute.

Even though the arrival of the snow is a mere day away, there is still a huge amount of uncertainty with the forecast, so there’s a good chance that the detail may change again.

Why is there so much uncertainty?

The difficulty for Thursday’s forecast is that cold air is coming down from the north, but at the same time, a small area of low pressure is moving in from the west across southern parts of the UK that has some milder air mixed in.

This means that initially, southern parts of the UK will see heavy rain, but as the colder air digs in and temperatures fall away into the evening, the rain will gradually turn to snow.

Such a scenario when rain is changing to snow makes it very hard to determine how much snow will settle – if it indeed settles at all – because heavy rain preceding the snow will make the ground wet beforehand.

However, the snow that falls will be wet snow, with big fluffy flakes that are capable of cooling the ground quite quickly, as well as giving very poor visibility.

How much snow and where?

For Wales, the West Country and west Midlands, heavy rain is most likely to turn to snow over the hills, mountains and moors as it clears during the afternoon and early evening.

There could be a slushy covering at sea level, but settling snow is most likely over the hills, mountains and moors, where there could be 1-5cm in places.

For the rest of southern England, the east Midlands and East Anglia, the heavy rain is likely to turn to sleet and snow through the evening rush hour – between 3pm and 6pm.

Snow is most likely to settle across the hills in these areas, where 2-7cm, locally 10cm is possible. At sea level, a covering of a 1-5cm is possible, but some places won’t see any settling snow at all.

For London, falling snow is likely anywhere in the city, but the urban heat island effect means it’s unlikely to settle in zones 1-3. In zones 4 and beyond, snow may settle in places – especially in higher up places such as Harrow-on-the-Hill, Highgate, Crystal Palace and out towards the Surrey Hills.

Snow elsewhere in the UK

There are also snow warnings for Thursday across Scotland, Northern Ireland, north west England and Wales.

In these areas, as much as 1-5cm of snow could settle at sea level, with 5-10cm possible over the hills and mountains, rising to as much as 10-20cm over the Scottish mountains.

For Scotland, gale-force winds accompanying the snow will give blizzards over the mountains, with the possibility of lightning giving thundersnow.

Lots of talk about thundersnow. It's quite rare, but here's more about how and why it happens. pic.twitter.com/z4VdoMaAyJ

— Liam Dutton (@liamdutton) January 11, 2017

How long will the cold weather last?

At the moment, the cold weather is set to last until the end of the weekend across much of the UK, with further sleet and snow likely in places.

With temperatures falling below freezing at night, it means that ice will be an additional hazard that will continue well into the daytime where snow is lying on the ground.

There is also some suggestion that the cold weather could continue through next week across southern parts of the UK, although at this stage, that is still fairly uncertain.

Don’t forget, you can get the latest forecast on the Channel 4 Weather website. I’ll also be posting regular update on Twitter – @liamdutton