In my blog last week, I wrote that I expected the weather across the UK to turn colder towards the middle of January – something that is still on the cards. However, what still remains very uncertain is how cold it will get.

The reason for the unusually high level of uncertainty relates to something else that wrote about last week, called sudden stratospheric warming.

This is where a sudden, sharp rise in temperature occurs high up in the atmosphere at 30-50km and can cause the normal west-to-east flow of air to slow down or reverse direction completely.

Initially, this slowing down or reversal of the west-to-east flow of air only happens high up in the atmosphere and doesn’t really affect the weather at the surface.

snowy church g wp Turning colder this week, but how cold?

However, eventually, the same effects can gradually percolate down into the lower parts of the atmosphere, which then brings a big change in weather pattern – often favouring blocking areas of high pressure to form.

The position and orientation of these blocking areas of high pressure in relation to the UK are crucial because they determine wind direction and where our weather comes from.

It is this aspect of the forecast that the weather computer models are struggling to get to grips with – hence the uncertainty as to how cold it will get.

Why do weather computer models struggle in these cases?

stratosphere earth g wp 300x199 Turning colder this week, but how cold?Most weather computer models tend to focus on what happens in the lowest part of earth’s atmosphere (the troposphere) because this is where weather happens.

As a result, they are not very good in taking into account sudden stratospheric warming events as they occur so much higher up. Also, the link between stratospheric warming and blocking high pressures is a relatively new discovery in meteorology, with research ongoing.

The Met Office has tackled this problem by developing an additional model, called GloSea4. It takes into account what is going on higher up in the stratosphere in an attempt to better predict the effects it may have on the weather lower down.

How cold will it get this week?

After the mild start to January, with temperatures widely reaching 10-13C, it will turn colder for all parts of the UK by Wednesday.

frostyfield wales g wp 300x199 Turning colder this week, but how cold?A cold front will bring colder air in from the north west, with temperatures coming back down to average for this time of year – around 4-7C by day.

Following the gloomy damp weather we’ve had during the last week, there’ll also be a change to brighter conditions with some sunshine at times, although frost and fog is possible at night.

Next week – even colder?

While the cooling trend down to average temperatures this week is certain, the uncertainty kicks in from this weekend onwards.

It is beyond this weekend that the weather computer models are really having difficulty in working out what the weather will do next, largely due to the reasons I have mentioned above.

However, one possible outcome is that high pressure will become established to the north east of the UK, which would bring very cold air from Europe with some snow. At this stage, I’d rate the chance of this happening at 30 per cent – so low but not insignificant.

As always, I’ll keep you updated with the latest here on my blog. You can also follow my updates on Twitter – @liamdutton